To this day, vegetarian diets are considered to be deficient in many ways. Invariably the negative issues raised are protein, vitamin A, vitamin B-6, vitamin B12, vitamin D, iron, zinc and calcium adequacy. Omnivorous diets, on the other hand, are regarded as exemplars of dietary adequacy, and the impression is created that without adequate consumption of at least dairy products, serious dietary shortages will arise. Modern research has, however, shown that most of these assumptions are incorrect, and the reverse may indeed be true. Most vegetarian diets fare well when they are compared in their adequacy with dietary patterns of non-vegetarians. Vegans, vegetarians and whole-food omnivores whose diets were compared with those of the general public, were found to be exemplars of balance, variety and moderation. Also they were more in line with current dietary recommendations for nutrient intakes than were omnivorous diets.1
The explosion of knowledge in the last decades has made mankind acutely aware of all the possible dietary shortages which may arise. The fear of deficiency diseases, together with the inroads made by manufacturers of dietary supplements has led to a large-scale increase in intake of dietary supplements. However, dietary deficiencies in vitamins and minerals will only arise if impoverished diets are followed or if stress and disease place additional demands on the system. It is possible to exist without vitamin and mineral supplementation (as has mankind for much of its existence), a fact that has also been recognized by health organizations.2
Most people in the Western world consume huge quantities of vitamins and minerals in the form of supplements, and it has been found that a very real danger of potential toxicity exists in some of these cases. Supplement use is greater in females than in males, and the data suggests potentially harmful levels of vitamins A, C and E were being consumed, with some people consuming up to 275 000 IU/day of vitamin A. This is 55 times the Recommended Dietary Allowance (RDA). It was also found that more than 50% of people who consumed mineral supplements exceeded the RDA of iron, zinc and calcium whilst many consumed more than five times the RDA for calcium and iron.3 Excessive mineral intake can also impact negatively on health.4 Nevertheless, moderate dietary supplementation has its place, particularly in the case of children, and in cases where research has indicated that additional vitamins might be required. A detailed analysis of the vitamin and mineral composition of specific foods will be presented in chapter 7, and only those micronutrients which are of special interest to vegetarians will be discussed here.
Vitamin A
The World Health Organisation (WHO) estimates that 40 million children in the world suffer from vitamin A deficiency, but the incidence varies greatly from region to region. Approximately 350 000 infants and young children become blind annually because of vitamin A deficiency, and 70% of these die within one year.5 In a number of supplementation studies carried out in affected areas, it was found that there was on average a 33% reduction in mortality after supplementation, even though some studies reported no effect.6 Vitamin A (Retinol) is one of the fat-soluble vitamins, and it has a variety of functions. Vitamin A is one of the recognized anti-cancer vitamins in view of its antioxidant properties, but it also affords protection against infectious diseases, is essential for the formation of eye pigment, plays a critical role in growth and bone remodelling, maintenance of a healthy skin and epithelia, such as those of the respiratory tract and gastrointestinal tract, and it also plays a role in spermatogenesis and embryonic development.7 The need for vitamin A is increased in adolescents, and pregnant women require some 25% more vitamin A than non-pregnant women.8,9
Preformed vitamin A is found only in animal sources as it is usually associated with lipids, but provitamin A (β-carotene), the original source of retinol, is found in plant pigments. Vitamin A concentrations in the literature are normally given in International Units (IU). One IU is the equivalent to the biological activity of 0.3 mg of retinol or 0.6 mg of β-carotene. In 1974 the food and Nutrition Board of the US National Research council decided to replace the IU with the Retinol Equivalent (RE), as this measure accounts for the absorption and conversion of carotene to retinol. The equivalents of this unit are:
1 RE = 1mg retinol (3.33 IU)
1 RE = 6mg beta carotene (10 IU)
1 RE = 12mg other carotenoids (10 IU)
It thus takes six times more -carotene than retinol to meet the bodies needs, but on a vegan diet with plenty of fruits and vegetables, there is virtually no possibility of suffering from a vitamin A deficiency. In fact, oversupply can be a real danger especially in cases where large doses are taken as supplements. In its final state (as retinol) vitamin is toxic, and taking more than the recommended amount will lead to hypervitaminosis A the symptoms of which are fatigue, nervousness, headaches, dizziness and decalcification of bones. Vitamin A is poorly supplied in most meat dishes with the exception of liver, some fish and dairy products. RDA of vitamin A for adults is 1 000 RE/day for men and 800 RE/day for women. Vitamin A is moderately unstable and it is destroyed by heat, light, exposure to oxygen and acids. Diets rich in cooked animal products are thus not the best source of vitamin A. Provitamin A is, however, well retained in cooked vegetables, and a varied diet which includes pigmented fruits (particularly red and yellow) and green and yellow vegetables will supply ample vitamin A. Deficiency diseases in poor countries can be ascribed to a lack of fruits and vegetables in the diet and the fact that the diets of the underprivileged consist largely of grains which on the whole are poor suppliers of vitamin A.
