Food supplies both the energy for all of the body’s functions and the building blocks for growth and maintenance. Even in fully grown adults there is a requirement for energy and to build and maintain body components that are being replace. For example, the human stomach is constantly being lost and replaced. Also, there is increasing evidence that diet plays a major role in our defense against disease, including chronic diseases such as cancer and heart disease. Mental processes and behavioral attitudes appear to be influenced by nutritional status and specific nutrients.
The food scientist must consider the nutritive aspects of food rom two points of view: first, what nutrients do foods contain and what is a human’s requirement for these; and second, what are the relative stabilities of these nutrients and how are they affected by food processing, storage, and preparation. The science of nutrition, concerned with these broad areas, also deals with the physiological biochemical phenomena of food utilization as related to health.
The nutrients in food, required in balanced amounts to produce and maintain optimum health, belong to the broad group of carbohydrates, proteins, fats, vitamins, and minerals. Water, not generally classified as a nutrient, must not be overlooked, because a lack of water even for a short period is life threatening.
FOOD AND ENERGY
Food is the “fuel” which supplies chemical energy to the body to support daily activity and synthesis of necessary chemicals within the body. The major sources of energy for humans and other animals are carbohydrates, fats, and proteins. In addition to supplying energy, these nutrients have other specific functions, but their conversions to energy are of fundamental importance. The energy value of foods is measured in heat units called calories.
Calories
A calorie is the amount of heat required to raise the temperature of one gram of water one degree Celsius (from 14.5 to 15.5C). The kilocalorie (1000) is the unit commonly used in expressing energy values of foods. In an effort to standardize nonmetric and metric measurement under the International System of Units (SI), the kilojoule is sometimes used in place of the kilocalorie (kilocalories X 4.2=kilojoules). Calories remain, however, the more common unit of nutritionists, and so this term will be used in subsequent discussions.
The total potential energy of foods and food components is determined by burning the food in a steel bomb calorimeter under elevated oxygen pressure. The bomb, and water that it is immersed in, rise in temperature to an extend that is directly related to the gross energy content of the food. This is termed “calorimetry.”
The total potential energy of a food as determined by calorimetry may not be equal to the energy that can be derived from it by an animal or human. If a food or food constituent is not totally digestible, or if the food is not completely oxidized within the body, then its caloric value in metabolism will be less than its theoretical total energy content.
Not all carbohydrates (apart from their relative utilization in the body) yield equal amounts of energy when burned in a calorimeter. This is true of fats and proteins also. A fat that is more highly oxidized chemically than another will yield less energy on further combustion in a calorimeter than a corresponding un oxidized fat. Nevertheless, common averages form calorimeter studies in kcal/g generally are given as 4.1 for carbohydrates, 9.4 for fats, and 5.7 for proteins.
Carbohydrates such as sugars and starches, which generally are about 98% digested and fully oxidized by humans, provide about 4 kcal/g. Most fats are generally digested to the extent of 95%, yielding 9 kcal/g. proteins, due to incomplete digestion and oxidation, generally also yield an energy equivalent of 4 kcal/g. Thus, on an equal weight basis, fat generally yields 2.25 times as many calories as protein or carbohydrate. These simple relationships permit approximate calculations of the caloric values of foods when their compositions are known. Calories are needed to satisfy the body’s energy requirements for production of body heat, synthesis of body tissue, and performance of work. The greater part of the food we consume goes to satisfy these energy requirements. When the body performance little work, a greater proportion of this energy is conserved and stored in the form of fat. Likewise, when energy demands exceed the intake of calories, fat and other tissues are oxidized to provide this energy and the body loses weight. An excess intake of about 9 g (1/3oz) of butter or margarine daily can result in the deposition of about 3.2 kg (7 lb.) of fat in a year. This can be counteracted by walking approximately an additional 2.4 km (1.5 miles) daily.
Comprehensive calorie charts for common foods are readily available and are not included here, although the caloric content of a few representative foods are given in Table 4.1, which also includes the contents of other nutrient in these foods. while the caloric contents of foods are relatively fixed, a human’s caloric requirements vary widely, depending upon such factors as physical activity, climatic conditions, weight, age, sex, and individual metabolic differences. Table 4.2 gives daily dietary allowances for each of the major nutrients, but not calories, recommended by the Food and Nutrition board, National Academy of Science National Research Council. These recommended daily dietary allowance are intended to cover most normal persons as they live in the United States under usual work and environmental stresses. Caloric needs are related to energy expenditure and are dealt with in a separate section of the recommended allowances. Especially depending on physical activity, an adult male’s daily requirement can range from about 2500 to 5000 kcal. If he is a laborer and needs5000 kcal per day, he must eat some fat since a human stomach is not large enough to hold sufficient carbohydrates and proteins, consumed at usual mealtimes, containing this many calories.
Whereas fats are the most concentrated source of food calories, carbohydrates are the cheapest source and proteins the most expensive. It generally is agreed that quite apart from the other nutritional demands of the body, and except for very young children and the aged, a daily intake of less than about 2000 kcal represents dietary insufficiency. It is one of the sad contrasts of our time that while so many of the world’s people go hungry, in the United States and certain other countries obesity form excess calories intake is a major nutritional disease.
ADDITIONLA ROLES OF CARBOHYDRATES, PROTEINS, AND FATS IN NUTRITIOIN
Carbohydrates, proteins, and fats in many ways are interrelated and intercom veritable in animal metabolism. Although dietary carbohydrate is an economical source of calories and provides rapidly available energy for a variety of physiologic functions, the body can fulfill its energy and carbon requirements from proteins and fats. It also can synthesize blood glucose, liver glycogen, the ribose sugar components of nucleic acids, and other important biological carbohydrates from proteins and fats.
