The reasons for choosing vegetarianism may be related to one or more of the following: morality, religion, culture, ethics, aesthetics, environment, society, economy, politics, taste, or health. A generic term for both vegetarianism and veganism, as well as for similar diets, is "plant-based diets".

Some vegetarians, particularly those who do not consume any food of animal origin, can experience deficiencies in calcium, vitamin D, riboflavin, vitamin B12 or complete proteins. Vegetarians are also at risk of energy deficiency in the form of calories, particularly in children.

An adult on a vegetarian diet for a prolonged period can be at increased risk for periodontal (gum) disease from a lack of vitamin D and calcium. Teeth may soften when there is a shortage of vitamin D, becoming more susceptible to decay and periodontal disease. Vitamin D is produced in the body with sun exposure, so deficiencies are rare, but it can develop in those who do not consume milk or fish. Adding vegetable margarines or soy milk to the diet may solve the problem.

Another risk reviewed by scientists from Australia and Vietnam of all the peer-reviewed literature on the issue has found that people on vegetarian diets have approximately 5% lower bone mineral density (BMD) than non-vegetarians.
The researchers selected nine studies for analysis which compared the BMD of 2749 men and women - meat eaters and vegetarians - from around the world and the study was led by Professor Tuan Nguyen from Sydney's Garvan Institute of Medical Research and Dr. Ho-Pham Thuc Lan from the Pham Ngoc Thach University of Medicine in Ho Chi Minh City, Vietnam.
Their findings are published online in the American Journal of Clinical Nutrition.

Iron-Deficiency Anemia due to low iron storage may be another issues. One study found that 27% of women and 5% of men who were lacto-ovo-vegetarians had low serum ferritin levels (iron storage).

Balancing vegetarian food ( variety and quality ) and nutrition is vital to maintaining a healthy vegetarian diet.