Vitamin B-6
Vitamin B-6 is the name for three compounds that are precursors for this vitamin (pyridoxine,
pyridoxal and pyridoamine). Pyridoxal phosphate functions as a co-enzyme and it plays an important role in many enzyme reactions. Vegan vegetarians may have a lower vitamin B-6 status than the general public because certain factors in plants may influence the bio-availability of this vitamin. The presence of certain fibre types in the diet may inhibit the uptake of vitamin B-6. Some investigations show that the addition of cooked wheat-bran can reduce the bio-availability of the vitamin by 17% or less.10 However, the presence of cellulose, lignin or pectin has little effect on the absorption of vitamin B-6. Another factor which seems to affect the bio-availability of the vitamin is the presence of pyridoxine glucoside which is found in some plant foods, particularly the crucifers (cabbage family), and which can substantially reduce the availability of vitamin B-6. Finally the processing of foods also impacts on the availability of vitamin B-6. Thermal processing reduces the availability from animal products, and processing foods with a high vitamin C content has the same effect.11 Good plant sources of vitamin B-6 include whole grains, legumes and green leafy vegetables. In view of the factors that influence the availability of the vitamin, it is advisable to follow a varied whole-food diet. Whole foods are rich in fibre, and such a regime would eliminate the need for added bran. If care is taken to prepare vegetables in waterless cookware or by steaming, then the loss of vitamins through leaching will also be reduced to a minimum. Variety is, however, the most important watchword, and if practised, then there is no need for the elimination of any foods which may on occasion interfere with the uptake of vitamin B-6.
Vitamin B-12
Vitamin B-12 (cobalamin) is a complex molecule that contains the minerals cobalt and phosphorous. The vitamin is an important co-enzyme required for the metabolism of carbohydrates, proteins and fats. It functions together with folacin and plays an important role in DNA synthesis and the maturation of red blood cells. Severe vitamin B-12 deficiency can result in pernicious anaemia and can lead to irreversible neurological deterioration.
Plants do not produce vitamin B-12 and neither do animals. The vitamin is produced by bacteria, and animals thus obtain their supply from their intestinal bacteria, or from eating the flesh of animals. The bacteria that produce vitamin B-12 are very sensitive to acid and so they are confined to intestinal regions that have a low acidity. For ruminants there is thus no problem, as the rumen contains an alkaline medium and ample bacteria which can produce the vitamin. In non-ruminant plant eaters such as rodents and rabbits, the bacteria that produce vitamin B-12 are mainly confined to the posterior portions of the intestinal tract where absorption of the vitamin is minimal. They solve this problem through the phenomenon known as coprophagy (eating one’s own excreta), and thus satisfy their vitamin B-12 demands. Carnivores, in turn, obtain their supply of vitamin B-12 from the stored vitamin in the flesh of animals, but they also prefer to eat the rumen content of their prey, which is rich in nutrients, including vitamin B-12.
The human digestive tract also contains the bacteria which produce vitamin B-12, but these bacteria are again largely confined to the colon, where absorption is minimal or non existent because of the absence of the intrinsic factor required for its absorption.. The bacteria are there, and b -12 is produced in the colon as was proved by correcting vitamin B-12 shortages with extracts of human stools.12 Short of coprophagy, there is thus no other way of obtaining vitamin B-12 but through the diet. As plants do not produce vitamin B-12 (although there is evidence that some plants may produce small amounts), the vegan vegetarian can only obtain this vitamin from food that is contaminated with bacteria, or from the small amounts which are available from the intestinal bacteria. Vegan vegetarians have a high consumption of fibre, and as a consequence, they have higher concentrations of bacteria in the lower portions of the small
intestine where B-12 can still be absorbed because of a high enough concentration of the necessary intrinsic factor.13 The bacterial flora in the mouth can also contribute to the vitamin B-12 requirements of vegans.13 The more alkaline the diet, the higher the intestinal bacterial concentration will be, and great care should thus be taken to ensure proper food combinations and to consume correct proportions of alkaline to acid forming foods.
