Plant sources of protein alone can provide adequate amounts of the essential and nonessential amino acids, assuming that dietary protein sources from plants are reasonably varied and that caloric intake is sufficient to meet energy needs. Whole grains, legumes, vegetables, seeds, and nuts all contain essential and nonessential amino acids. Conscious combining of these foods within a given meal, as the complementary protein dictum suggests, is unnecessary. Additionally, soy protein has been shown to be nutritionally equivalent in protein value to proteins of animal origin and, thus, can serve as the sole source of protein intake if desired¹².(table 1 here)

Although most vegetarian diets meet or exceed the Recommended Dietary Allowances¹³ for protein, they often provide less protein than nonvegetarian diets. This lower protein intake may be associated with better calcium retention in vegetarians and improved kidney function in individuals with prior kidney damage. Further, lower protein intakes may result in a lower fat intake with its inherent advantages, because foods high in protein are frequently high in fat also.

Plant carbohydrates are usually accompanied by liberal amounts of dietary fiber. This is in contrast to animal products, which are devoid of fiber. Fiber has been shown to be important in the prevention and treatment of certain conditions and diseases.

Vegetarian diets that are low in animal products are typically lower than nonvegetarian diets in total fat, saturated fat, and cholesterol, factors associated with reduced risk of coronary artery disease and some forms of cancer.

Adequate iron nutriture depends on both the amount of dietary iron consumed and the amount absorbed. Inhibitors and enhancers affect the absorption of nonheme iron, the form of iron found in plants. However, inhibitors and enhancers can offset each other when a variety of foods is consumed. Vegetarians are not at greater risk of iron deficiency than nonvegetarians, but Western vegetarians generally have better iron status than those in developing countries. Western vegetarians generally have an adequate intake of iron from plant products. They also consume greater amounts of ascorbic acid, an important enhancer of nonheme iron absorption. In contrast, vegetarians in developing countries rely on food staples that are low in iron; consume less ascorbic acid; and consume more tea, which contains tannin, an inhibitor of iron absorption.

The Recommended Dietary Allowance¹³ for vitamin B12 is minute. Vitamin B12 is produced by microorganisms present in the guts or gastrointestinal tracts of animals and human beings, as well as in dirt on the surface of unwashed plants. Vitamin B12 is found in all animal products; hence, a pattern that includes animal products such as milk Bacteria produce vitamin B12 in the human gut, but it appears to be produced beyond the ileum, the site of vitamin B12 absorption in the intestine¹⁴.

Lack of intrinsic factor in the stomach, rather than diet, however, is he most common cause of vitamin B12 deficiency. Atrophic gastritis, with the consequent bacterial overgrowth of the upper gut, may also contribute to vitamin B12 deficiency, especially in the elderly. Plants provide no vitamin B12. In countries where sanitation is poor, vegans may derive vitamin B12 from foods that are contaminated with microbes, organisms that produce the vitamin, such as on the surfaces of unwashed fruits or vegetables. In Western countries, however, where sanitary practices are better, the risk of vitamin B12 deficiency may be far greater.

Vegans should include a reliable source of the vitamin in their diets. Spirulina, seaweed, tempeh, and other fermented foods are not reliable sources of vitamin B12. As much as 80% to 94% of the so-called vitamin B12 in these foods, as measured by microbiological assay, may be inactive analogs. Cyanocobalamin, the form of vitamin B12 that is physiologically active for human beings, is available from vitamin fortified foods such as some commercial breakfast cereals, soy beverages, some brands of nutritional yeast, and other products.

Certain plant constituents appear to inhibit the absorption of dietary calcium, but within the context of the total diet, this effect does not appear to be significant. Calcium from low-oxalate vegetable greens, such as kale, has been shown to be absorbed as well or better than calcium from cow's milk¹⁵. Calcium deficiency in vegetarians is rare, and there is little evidence to show that calcium intakes below the Dietary Allowance¹³ cause major health problems in the vegetarian population. The relatively high US recommendations for calcium intake, compared with those for populations consuming a more basic diet, are designed to compensate for the calciuric effect of high intakes of animal protein, which are customary in the United States. Studies have shown that vegetarians, on the other hand, absorb and retain more calcium from foods than do nonvegetarians^16,17.

Zinc is necessary for proper growth and development. Good plant sources include grains, nuts and legumes. Western vegetarians usually have satisfactory zinc status¹⁸.

Next: Part 3 (this document continued)

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