On the other hand, carbohydrates from the foods consumed help the body use fat efficiently. They do this by supplying an organic acid formed as an intermediate in the oxidation of carbohydrates. This organic acid is required for the complete oxidation of fat to CO2 and water. When fat is not efficiently oxidized, ketone bodies can accumulate in the blood and produce the disease condition known as ketone.
Carbohydrates also exert a protein-sparing effect. When carbohydrates are depleted in the animal body and the animal needs additional energy, it gets this energy by oxidizing fats and proteins. In the case of proteins, this energy requirement is thus satisfied at the expense of the body’s requirement for proteins and amino acids as components of body tissues enzymes antibodies, and other essential nitrogen containing substances. However, if carbohydrates are supplied, the body oxidizes them fats can exert a protein-sparing effect.
The role of carbohydrates such as cellulose and hemicellulose in providing fiber and bulk is essential to a healthy condition of the intestine. In addition, the microflora of the intestine is much influenced by the nature of carbohydrates in the diet. When these carbohydrates are comparatively slow to dissolve, as in the case of starch and lactose, they remain in the intestinal tract for longer periods than the more highly soluble sugars. In this case, they serve as readily available nutrients for growth of microorganisms that synthesize several vitamins of the B complex. On the other hand, the slow rate of absorption of lactose from the intestine can cause diarrhea in some adults concerning excessive amounts of this sugar. Lactose also appears to increase calcium retention in children.
The role of protein in supplying chemical building materials for the synthesis of body tissues and other constituents of life and in providing those essential amino acids that the body cannot itself synthesize have been mentioned.
The nutritional value of “The allowances, expressed as average daily intakes over time, are intended to provide for individual variations among most normal persons as they live in the United States under usual environmental stresses. Diets should be based on variety of common foods I order to provide other nutrients for which human requirements have been less well defined. See text for detailed discussion of allowances and of nutrients not tabulated.
Weights and heights of reference adults are actual medians for the U.S. population of the designated age, as reported by National Health and Nutrition Examination Survey II. The use of these figures does not imply that the height-to-weight ratios are ideal.
Because there is less information on which to base allowances, these figures are not given in the main table of RDA and are provided here in the form of ranges of recommended intakes.
Since the toxic levels for many trace elements may be only several times usual intakes, the upper levels for the trace elements given in this table should not be habitually exceeded.
Recommended Dietary Allowances, Revised 1989. Designed for the maintenance of good.
Different proteins depend on their different amino acid compositions. A complete protein is one that contains all of the essential amino acids in amounts and proportions to maintain life and support growth when used as the sole source of protein. Such a protein is said to have high biological value. Many animal proteins such as those found in meat, poultry, fish, milk, and eggs generally are of high biological value. An exception is gelatin, which contains limited amounts of isoleucine, threonine, and methionine, and no tryptophan. Plant proteins generally are not as high in biological value as animal proteins because of amino acid limitations. Thus, for example, most varieties of wheat, rice, and corn lack lysine; corn also lacks tryptophan; legumes are of somewhat higher protein quality but have limited amounts of methionine.
Incomplete proteins can be supplemented with the missing essential amino acids either in the form of synthetic compounds or as protein concentrates from natural sources. Blends of plant and animal products also can overcome essential amino acid limitations and produce nutritional adequacy, but complementary components should best be given at the same feeding since the body has very limited protein storage capacity and all amino acids are needed for daily protein synthesis. Much protein supplementation is now being practiced to improve world food resources.
The amount of protein required daily, which beyond early childhood may range from about 40 to 60 g (Table 4.2), depends on the body demand –the demand being greatest during growth, pregnancy, and lactation.
One of the severest needs for protein on a world population basis is in infant after weaning and in young children. Protein shortage or protein malnutrition can be dramatically reversed by proper diet. However, in instances where adequate protein and proper diet are withheld too long, recovery may not be complete due to irreversible damage and possible mental retardation.
In addition to supplying calories for energy, fats supply polyunsaturated fatty acids, at least one of which, linoleic acid, is an essential fatty acid. As in the case of the essential amino acids, linoleic acid is called an essential fatty acid because animals cannot adequately synthesize it and so it must be supplied by the diet as such. In rats and in human infants, absence of linoleic acid interferes with normal growth rates and results in skin disorder. Two other polyunsaturated fatty acids, linoleic acid and arachidonic acid, formerly were listed also as essential fatty acids. However, since the body can convert linoleic acid to arachidonic acid and since linoleic acid can only partially replace linoleic acid, we now regard only linoleic acid as an essential fatty acid. Good sources of linoleic acid include grain and seed oils, fats from nuts, and fats from poultry. Linoleic and other unsaturated fatty acids when present in high proportion of dietary fats can lower blood cholesterol levels under certain dietary conditions; more will be said about this in the last section of this chapter.
Vitamins A, D, E, and K are fat soluble and so are to be found associated with the fat fractions of natural foods. Additionally, phospholipids, which are organic esters of fatty acids and also contain phosphoric acid and usually a nitrogenous base, are partially soluble in fats. The emulsifying properties of lecithin were disused in Chapter 3. Lecithin, cephalic, and other phospholipids are found in brain, nerve, liver, kidney, heart, blood, and other tissues in addition to their presence in egg yolk. Because of their strong affinity for water, they facilitate the passage of fats in and out of the cells and play a role in fat absorption from the intestine and the transport of fats from the liver. Fat also physically insulates the body from rapid changes in temperature and helps cushion organs from sudden injury. Excess dietary fat is stored in the body’s adipose (fatty) tissue, as are fats formed from the metabolism of excess carbohydrates and proteins. These stored fats can be drawn upon as a reserve source of energy. In excessive amounts they contribute to obesity.
