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The Vegetarian Diet

Source: The American Dietetic Association

Scientific data suggest positive relationships between a vegetarian diet and reduced risk for several chronic degenerative diseases and conditions, including obesity, coronary artery disease, hypertension, diabetes mellitus, and some types of cancer.

It is the position of The American Dietetic Association (ADA) that appropriately planned vegetarian diets are healthful, are nutritionally adequate, and provide health benefits in the prevention and treatment of certain diseases.

The eating patterns of vegetarians vary considerably. The lacto-ovo-vegetarian eating pattern is based on grains, vegetables, fruits, legumes, seeds, nuts, dairy products, and eggs, and excludes meat, fish, and fowl. The vegan, or total vegetarian, eating pattern is similar to the lacto-ovo-vegetarian pattern except for the additional exclusion of eggs, dairy, and other animal products. Even within these patterns, considerable variation may exist in the extent to which animal products are avoided. Therefore, individual assessment is required to accurately evaluate the nutritional quality of a vegetarian's dietary intake.

Studies indicate that vegetarians often have lower morbidity (1) and mortality (2) rates from several chronic degenerative diseases than do nonvegetarians. Although nondietary factors, including physical activity and abstinence from smoking and alcohol, may play a role, diet is clearly a contributing factor. In addition to the health advantages, other considerations that may lead a person to adopt a vegetarian diet pattern include concern for the environment, ecology, and world hunger issues. Vegetarians also cite economic reasons, ethical considerations, and religious beliefs as their reasons for following this type of diet pattern. Consumer demand for vegetarian options has resulted in increasing numbers of foodservices that offer vegetarian options. Presently, most university foodservices offer vegetarian options.

Health Implications of Vegetarianism

Vegetarian diets low in fat or saturated fat have been used successfully as part of comprehensive health programs to reverse severe coronary artery disease (3,4). Vegetarian diets offer disease protection benefits because of their lower saturated fat, cholesterol, and animal protein content and often higher concentration of folate (which reduces serum homocysteine levels) (5), antioxidants such as vitamins C and E, carotenoids, and phytochemicals (6). Not only is mortality from coronary artery disease lower in vegetarians than in nonvegetarians (7), but vegetarian diets have also been successful in arresting coronary artery disease (8,9). Total serum cholesterol and low-density lipoprotein cholesterol levels are usually lower in vegetarians, but high-density lipoprotein cholesterol and triglyceride levels vary depending on the type of vegetarian diet followed (10).

Vegetarians tend to have a lower incidence of hypertension than nonvegetarians (11). This effect appears to be independent of both body weight and sodium intake. Type 2 diabetes mellitus is much less likely to be a cause of death in vegetarians than nonvegetarians, perhaps because of their higher intake of complex carbohydrates and lower body mass index (12).

Incidence of lung and colorectal cancer is lower in vegetarians than in nonvegetarians (2,13). Reduced colorectal cancer risk is associated with increased consumption of fiber, vegetables, and fruit (14,15). The environment of the colon differs notably in vegetarians compared with nonvegetarians in ways that could favorably affect colon cancer risk (16,17). Lower breast cancer rates have not been observed in Western vegetarians, but cross-cultural data indicate that breast cancer rates are lower in populations that consume plant-based diets (18). The lower estrogen levels in vegetarian women may be protective (19).

A well-planned vegetarian diet may be useful in the prevention and treatment of renal disease. Studies using human being and animal models suggest that some plant proteins may increase survival rates and decrease proteinuria, glomerular filtration rate, renal blood flow, and histologic renal damage compared with a nonvegetarian diet (20,21).

Nutrition Considerations for Vegetarians

Plant sources of protein alone can provide adequate amounts of essential amino acids if a variety of plant foods are consumed and energy needs are met. Research suggests that complementary proteins do not need to be consumed at the same time and that consumption of various sources of amino acids over the course of the day should ensure adequate nitrogen retention and use in healthy persons (22). Although vegetarian diets are lower in total protein and a vegetarian's protein needs may be somewhat elevated because of the lower quality of some plant proteins, protein intake in both lacto-ovo-vegetarians and vegans appears to be adequate (16).

