Here a quote from a book. I don't know if this is really true or not, because I am also a vegeterian.
Unless they choose a proper balance of foods, strict vegetarians are at risk for several deficiencies, especially vitamin B12. The other nutrients at risk are riboflavin, calcium, iron, and the essential amino acids lysine and methionine. Vegetarian children not exposed to sunlight are at risk for vitamin D deficiency. Zinc deficiency can occur in vegans because the phytic acid in whole grains binds zinc, and there is little zinc in fruits and vegetables. Since B12 is present only in animal foods and a limited number of specially fortified foods, vegans should probably take B12 supplements prescribed by a physician. Further, the symptoms of vegetarianism
may include protein deficiency, amino acid and mineral depletion coupled with chronic low levels of energy.