The requirements for vitamin B-12 are extremely low, and nobody needs more than 1 mg/day. It has even been found that doses as low as 0.1 mg/day could reverse symptoms of deficiency.14 Moreover, vitamin B-12 is reabsorbed very efficiently from bile and thus has the longest reserve capacity of all vitamins, and this explains why it takes up to 20 years to run out of vitamin B-12 after one stops consuming it. In cases of disturbed absorptive capacity, mostly because of intestinal infections or a reduction in the production of the intrinsic factor required for the absorption of vitamin B-12 it will however take only 3 years to run out of vitamin B-12.14 The production of the intrinsic factor is normally impaired when portions of the stomach have been operatively removed, or if there is an infection of the stomach mucosa (gastritis). This probably explains why cases of vitamin B-12 deficiency are rarely reported in the literature.6 Vegan vegetarians need thus not panic over the issue, but they should be aware of the possible shortages which may arise, particularly in small children, and should supply the lack in the form of a supplement or foods fortified with cobalamin. Fortified soy milk or vitamin B-12 fortified nutritional yeast are possible sources for this purpose. It is important to ensure sufficient dietary B-12 in the case of infants that are breast fed, as B-12 reserves will decline over time in mothers that breast feed if reserves are not maintained.13 For peace of mind in this regard, it would be prudent to have a serum analysis done to determine the concentrations of B-12 in the blood, as this is the most accurate way of determining B-12 status.
When purchasing fortified foods or supplements, it is important to note that the product contains cobalamin, and not some analogue of the vitamin. Most of the claims of B-12 content on products are incorrect, as the analysing techniques used do often not distinguish between analogues and the active B-12 which is cobalamin. Fermented soy foods, such as tempeh, also do not contain vitamin B-12, and neither does Spirulina, which is often sold in health shops as a source of vitamin B-12.14 Spirulina may even make matters worse because it contains analogues which may interfere with normal absorption of cobalamin.
Vitamin D
Vitamin D is really a pro-hormone, and its active form is the hormone 1,25-dihydroxycholecalciferol [1,25 (OH)2D3]. The two compounds with vitamin D activity are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Vitamin D2 is found in ergot, a fungus growth on cereals, and vitamin D3 is formed when the skin is exposed to sunlight. The vitamins are also found in yeast and the oil of fishes. Vitamin D is fat-soluble, and requires the presence of bile salts in order to be absorbed from the intestines. Vitamin D is essential for the absorption and transport of phosphorous and calcium, and it promotes normal bone mineralisation. Deficiencies result in malformation of the bone structure in growing children, a condition known as rickets.
Since vitamin D does not occur in plants per se, it is possible to develop shortages of this vitamin if exposure to the sun is minimal. Vegans living in sunshine-poor areas should thus provide some form of supplementation to children and infants in particular. Nutritional rickets may occur in infants if the nursing mothers are themselves deficient in vitamin D.8 It is therefore, advisable for nursing mothers, in sunshine-poor areas, to also take a vitamin D supplement. Margarines are normally fortified with vitamin D, but if oils and margarines are
avoided, then supplementation through tablets seems desirable in sunshine poor areas.
Iron
Iron deficiency is the most widespread nutrition problem in the world today. According to the WHO the incidence of iron deficiency in developing countries is 26% for men, 42% for women, 46% for school-age children and 51% for children 0 to 4 years of age.6 Iron is essential for the formation of haemoglobin and shortages will lead to anaemia. Iron, together with zinc and copper is also essential for maintaining immunocompetence. Deficiencies in either of these minerals will lead to increased susceptibility to infectious illnesses.15 An early symptom of iron deficiency is chronic tiredness, whereas dizziness, breathlessness, interference with body temperature regulation and constipation are further warning signs. In infants, iron-deficiency anaemia has been shown to delay psychomotor development and impair the cognitive performance, and deficiency in mothers during pregnancy increases maternal mortality, prenatal and perinatal infant loss and premerturity.6
Iron can enter into the body in two forms, nonhaeme iron and haeme iron. All the iron in plant foods is in the form of nonhaeme iron, whereas in animal tissues three fifths of the iron is in the form of nonhaeme iron and two fifths is in the form of haeme iron. Haeme iron is more readily absorbed than nonhaeme iron, and this has prompted the concern that vegetarians may suffer from iron deficiency. The RDA for iron is 10 mg/day for men and 18 mg/day for women in their childbearing years. In a recent survey of the relevant literature it was found that adult menstruating women need to absorb 2,84 mg/day of iron and, as only a fraction of the dietary iron is absorbed, this would require a dietary intake of 18,9 mg/day. Teen-age girls would require a somewhat higher intake of 21,4 mg/day. However, many factors, such as the type of contraceptive used and the type of diet consumed, impacted on iron requirements.16
Most vegetarians living in Western countries have less iron deficiencies than might be expected from the type of iron which they consume. One of the reasons for this phenomenon is that nonhaeme iron absorption is enhanced by other factors in the vegetarian diet. Vitamin C intake in vegetarians is normally high, and this enhances the uptake of iron, and can counteract the effect of absorption inhibitors such as phytate which acts as a chelator of iron. The mineral content of plant foods is often high, and this can also offset the effects of inhibitors. Black tea also contains high concentrations of inhibitors, which impact negatively on iron absorption and avoidance of this beverage is thus advisable. A further factor which can lead to inadequate iron absorption is a high calcium level in the diet.17 Finally it can be said that a vegetarian whole-food diet can supply all the iron requirements, provided a varied diet is followed, which includes a regular consumption of iron-rich foods as outlined in chapter 7.