PROTEIN QUALITY
It previously was stated that the comparative value of different protein depends on their different amino acid compositions, especially their contents of the essential amino acids, leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine, plus histidine to meet the demands of growth during childhood. Although this statement is essentially true, it requires some further consideration.
Protein quality, or the nutritional value of a protein, is meaningful only in terms of the usefulness of a protein for specific vital purposes such as growth, replacement of metabolic losses and damaged tissue, reproduction, lactation, and general well-being. The usefulness of a protein may differ for several of these functions. Further, a measure of usefulness of a protein based on chemical analysis of it amino acid makeup is complicated by a number of factors. These include accuracy of the analytical method under conditions that can preclude the detection of one or another amino acid or cause its destruction, availability and digestibility of the protein from foods that may not be readily broken down by digestive enzymes or absorbed through the intestine, and factors contributing to unpalatability of the protein-containing food. Yet another factor has to do with amino acid imbalance. It is possible to have an excess of one or more amino acids relative to others in a protein. This can have a negative effect on growth rate.
These objections to chemical analysis of amino acid content to determine protein quality are not encountered when a biological method, such as an animal feeding study, is used. However, in this case a different set of obstacles is encountered. One of the most obvious is how closely results obtained with laboratory animals apply to humans. Even where question of digestibility and vital response are ruled out, the palatability differences between species must be considered in arriving at a valid measure of the nutritional usefulness of a protein source. Notwithstanding these difficulties, research has shown that results obtained with young rats generally are applicable to humans, and feeding tests under controlled conditions are far more easily carried out on rats than with humans. Such tests may be run under a variety of experimental conditions which then influence the interpretations that may be given the nutritional findings.
Several test methods using the rat have been developed and result in specific terms that the food scientist encounters in the area of protein evaluation. One of the commonest methods involves measurement of weight gain of rats per gram of protein eaten. This is known as Protein Efficiency Ratio (PER). Typical PER values representing protein quality from various foods are given in Fig. 4.1. One of the principal limitations of PER values is that results depend on the amount of food eaten, which can give an erroneous picture if the food is unpalatable to the test animal. The modification known as Net Protein Retention (NPR) improves upon. If two groups of animals are used and one is placed on the test protein diet and the other is placed on a protein-free diet, then the weight loss of the protein-free group can be compared with the weight gain of the protein-receiving group. Properly controlled, the test becomes independent of food intake. Another measure is the proportion of absorbed nitrogen that is retained in the body for maintenance and/or growth. This is known as Biological value (BV). It requires measurement of protein consumed and the fraction that is excreted in the urine and feces. Since Biological Value (BV) is corrected for the digestibility of the protein. Digestibility (D) also can be measured and is the proportion of consumed food nitrogen that is absorbed. When Biological Value (BV) is corrected for the digestibility factor, we get the proportion of nitrogen intake that is retained and this is termed Net Protein Utilization (NPZ), which is BV X D. Since the nutritive value of a protein food involves both the quality and quantity of protein contained, still another measure employs NPU multiplied by the amount of protein in the food-this is known as Net protein value. These are but a few of the approaches for measuring protein usefulness.
The complexity of the assessment of protein quality and usefulness has had particular relevant to efforts to develop high-protein foods and supplements to correct nutritional delicious in underdeveloped regions of the world. Here the importance of field studies with ……...subjects under real life conditions has been repeatedly observed. Many new facts of excellent protein content measured by sophisticated laboratory procedures have full in their purpose because of poor palatability, having been manufactured in a …… physical form or having been presented in a manner in conflict with acceptable custom or social status.
BIOABAILABILITY OF NUTRIENTS
As with protein, the contents of other nutrients in foods determined by chemical or physical analysis may be quite misleading in terms of the nutrient status of a food. Apart from amount, what is important is whether the nutrient is in a form that can be utilized in metabolism; that is, whether the nutrient is bioavailable. For example, adding small iron pellets to cereals would increase their iron content, but the iron would not be very available to people eating the cereal and, therefore, be of little value.
Many factors influence a nutrient’s bioavailability, including the food’s digestibility and the nutrient’s absorbability from the intestinal tract, which are affected by nutrient binding to indigestible constituents and nutrient-nutrient interactions in food raw materials. Processing and cooking procedures also can influence nutrient bioavailability. Apart from the food itself, different animal species exhibit variations in bioavailability of specific nutrients form a particular food. The age, sex, physiological health, consumption of drugs, general nutritional status, combination of foods eaten together, and clear factors all influence the ability of an individual to make use of a particular nutrient.
Bioavailability of carbohydrates, proteins, fats, vitamins, and minerals may be increased or decreased since all nutrients are reactive and generally present in varying ……. In food system. There are many example of how food composition, processing, and for age affect nutrient bioavailability. One example is the essential mineral iron. Under practical conditions its bioavailability from foods may be only 1-10% of its total level determined by chemical analysis. The recommended dietary allowances for nutrients in the United States and other countries attempt to take bioavailability into account. However, the many factors influencing nutrient bioavailability and the difficulties inherent in meaningful evaluation procedures leave much research in this area still to be done.
VITAMINS
Vitamins are organic chemicals, other than essential amino acids and fatty acids, that must be supplied to an animal in small amounts to maintain health. An exception to this is vitamin D, the only major vitamin the human body is known to be capable of manufacturing. Under certain circumstances, however, vitamin D may not be synthesized in adequate amounts and then it too must be supplied by diet or as a dietary supplement if life and health are to be sustained. Vitamins function in enzymes systems which facilitate the metabolism of proteins, carbohydrates, and fats, but there is growing evidence that their roles in maintaining health extend yet further.