Plant foods contain only nonheme iron, which is more sensitive than heme iron to both inhibitors and enhancers of iron absorption. Although vegetarian diets are higher in total iron content than nonvegetarian diets, iron stores are lower in vegetarians because the iron from plant foods is more poorly absorbed (23). The clinical importance of this, if any, is unclear because iron deficiency anemia rates are similar in vegetarians and nonvegetarians (23). The higher vitamin C content of vegetarian diets may improve iron absorption.

Although plant foods can contain vitamin B-12 on their surface from soil residues, this is not a reliable source of B-12 for vegetarians. Much of the vitamin B-12 present in spirulina, sea vegetables, tempeh, and miso has been shown to be inactive B-12 analog rather than the active vitamin. Although dairy products and eggs contain vitamin B-12, research suggests that lacto-ovo-vegetarians have low blood levels of vitamin B-12. Supplementation or use of fortified foods is advised for vegetarians who avoid or limit animal foods (24).Because vitamin B-12 requirements are small, and it is both stored and recycled in the body, symptoms of deficiency may be delayed for years. Absorption of vitamin B-12 becomes less efficient as the body ages, so supplements may be advised for all older vegetarians. Lacto-ovo-vegetarians have calcium intakes that are comparable to or higher than those of nonvegetarians (25,26).

Calcium intakes of vegans, however, are generally lower than those of both lacto-ovo-vegetarians and omnivores (26). It should be noted that vegans may have lower calcium needs than nonvegetarians because diets that are low in total protein and more alkaline have been shown to have a calcium-sparing effect (27). Furthermore, when a person's diet is low in both protein and sodium and regular weight-bearing physical activity is engaged in, his or her calcium requirements may be lower than those of a sedentary person who eats a standard Western diet. These factors, and genetic influences, may help explain variations in bone health that are independent of calcium intake. Because calcium requirements of vegans have not been established and inadequate calcium intakes are linked to risk for osteoporosis in all women, vegans should meet the calcium requirements established for their age group by the Institute of Medicine (28). Calcium is well absorbed from many plant foods, and vegan diets can provide adequate calcium if the diet regularly includes foods rich in calcium (29). In addition, many new vegetarian foods are calcium-fortified. Dietary supplements are advised for vegans only if they do not meet calcium requirements from food.

Vitamin D is poorly supplied in all diets unless vitamin D-- fortified foods are consumed. Vegan diets may lack this nutrient because fortified cow's milk is its most common dietary source. However, vegan foods supplemented with vitamin D, such as soymilk and some cereals, are available. Furthermore, findings indicate that sunlight exposure is a major factor affecting vitamin D status and that dietary intake is important only when sun exposure is inadequate (30). Sun exposure to hands, arms, and face for 5 to 15 minutes per day is believed to be adequate to provide sufficient amounts of vitamin D (31). People with dark skin or those who live at northern latitudes or in cloudy or smoggy areas may need increased exposure. Use of sunscreen interferes with vitamin D synthesis. If sun exposure is inadequate, vitamin D supplements are recommended for vegans. This is especially true for older persons who synthesize vitamin D less efficiently and who may have less sun exposure.

Studies show zinc intake to be lower or comparable in vegetarians compared with nonvegetarians (16). Most studies show that zinc levels in hair, serum, and saliva are in the normal range in vegetarians (32). Compensatory mechanisms may help vegetarians adapt to diets that may be low in zinc (33). However, because of the low bioavailability of zinc from plant foods and because the effects of marginal zinc status are poorly understood, vegetarians should strive to meet or exceed the Recommended Dietary Allowances for zinc.

Diets that do not include fish or eggs lack the long-chain n-3 fatty acid docosahexanoic acid (DHA). Vegetarians may have lower blood lipid levels of this fatty acid, although not all studies are in agreement with this finding (34,35). The essential fatty acid linolenic acid can be converted to DHA, although conversion rates appear to be inefficient and high intakes of linoleic acid interfere with conversion (36). The implications of low levels of DHA is not clear. However, it is recommended that vegetarians include good sources of linolenic acid in their diet.