Calcium (See also calcium in dairy products in chapter 4)
Calcium is the most abundant mineral in the human body. It is essential for a host of physiological functions and for the normal growth and development of the skeletal system. One of the most misrepresented issues in human nutrition is the issue of calcium, and some of these issues have already been discussed. In chapter 1 the negative impact of high protein, particularly animal-protein diets, on calcium absorption and storage was discussed, and in chapter 4 the availability of calcium from dairy products was also investigated. It was shown that most diets in affluent societies are directly responsible for calcium loss from bone, and that distinct correlations exist between the consumption of dairy products and the incidence of
osteoporosis. High-protein diets cause calcium loss in the urine, and animal proteins pose a greater risk than plant proteins.18,19,20,21,22 The reason for this is that sulphate (a product of protein metabolism) excretion is linked to calcium excretion.
High sodium diets also cause calcium loss via the urine as do diets rich in chloride.23 Calcium homeostasis is best achieved by a balanced relationship between macro- and micro-nutrients. Dietary excesses of anions seem to inhibit absorption of calcium, and dietary excesses of cations seem to cause calcium loss. Affluent diets and diets rich in animal products are usually high in sodium and can thus account for substantial calcium loss in the urine. In contrast, whole-food programmes will provide a superb relationship between the macro and micro nutrients, and will curtail the loss of urinary calcium. Some investigators have found that phytic acid (found in grains and legumes), may impact negatively on calcium absorption, but this issue has not yet been satisfactorily resolved as other investigators did not find similar results.23 Grains, legumes, nuts, some seeds and dark green vegetables, are excellent sources of calcium, and if care is taken to regularly include these calcium-rich foods in the diet, then concern for calcium deficiencies is unwarranted.
Zinc
The significance of zinc in the diet has only recently been appreciated. Zinc forms an important component of many enzymes known as metaloenzymes, of which many participate in the digestion and assimilation of nutrients. Zinc is also essential to the proper function of the endocrine system because it forms a structural component of the hormone receptor system.24 Moreover, zinc plays a role in the synthesis of RNA and DNA, and is essential for the proper functioning of the immune system.15
Most dietary guidelines will recommend the consumption of animal products as a source of zinc, but it has been found that, as in the case of calcium, the high protein content of diets rich in animal products impacts negatively on zinc availability.23,25 Incomplete hydrolysis of casein, the protein in milk, also inhibits the uptake of zinc.23 As in the case of calcium, phytate, found in grains and legumes, may inhibit the absorption of zinc, but this may be partially offset by the protein composition of these foods. Finally, it seems as if the toasting of foods renders the protein-phytate mineral complexes less digestible and leads to lower mineral availability.26
A vegetarian whole-food diet will supply more zinc than an omnivorous diet. Legumes and seeds have a relatively high zinc content ranging from 2.7-3.2mg/100g, and grains are also rich in zinc. The bran and the germ of wheat are the main storage area of zinc in grains and contain an average of 9.8 and 14.3mg/100g respectively.27 It is better to obtain zinc from whole foods, however, than from added bran and wheat-germ, as the high fibre content of such fortified meals will make the zinc less available. Vegetables and fruits are relatively low in zinc content, emphasizing the need for a varied diet which, beside fruits and vegetables, includes grains, seeds and legumes. The zinc content of selected plant foods is presented in table 6.1, and values for some animal products are given for comparison.
Table 6.1. The zinc content of selected foods. (Adapted from reference 27)