The vitamins are conveniently divided into two major groups, those that are fat soluble and those that are water soluble. Fat-soluble vitamins are A, D, E, and K. Their absorption by the body depends on the normal absorption of fat from the diet. Water-soluble vitamins include vitamin C and the several members of the vitamin B complex.
Vitamin A (Retinol)
Vitamin A as such naturally occurs only in animal materials-meat, milk, eggs, and the like. Plants contain no vitamin A but contain its precursor, β-carotene. Humans and other animal need either vitamin A or β-carotene, which they easily convert to vitamin A, β-Carotene is found in orange and yellow vegetables, as well green leafy vegetables.
A deficiency of vitamin A leads to blindness, failure of normal bone and tooth development in the young, and diseases of epithelial cells and membranes of the nose, throat, and eyes, which can decrease are rarely seen in the developed world but are sadly too common in some parts of the world.
Food sources rich in vitamin A are liver, fish oils, dairy products containing butterfat, and eggs. Sources of its main precursor, β-carotene, are carrots, squash, sweet potatoes, spinach, and kale. Vitamin A and β-carotene also are made synthetically, as are other vitamins.
Until recently, vitamin A activity in food was expressed in terms of International Units (IU). The IU is a measure of a vitamin’s biological activity. Because the biological activity of preformed vitamin A (retinol), β-carotene, and other carotenoids differs, confusion can be avoided by expressing total vitamin A activity in terms of the equivalent weight of pure retinol. Thus, several countries have replaced IU with “retinol equivalents.” A retinol equivalent is equal to 1µg of retinol or 6 µg of the β-carotene. It is also equal to 3.33 IU of vitamin activity from retinol and 10 IU of vitamin A activity from β-carotene. In the United States, the recommended allowance is 80% of this, but it is increased during lactation. Like many other nutrients, excessive doses of preformed vitamin A can be toxic. Large intakes of carotene are not similarly harmful since the body will limit the conversion to vitamin A; however, yellow coloration of the skin may result to combat vitamin A deficiency, several countries in South America have passed laws that all sugar for home consumption be fortified with this vitamin.
Vitamin D
Vitamin D is formed in the skin of humans and animals by activation of sterols y ultraviolent light from the sun or by ultraviolent activation of sterols artificially. Such sterols as cholesterol and ergo sterol are involved. Cholesterol is found in and under the skin of animals. Irradiated ergo sterol from yeast has served as a vitamin D source phosphorus from the intestinal tract and is necessary for their efficient utilization. Shortage of vitamin D results in bone defects, the principal one being rickets. This shortage may occur when exposure to the sun is limited. Most foods are low in vitamin D, although good sources are liver, fish oils, dairy products, and eggs. In children, 400 IU of vitamin D per day is considered optimum, and this is the basis of fortifying milk with added vitamin D at the level of 400 IU per 0.946 liter (1 qtr.). in the case of vitamin D, 400 IU is equivalent to µg of naturally occurring form of the vitamin in animal tissues. Excessive intake of vitamin D provides no benefits and is potentially harmful.
Vitamin E
Also known as α-tocopherol, vitamin E is an ant sterility factor in rats and is essential for normal muscle tone is dogs and other animals, but its significance for humans is still uncertain. Vitamin E is a strong antioxidant and probably functions as such in human metabolism. Diets excessive in polyunsaturated fats can lead to the formation of per oxidized fatty acids that may reach harmful levels. There is evidence that vitamin E can prevent this. Further, vitamin E favors the absorption of iron and may play a role in maintaining stability of biological membranes. Because of its antioxidant properties, vitamin E also is able to spare carotene and vitamin A form oxidative destruction.
Vegetable oils are good sources of vitamin E, but vitamin E deficiency under practical conditions of human nutrition is rare. Vitamin E in large doses has been promoted as a remedy for numerous diseases and as an agent to prolong youth and increase sexual potency. There is little scientific evidence for such claims.
Vitamin K
Vitamin K is essential for normal blood clotting. Its deficiency generally parallels liver disease where fat absorption is abnormal. It also can be deficient in infants. This is prevented by giving infants vitamin K with their formulas. Good sources of vitamin K are green vegetables such as spinach and cabbage. Vitamin K also is synthesized by bacteria in the human intestinal tract. Thus, antibiotic therapy that destroys intestinal organisms can produce deficiencies of vitamin K and certain other vitamins synthesized by bacteria.
Vitamin C (Ascorbic Acid)
Vitamin C is the antiscurvy vitamin. Its deficiency causes fragile capillary walls, easy bleeding of gums, loosening of teeth and bone joint diseases. It is necessary for the normal formation of the protein collagen, which is an important constituent of skin and connective tissue. Like vitamin C favors the absorption of iron.
Vitamin C, also known as ascorbic acid, is easily destroyed by oxidation, especially at high temperatures, and is the vitamin most easily lost during food processing, storage, and cooking. Vitamin C-containing foods must be protected against exposure to oxygen to prevent losses.
The recommended daily allowance for vitamin C in the United States for the male and female adults is 60 mg. in the United Kingdom and Canada, the recommended daily allowance has been 30 mg. this is true with other vitamin and nutrient recommendations there is not complete international agreement.
Excellent sources of vitamin C are citrus fruit, tomatoes, cabbage, and green peppers. Potatoes also are a fair source (although the content of vitamin C is relatively low) because we consume large quantities of potatoes. Milk, cereals, and meats are poor sources.
Two of the more recent claims for vitamin C are that it removes high levels of cholesterol from the blood of rats and prevents clods in humans. The significance of the rate studies in relation to humans has not yet been established. A very high level of one or more grams of vitamin C taken daily in the form of tablets has been advocated by some as a way to prevent colds. However, the effectiveness of this treatment has not been supported by the medical profession or the FDA.
