Dietary supplement use is on the rise. More than half of American adults use supplements, most often multivitamins and minerals. In particular, more Americans are taking vitamin D and calcium supplements than in the past.

However, studies have found no difference in mortality rates between people who take vitamin supplements and those who don't take supplements. Most people who eat a healthy diet do not need vitamins, but there are some exceptions.

Pregnant and Breastfeeding Women. Women who are pregnant or breastfeeding generally need additional vitamins. Folic acid and vitamins B6 and B12 are particularly important during pregnancy. Women who are vegetarians need to take enough of vitamin B12. A deficiency in this vitamin can harm their baby. Folic acid reduces the risk for neural tube defects and possibly deformities of the face, such as cleft palate. Studies also link low folate levels during pregnancy with low birth weight, which may increase the risk of heart disease in adulthood. A woman's best approach is to start taking extra folic acid plus multivitamin supplements before she becomes pregnant.

Vitamin B12 source
The human body stores several years' worth of vitamin B12, so deficiency of this vitamin is extremely rare. However, people who follow a strict vegetarian diet and do not eat eggs or dairy products may need to take vitamin B12 supplements.

Pregnant women who eat a healthy diet may still have low folate levels and need to take folic acid supplements. Requirements are as follows:

The recommended dietary allowance (RDA) for folic acid before getting pregnant is 400 mcg.
During pregnancy the RDA is 600 mcg.
Women who are breast-feeding should get 500 mcg of folic acid.
Some women have low vitamin A reserves in their liver. However, getting too much vitamin A from food or supplements significantly increases the risk for birth defects. Experts recommend that pregnant women get no more than 3,000 mcg of vitamin A each day.

Infants and Children. Infants who are breastfed by healthy mothers should receive enough vitamins. However, in some cases, infants may not get enough of vitamins K and D.

Human milk contains low levels of vitamin K, and the newborn's immature intestinal tract may not produce enough of the baby's own supply. Most babies are given an injection of this vitamin at birth.
Infants who are breast-fed by malnourished women or who do not get enough sunlight exposure may be deficient in vitamin D. In these cases, supplements of 200 - 300 IU are recommended.
Formulas are required to contain enough vitamins and minerals. After infancy, most American children receive all the vitamins they need from their diet, unless they are severely deprived. However, research suggests that many healthy children ages 1 to 11, especially African-American and Hispanic children, are not getting enough vitamin D.

Smokers. Smoking interferes with the absorption of several vitamins, especially vitamins C and D. Smoking can also interfere with the metabolism of vitamin D, resulting in poor muscle function.

Taking supplements of antioxidant vitamins, especially beta-carotene, is harmful to smokers. Instead of taking supplements, smokers should eat a diet rich in fresh fruits, vegetables, and whole grains. Smoking cessation is the most important intervention.