Women who are pregnant or breastfeeding need more of certain nutrients, especially iron, folic acid and calcium. People with very low calorie intakes frequently consume diets that do not meet their needs for all nutrients. Some vegetarians may not be receiving adequate calcium, iron, zinc and vitamin B-12. Newborns are given, under the direction of a physician, a single dose of vitamin K to prevent abnormal bleeding. Certain disorders or diseases and some medications may interfere with nutrient intake, digestion, absorption, metabolism or excretion and thus change requirements.
Nutrients are potentially toxic when ingested in sufficiently large amounts.
Safe intake levels vary widely from nutrient to nutrient and may vary with the age and health of the individual. In addition, high dosage vitamin and mineral supplements can interfere with the normal metabolism of other nutrients and with the therapeutic effects of certain drugs. The Recommended Dietary Allowances represent the best currently available assessment of safe and adequate intakes, and serve as the basis for the U.S. Recommended Daily Allowances shown on many product labels. There are no demonstrated benefits of self supplementation beyond these allowances.