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Dietary supplements may act differently in different people. As noted by Christine Lewis-Taylor, former director of the FDA’s Office of Nutritional Products, Labeling, and Dietary Supplements, “one man’s dose can be another man’s poison.” An individual’s genes and health status help to explain this difference.
Nutritional genomics and metabalomics are growing areas of research that explain the relationship between genetic makeup and nutrition- and metabolism-related outcomes.Certain relationships between nutrients and health outcomes are well characterized. Folate supplementation, for example, is recommended for all women of childbearing age to prevent fetal neural tube defects. By isolating and analyzing the function of the genes responsible for folate metabolism, however, scientists have identified a genetic variation that predisposes some women to give birth to babies with neural tube defects. This knowledge may eventually allow scientists to target folate supplementation to at-risk individuals.
Stage of life, medical history, and environmental factors such as diet and exercise can affect how a dietary supplement behaves in the body. Several populations are at increased risk of adverse effects of dietary supplements:
• children,
• people of smaller stature,
• elderly people,
• women who are pregnant or breastfeeding,
• people with immune disorders (for example, HIV or AIDS),
• people with cancer,
• malnourished people,
• people with existing or subclinical liver or kidney disease,
• people with a history of gastrointestinal health conditions (such as Crohn’s disease) or surgery (such as gastric bypass surgery),
• people who are hospitalized,
• transplant recipients,
• surgical patients.
While research has revealed the unique nutritional needs in women of childbearing age (for example, 400 microgramsof folate are recommended to prevent neural tube defects in offspring, and if pregnant, supplemental vitamins are recommended), information on the effects of dietary supplements during pregnancy and breastfeeding are extremely limited. This is largely due to ethical considerations:
scientific studies are not conducted during pregnancy and breastfeeding because of potential risks to the women and their babies. Several herbal dietary supplements historically used during pregnancy have been identified as harmful.
Certain dietary supplements may cause adverse effects during surgery, including increased or decreased bleeding. Medical or surgical professionals should be consulted several weeks before any scheduled surgery to determine whether dietary supplements should be discontinued.
Perhaps the most common adverse effects of dietary supplements are gastrointestinal, including constipation, diarrhea, or nausea. Gastrointestinal adverse effects are often present when a dietary supplement is first added to the diet. Over time, these symptoms may decrease or may cause an individual to discontinue use.
Many dietary supplements are metabolized by the liver and/or kidneys and may therefore cause adverse effects in people with overt or subclinical liver or kidney disease. Adverse effects could include worsening of disease complications or presentation of a previously undiagnosed disease. Other dietary supplements may cause adverse effects in other parts of the body; for example, calendula may cause eye irritation, Ginkgo bilobacan cause skin to become dry, and St. John’s wort may increase sensitivity to the sun.

Dietary supplement safety should be of primary concern to consumers. Because they are assumed to be safe unless proven otherwise, dietary supplements present special safety challenges. In 2000 the FDA received 500 dietary supplement adverse event reports; that number rose to 553 in 2001 and to 1214 in 2002. Adverse events caused by dietary supplements vary from mild to severe and may be influenced by individual health status (medical history and genetic makeup), dietary supplement composition and dosage (including levels of active and inactive ingredients), and concomitant consumption of other substances (including foods, beverages over-the-counter and prescription drugs, and other dietary supplements).