Vegetarianism Throughout the Life Cycle

Well-planned vegan and lacto-ovo-vegetarian diets are appropriate for all stages of the life cycle, including during pregnancy and lactation. Appropriately planned vegan and lacto-ovo-vegetarian diets satisfy nutrient needs of infants, children, and adolescents and promote normal growth (37). Dietary deficiencies are most likely to be observed in populations with very restrictive diets. All vegan children should have a reliable source of vitamin B-12 and, if sun exposure is limited, vitamin D supplements or fortified foods should be used. Foods rich in calcium, iron, and zinc should be emphasized. Frequent meals and snacks and the use of some refined foods and foods higher in fat can help vegetarian children meet energy needs. Guidelines for iron and vitamin D supplements and for the introduction of solid foods are the same for vegetarian and nonvegetarian infants. When it is time for protein-rich foods to be introduced, vegetarian infants can have pureed tofu, cottage cheese, and legumes (pureed and strained). Breast-fed vegan infants should receive a source of vitamin B-12 if the mother's diet is not supplemented and a source of vitamin D if sun exposure is inadequate.

Vegetarian diets are somewhat more common among adolescents with eating disorders than in the general adolescent population; therefore, dietetics professionals should be aware of young clients who greatly limit food choices and who exhibit symptoms of eating disorders (38). However, recent data suggest that adopting a vegetarian diet does not lead to eating disorders (39). With guidance in meal planning, vegetarian diets are appropriate and healthful choices for adolescents.

Vegetarian diets can also meet the needs of competitive athletes. Protein needs may be elevated because training increases amino acid metabolism, but vegetarian diets that meet energy needs and include good sources of protein (eg, soyfoods, legumes) can provide adequate protein without use of special foods or supplements. For adolescent athletes, special attention should be given to meeting energy, protein, and iron needs. Amenorrhea may be more common among vegetarian than nonvegetarian athletes, although not all research supports this finding (40,41). Efforts to maintain normal menstrual cycles might include increasing energy and fat intake, reducing fiber, and reducing strenuous training.

Lacto-ovo-vegetarian and vegan diets can meet the nutrient and energy needs of pregnant women. Birth weights of infants born to well nourished vegetarian women have been shown to be similar to birth-weight norms and to birth weights of infants of nonvegetarians (42). Diets of pregnant and lactating vegans should be supplemented with 2.0 micrograms and 2.6 micrograms, respectively, of vitamin B-12 daily and, if sun exposure is limited, with 10 micrograms vitamin D daily (43,44). Supplements of folate are advised for all pregnant women, although vegetarian women typically have higher intakes than nonvegetarians.

Meal Planning for Vegetarian Diets

A variety of menu-planning approaches can provide vegetarians with adequate nutrition. Figure 2 suggests one approach. In addition, the following guidelines can help vegetarians plan healthful diets. Choose a variety of foods, including whole grains, vegetables, fruits, legumes, nuts, seeds and, if desired, dairy products and eggs. Choose whole, unrefined foods often and minimize intake of highly sweetened, fatty, and heavily refined foods. Choose a variety of fruits and vegetables. If animal foods such as dairy products and eggs are used, choose lower-fat versions of these foods. Cheeses and other high-fat dairy foods and eggs should be limited in the diet because of their saturated fat content and because their frequent use displaces plant foods in some vegetarian diets. Vegans should include a regular source of vitamin B-12 in their diets along with a source of vitamin D if sun exposure is limited. Solely breast-fed infants should have supplements of iron after the age of 4 to 6 months and, if sun exposure is limited, a source of vitamin D. Breast-fed vegan infants should have vitamin B-12 supplements if the mother's diet is not fortified. Do not restrict dietary fat in children younger than 2 years. For older children, include some foods higher in unsaturated fats (eg, nuts, seeds, nut and seed butters, avocado, and vegetable oils) to help meet nutrient and energy needs.

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