Vitamins of the B Complex Group
All members of the vitamin B complex generally are found in the same principal food sources, such as liver, yeast, and the bran of cereal grains. All are required for essential metabolic activities and several function as parts of active enzymes. Absence of a particular B vitamin results in a specific deficiency disease.
Thiamin (vitamin B1).
Thiamin was the first of the B vitamins to be recognized. The disease beriberi, caused by a deficiency of thiamin, is common where polished rice is a major dietary item. Fortification of rice or white bread with thiamin corrects this disease. A most important role of thiamin is in the utilization of carbohydrate to supply energy, where it functions as the coenzyme thiamin pyrophosphate, or carboxylase, in the oxidation of glucose.
Important to the food technologist is the sensitivity of thiamin to sulfur dioxide (SO2), a common food preservative chemical, and to sulfite salts. Sulfur dioxide destroys the vitamin activity and should not be used to preserve foods that are major sources of thiamin a practice that is prohibited by the FDA and the meat inspection laws. The recommended adult daily allowance for thiamin is about 1.0-1.5 mg, depending on age and sex: Best sources are wheat germ, whole cereals containing bran, livor, pork, yeast, and egg yolk. Thiamin is stable to heat in acid foods but less so in neutral and alkaline foods, and this is taken into account in food processing.
Riboflavin (Vitamin B2).
Riboflavin is the yellow-green pigment of skim milk and whey. It functions in the oxidative processes of living cells and is essential for cellular growth and tissue maintenance.
Deficiency in humans generally results in skin conditions, such as cracking at the corners of the mouth. Recommended daily allowance for adults in 1.2-1.7 mg, depending on sex and age. Liver, milk, and eggs and good sources. Meats and green leafy vegetables are moderate sources of riboflavin.
Riboflavin is quite resistant to heat but very sensitive to light and this is why brown milk bottles have seen limited use in the past. Paper cartons, which protect milk from light, are more practical.
Niacin (Nicotinic Acid).
Niacin, also referred to as nicotinamide in the United Kingdom, is not to be confused with nicotine from tobacco. A deficiency of niacin adversely affects tissue respiration and oxidation of glucose and results in the disease known as pellagra in humans. This is characterized by skin and mucous membrane disorders as well as depression and confusion. Pellagra can be cured by feeding niacin or by feeding the essential amino acid tryptophan from which niacin can be made in the body. The adult recommended daily allowance is 13-20 niacin, depending on sex and age. Good sources of this vitamin are yeast, meat, fish, poultry, peanuts, legumes, and whole grain cereals. Niacin is very stable to heat, light, and oxidation, but like other water-soluble nutrients it can be leached from foods during processing and cooking.
Vitamin B6.
Vitamin B6 is the name given to the closely related substances pyridoxine, pyridoxal, and pyroxamine. Although essential in the human diet for specific enzyme systems and normal metabolism, a deficiency of this vitamin does not cause a well-recognized disease. Vitamin B6 is widely distributed in foodstuffs-good sources being muscle meat, liver, green vegetables, and grain cereals with bran. The recommended daily allowance for adults is approximately 2 mg, and 2.2 mg during pregnancy and lactation. Women taking steroid contraceptive pills may require higher levels.
Pantothenic Acid.
Because pantothenic acid is widespread in foods, obvious symptoms of its deficiency are rare in humans. But a deficiency may appear in experimental animals on limited diets or in severely malnourished individuals. In this case there is a general lowering in the state of well-being of the individual with signs of depression, less resistance to infection, and possibly less tolerance to stress. The human requirement for this vitamin is not well established but is believed to be about 5 mg per day, including pregnant and lactating women. This is easily supplied in a normal diet.
Vitamin B12.
Called the anti-pernicious anemia factor, vitamin B12 also is important in nucleic acid formation and in fat and carbohydrate metabolism. Vitamin B12 also called Ciano-cobalamin, is the largest vitamin molecule and contains cobalt in its structure, giving rise to an essential requirement for the mineral cobalt in nutrition.
Vitamin B12 is synthesized by bacteria and molds and is a commercial by-product of antibiotic production. Good natural sources of this vitamin are liver, meats, and seafood. Strict vegetarians may not get sufficient vitamin B12 from their diets since it is virtually absent from plant tissues. The recommended daily allowance for adults is 2.0 µg. vitamin B12 activity is not restricted to a single substance but is exhibited by several structurally related compounds.
Folacin.
Folacin and folate are the names given to related compounds exhibiting the vitamin activity of folic acid. Like vitamin B12 folacin prevents certain kinds of anemias, is involved in the synthesis of nucleic acids, and is synthesized by microorganisms. Folacin is present in animal and plant foods, especially liver, leafy vegetables, legumes, and cereal grains and nuts. The recommended daily allowance for folacin is about 200 µg for adult males, 180 µg for females, and 400 µg during pregnancy. This allowance recognizes a limited biological availability of vitamin from certain foods of a mixed diet.
Biotin and Choline.
Two additional substances that are water soluble and generally listed with the vitamins of the B complex are biotin and choline. Biotin is active in the metabolism of fatty acids and amino acids. Choline is a component of cell membranes and brain tissue, and it functions in the transmission of never impulses. Biotin and choline are seldom in short supply when the diet is adequate in the other B vitamins. Future, these and other growth factors, such as inositol and para-amino benzoic acid, are produced by the normal microflora of the intestine.
Daily allowance and Insufficiency.
