Vegetarian diets are often nutritionally inadequate

There is a possibility that some 'strict vegetarians' may be at risk of deficiencies. There are different degrees of- vegetarianism:

Vegetarians obtain relatively more of their protein from plant sources than people with a mixed diet. Therefore, it is important that they eat complementary proteins (a mixture of proteins that will provide all nutritionally essential amino acids) from grains, legumes, and sometimes nuts. This is because grains are relatively deficient in the essential amino acid lysine, legumes (peas, beans and lentils) in, the essential amino acid methionine, and nuts can be relatively deficient in both lysine and methionine. But when eaten together these plant foods yield high-quality protein.

Vegans, but not vegetarians who eat some animal derived food (e.g., milk or eggs), are theoretically at risk of vitamin B-12 deficiency. Vitamin 8-12 is not found in the plant world in a form that humans can use. It is made by microorganisms, from which animals obtain it. Thus, in less than hygienic circumstances, microorganisms could well provide vitamin 8-12 for the vegan. Other possible sources of vitamin B-12 for the vegan include microorganisms in the mouth, and water which has run over soil microorganisms. Perhaps these possible microorganism sources of vitamin B-12 explain why vitamin B-12 deficiency is rarely seen in those on traditional vegan diets.

A particular problem has arisen in breastfed babies of vegan mothers. The mothers may show no evidence of vitamin B-12 deficiency, due to marginal stores of the vitamin. However, their bodies may be unable to meet the extra demands of vitamin B-12 for breast milk. Such babies have developed abnormal brain function, some of which fortunately appears to be reversible in the early stages.