Position of the American Dietetic Association: Vegetarian Diets (1997)

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. Vegetarian diets, like all diets, need to be planned appropriately to be nutritionally adequate.

POSITION STATEMENT

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 and mortality 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.

Vegetarian diets low in fat or saturated fat have been used successfully as part of comprehensive health programs to reverse severe coronary artery disease. 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), antioxidants such as vitamins C and E, carotenoids, and phytochemicals. Not only is mortality from coronary artery disease lower in vegetarians than in nonvegetarians, but vegetarian diets have also been successful in arresting coronary artery disease. 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.

Vegetarians tend to have a lower incidence of hypertension than nonvegetarians. 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.

Incidence of lung and colorectal cancer is lower in vegetarians than in nonvegetarians. Reduced colorectal cancer risk is associated with increased consumption of fiber, vegetables, and fruit. The environment of the colon differs notably in vegetarians compared with nonvegetarians in ways that could favorably affect colon cancer risk. 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. The lower estrogen levels in vegetarian women may be protective.

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.

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. 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.

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. The clinical importance of this, if any, is unclear because iron deficiency anemia rates are similar in vegetarians and nonvegetarians. 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.

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. Calcium intakes of vegans, however, are generally lower than those of both lacto-ovo-vegetarians and omnivores. 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. 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. Calcium is well absorbed from many plant foods, and vegan diets can provide adequate calcium if the diet regularly includes foods rich in calcium. 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. 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. 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. Most studies show that zinc levels in hair, serum, and saliva are in the normal range in vegetarians. Compensatory mechanisms may help vegetarians adapt to diets that may be low in zinc. 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. 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. 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.

Figure 1 below presents food sources of nutrients that are often of concern for vegetarians.

FIG 1. Food sources of nutrients. Sources: Package information and data from: Pennington J. Bowe's and Church's Food Values of Portions Commonly Used.  16th ed.   Lippincott-Raven; 1994. Provisional Table on the Content of Omega-3 Fatty Acids and Other Fat Components in Selected Foods, 1988.  Washington, DC: US Dept of Agriculture: 1988:  Publication No. HNIS/PT-103.  Hytowitz DB, Matthews RH.  Composition of Foods: Legumes and Legume Products. Washington, DC:  US De