Recommended daily allowances for vitamins not only differ for children and adults, for different physiological states, and for different levels of physical activity, but a distinction also must be made between recommended levels and minimum acceptable levels. The recommended levels given in Table 4.2 provide a substantial margin of safety and may be as much as five times the minimum levels required to sustain life.
Although diet may provide liberal quantities of the various vitamins, several practices and life situations that have become common can result in vitamin inadequacies. As mentioned, women taking steroid contraceptive pills may require higher intakes of vitamin B6. Oral contraceptives also lower body levels of vitamins C, B1, B2, B12 and folacin. Heavy consumption of alcohol may result in vitamin B1, B6, and folacin insufficiency. Smoking reduces blood levels of vitamin C. Emotional stress can decrease absorption an increase excretion of vitamins and other nutrients. Prolonged use of certain drugs also can increase vitamin and other nutrient requirements.
MINERALS
Calcium and Phosphorus
Calcium and phosphorus are the minerals that humans require in the greatest amounts. Deficiencies result chiefly in bone and teeth diseases. Calcium also is necessary for clotting of the blood, for the function of certain enzymes, and for control of fluids through cell membranes. Phosphorus is an essential part of every living cell. It is involved in the enzyme-controlled energy-yielding reactions of metabolism. Phosphorus also helps control the acid-alkaline reaction of the blood. Highest requirements for calcium and phosphorus are for the young, and for pregnant and nursing mothers.
Not only is the dietary intake of these minerals important but also the percentage that is absorbed into the bloodstream. Because calcium and phosphorus can combine and precipitate one another, they actually interfere with the effective absorption of one another. Oxalates in foods like rhubarb also can precipitate calcium and make it unavailable for nutritional purposes. Milk and dairy products are excellent sources of calcium and phosphorus, and in normal diets there is seldom any deficiency of these minerals. In recent years the role of calcium in preventing the loss of calcium from bones (called osteoporosis) has been studied. This disease is especially prevalent in older women. There is some evidence that increasing calcium intake, especially when young, can help reduce osteoporosis late in life. Vitamin D is essential for absorption of calcium from the intestinal tract, and lactose also is effective in promoting this absorption. This makes milk, especially milk fortified with vitamin D, a particularly valuable source of calcium.
Magnesium
Magnesium is essential to the function of several enzyme systems, is important in maintaining electrical potential in nerves and membranes, is involved with liberation of energy for muscle contraction, and is required for normal metabolism of calcium and phosphorus. Deficiency symptoms are more common in farm and experimental animals, which may have a restricted diet, than in humans whose diets generally are adequate in magnesium.
Iron and Copper
Iron is required as a component of blood hemoglobin, which carries oxygen, and muscle myoglobin, which stores oxygen. Of all required nutrients, shortages of iron may be the most common inadequacy in the diets of the industrialized world. Copper aids in the utilization of iron and in hemoglobin synthesis. The need for iron and cooper is related to the rate of growth and to blood loss. Much of the iron in plant foods is bound in poorly soluble iron phytate and iron phosphates and is not bioavailable. Iron from animal sources generally is more readily absorbed in digestion, as is iron from soluble salts used in food enrichment and fortification.
Cobalt
As mentioned, cobalt is a part of vitamin B12. However, cobalt will not replace the need for vitamin B12 in humans.
Zinc
Zinc is an essential constituent of enzymes involved in carbohydrate and protein metabolism and nucleic-acid synthesis. Its deficiency results in impaired growth and development, skin lesions, and loss of appetite.
Sodium and Chloride
Sodium and chloride are the chief extracellular ions of the body. They are involved primarily with maintaining osmotic equilibrium and body-fluid volume. Chloride ion also is necessary for the production of hydrochloric acid of gastric juice. Great losses occur in sodium and chloride during loss of body fluids, such as perspiration during exercise, and these must be replaced to prevent weakness, nausea, and muscle cramps. A human’s daily intake from food of about 10 g of salt more than meets their needs, and indeed may be excessive since high sodium can contribute to elevating blood pressure. Vegetables are relatively low in salt and so vegetarians and grass-eating animals generally need salt supplementation to their diets.
Potassium
Potassium is the principal intracellular cation and with sodium helps regulate osmotic pressure and Ph. Equilibria. It also is involved with cellular enzymes function. Potassium is essential for life but rarely is limiting even in the most meager diets.
Iodine
Iodine is part of the thyroid hormone and is essential for the prevention of goiter in humans. There is never a shortage of iodine where saltwater fish are eaten. The central United States and parts of South America, away from the ocean, are short of indigenous iodine. Today, the common use of iodized salt prevents deficiency, and in the United States there is concern that iodine levels not become excessive.
Fluorine
The fluoride ion is required for the development of sound teeth with resistance to tooth decay. Diets of growing children appear to be low in fluorine since supplementation of water with about 1 ppm reduces incidence of tooth decay. No other dietary requirement for fluorine is well documented.
Other Elements
Several other trace minerals are required by humans in at least trace amounts, but normal diets generally provide these. Thus, manganese is needed for normal bone structure, reproduction, and functioning of the central nervous system. Chromium is required for normal glucose metabolism. Molybdenum is involved in protein metabolism and oxidation reactions. Requirements also have been demonstrated in experimental animals for selenium, nickel, tin, vanadium, arsenic, and silicon, but their roles in human nutrition remain to be determined.
FIBER
The role of indigestible components of plant materials in providing roughage and bulk and in contributing to a healthy condition of the intestine has long been recognized. Celluloses, hemicelluloses, pectin, lignin, and other plant substances that are not readily digested perform this role and are collectively referred to as fiber or dietary fiber. All of these substances hold water, tend to soften stools, and decrease stool transit time through the large intestine.
In addition to these benefits of a diet adequate in fiber, research over the past decade has revealed further physiological actions of fiber under specific conditions. These include the lowering of plasma cholesterol levels, decreasing the incidence of colon cancer, lowering insulin requirements of diabetics, and others. This has led to many exaggerated health claims for fiber beyond experimental findings and the promotion of many new high-fiber food products and supplements. Although the term fiber is used inclusively, it is clear that the fiber from different food sources contains varying proportions of the different indigestible components and these components are not equal in terms of physiological effect. Further, grinding and other processing can affect the physical properties (e.g., particle size) and, in turn, the water-holding capacity of a fiber from a particular source. Fiber also may bind minerals, making them unavailable for absorption; if excessive fiber is ingested, this binding could produce essential mineral imbalance and deficiency.
Diets that contain moderate quantities of cereal grains, fruits, and vegetables are not likely to be low in fiber nor excessive in mineral binding. Persons in good health consuming such diets would not be expected to benefit from high-fiber supplements.
WATER
About 60%, by weight, of a person’s body is water. A normal person experiences symptoms of dehydration when 5-10% of the body weight is lost as water and not soon replaced. Long before this occurs, thirst, weakness, and mental confusion are experienced. If the state of dehydration progresses further, the skin and lips lose elasticity, the cheeks become pale and the eyeballs sunken, the volume of urine decreases, and ultimately respiration ceases. Under certain conditions, a person may survive without food for about 5 weeks but can seldom live without water for more than a few days.
The need for water exists at the molecular level, the cellular level, and at the metabolic and functional levels. Water is the major solvent for the organic and inorganic chemicals involved in the biochemical reactions that are essential to life. Water is the principal medium that transports nutrients via body fluids to cell walls and through membranes. Water is the medium that carries nitrogenous waste products from the cells for ultimate elimination. The evaporation of water from the skin is own important mechanism for controlling and maintaining normal body temperature, essential for the controlled rate of metabolic reactions and physical comfort of the individual.
The quantitative requirement for water is directly related to the sum of water losses from the body. These include losses from excretion and elimination of body wastes, perspiration, and respiration. Any factors that increase the rates of these processes, such as exercise, excitation, elevated temperature, or low relative humidity, also increase the need for water replenishment.
An adult may consume 400 liters of water a year. About an equal amount is obtained from food. Given sufficient water, or water in excess, the body closely regulates its water content. Except in unusual cases of deprivation or illness, the body seldom suffers from a deficiency of water in the sense that there may be a deficiency of other essential nutrients. This is because, unlike many of the other nutrients, a decrease in body water causes almost immediate discomfort, driving the individual to correct the shortage.
STABILITY OF NUTRIENTS
One of the principal responsibilities of the food scientist is to preserve nutrients through all phases of food acquisition, processing, storage, and preparation. The key to doing this is knowledge of the stability of nutrients under different conditions. As shown in Table 4.3, vitamin A is highly sensitive (i.e., is unstable) to acid, air, light, and heat; on the other hand, vitamin C is table in acid but is sensitive to alkalinity, air, light, and heat. Because of the instability of nutrients under various conditions and their water solubility, cooking losses of some essential nutrients may be greater than 75% (Table 4.3). In modern food processing operations, however, losses seldom exceed 25%.
Where nutrient losses are unavoidably high, the law permits restoration or enrichment by the addition of vital nutrients. A common example is the enrichment of flour and white bread (Table 4.4). the standards for enrichment of food products are in a state of periodic revision as knowledge of nutrition increases.
The final nutritive value of a food reflects losses incurred throughout its history from farmer to consumer. Nutrients value begins with the genetics of the plant and animal. The farmland fertilization program affects tissue composition of plants, and animals consuming these plants. The weather and degree of maturity at harvest also affect tissue composition. Storage conditions before processing affect vitamins and other nutrients. Washing, trimming, and heat treatments affect nutrient content. Canning, evaporating, drying, and freezing alter nutritional values, and the choices of times and temperatures in these operations must be balanced between good bacterial destruction and minimum nutrient destruction. Packaging and subsequent storage affect nutrients. One of the most important factors is the final preparation of the food in the home and the restaurant the steam table can destroy much of what has been preserved through all prior manipulations.
With a few specific subpopulation exceptions such as the very poor or people with other medical problems, diseases resulting from insufficient nutrients in most of the developed world including the United States have nearly disappeared. This has led to a major shift in nutrition emphasis in the affluent countries. There is now much concern with the effects of consuming the wring amounts or mix of nutrients and how this may affect one’s risk from chronic diseases such as heart disease and cancer. The problems of obesity from overconsumption and its relationship to major degenerative diseases is also of concern. Increasingly, research, nutrition education, and food product development are focused on the relationship between diet and chronic disease and the problems associated with overconsumption.
Recent research indicates that diet is a significant factor in several diseases. A comprehensive report titled “Diet and Health” by the National Research Council found strong evidence that dietary patterns can influence several common diseases including atherosclerotic cardiovascular (heart) diseases and hypertension, and highly suggestive evidence that diet affects cancer. Certain dietary patterns seem to predispose for diabetes mellitus and dental caries.
The evidence for an effect on osteoporosis and chronic renal disease is insufficient. The U.S. Public. Health service has laid out large number of health objectives for the U.S. population in the year 2000 (“Healthy People 2000”). Prominent were a number of recommendations for changing American’s eating habits.
Atherosclerosis and Cardiovascular Disease
Atherosclerosis is used to describe several pathological processes occurring in a number of arteries and is responsible for coronary heart disease, stroke, and diseases of the peripheral circulatory system. Presently in the United States atherosclerosis and related cardiovascular diseases cause over one-half of all deaths, and the incidence of heart disease is greater than in any other country of the world. Atherosclerosis is a disease characterized by deposition of a fatty material on the walls of the arteries. This material consists essentially of cholesterol, triglyceride fats, fibrous tissue, and red blood cells. As the deposit continues to build, it restricts blood flow through the artery. When the coronary artery is involved, heart attack and death may follow. Coronary thrombosis refers to the presence of a blood clot in the coronary artery that blocks the normal flow of blood to the heart. Thus, atherosclerosis can contribute to a coronary thrombosis by narrowing the lumen of the coronary artery so that a clot is more likely to exert blockage.
Both animal studies and human studies have indicated a link between atherosclerosis and diet exists. The intake of saturated fats and cholesterol increases the likelihood of having elevated serum cholesterol which is associated with atherosclerosis. Other factors in addition to diet are associated with occurrence of atherosclerosis. Among them are obesity, hypertension, diabetes, sedentary living, cigarette smoking, and high bold cholesterol levels. The latter may be caused by diet or e of hereditary origin. Although diet does appear to be involved, it must be emphasized that its relative importance in contributing to atherosclerosis is not entirely clear. Since cholesterol, a sterol found in all animal tissues, eggs, milk, and other foods of animal origin, is a component of the atherosclerotic deposit, it has been reasonable to hypothesize that foods high in cholesterol can contribute to atherosclerosis. Such foods may increase the level of cholesterol in the blood. But other components of diet especially large quantities of saturated fats and sugars also can result in high levels of blood cholesterol. Further, some investigators find that high levels of blood triglyceride correlate even more closely with coronary disease than do high levels of blood cholesterol. High levels of blood triglycerides also result from the consumption of large quantities of saturated fats and sugars. Whereas consumption of large quantities of saturated fats can increase levels of both cholesterol and triglycerides in the blood, liberal quantities of polyunsaturated vegetable oils tend to decrease blood cholesterol.
Such considerations have influenced the thinking of many doctors and nutritionist. Until more is learned, many are advocating a diet less rich in fats and sugars, and the substitution of polyunsaturated vegetable oils for at least part of the saturated animal fats. A reduction in quantity of foods high in cholesterol also has been generally recommended.
All of this has had an influence on manufacturers of foods. Research has made it possible to lower the cholesterol content of egg and dairy products. Margarines and other fatty foods are available that have been manufactured with a high concentration of vegetable oils rich in polyunsaturated fatty acids. Such products are said to have a high polyunsaturated to saturated fat ration or high P/S ration. Because the complex interrelationships between heart disease and diet have not yet been fully explained, the Food and Drug Administration has been most cautious in regulating promotional claims for such products. In this regard, recognition also must be given to possible adverse effects from excessive levels of polyunsaturated fatty acids; these have been demonstrated under experimental conditions in test animals but remain uncertain in humans. However, as is discussed in Chapter 24, certain health claims regarding heart disease can now be made on food labels:
Hypertension
Hypertension, or high blood pressure, is a major contributor to death from cardiovascular diseases as well as diseases of other organs such as the kidney. It has a genetic component but is further augmented by obesity, lack of physical activity, emotional stress, cigarette smoking, and diet. Of dietary components, sodium has been most studied. Blood pressure is positively correlated with sodium intake in populations which routinely consume larger amounts of sodium. In 1989, the diet and Health committee of the National Academy of Sciences recommended that daily salt intake (as sodium chloride) be limited to 6 g or less. They also recommended that the use of salt in cooking be limited that foods be labeled with respect to their sodium content, and food processors modify products to contain less sodium. A number of low-sodium content foods have been introduced by the food industry in response to this concern.
Cancer
Among the many causes and contributors to various kinds of cancers, diet-related factors continue to be suggested and to promote controversy. A 1982 report titled “Diet, Nutrition, and Cancer,” issued by the National Academy of Sciences-National Research Council, summarized evidence leading to their that diet affects the risk of getting caner, especially specific kinds of cancer. The 1989 report of the Committee on Diet and Health of the National Academy of Science found sufficient epidemiological evidence that up to one-third of all cancers were related in some way to diet. This led to recommendations that diet be modified to decrease the risk of certain cancers.
The strongest casual relationships cited were between diets high in fat and low in fresh fruits and vegetables and the incidence of gastrointestinal-tract cancers (stomach and colon). This has led to the recommendations that fat intake should contribute no more than 30% of dietary calories. Fresh fruit and vegetable intake should be increased to at least five servings per day. Excessive consumption of cured and smoked foods, which are associated with increased incidence of cancers of the stomach and esophagus, and excessive consumption, of alcohol should be avoided. Less definite were positions relative to consumption, but not overconsumption, of vitamins A and C and certain other nutrients were noted also. However, it is not yet possible to say how effective diet alteration might be in reducing the incidence of cancers.
Dietary Guidelines and Recommendations
In addition to the Diet and Health report cited above, several other groups have issued dietary goas in order to arrive at a national nutrition policy which promotes better health. Dietary goals and guidelines have been issued and discussed by a number of health authorities including the Surgeon General’s Office, the U.S. Department of Agriculture and the Department of Health and Human Services, the National Academy of Science Board on Food and Nutrition, The National Cancer Institute, and others. Although there have been areas of disagreement, most reports recommend the following: avoidance of overweight; consumption of a variety of foods; reduction of total fat to less than 30% of calories and reduction in the amount of saturated fat and cholesterol consumed; moderation in the consumption of salt and alcohol; and increased consumption of fresh fruits and vegetable and other fiber-containing foods.
The general population in the United States has begun to alter their diet in response to some of these recommendations. The food industry has developed many new products in an attempt to respond to these needs. For example, a large number of reduced products have been introduced in recent years.
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