Supplements: what the evidence shows
Supplements are everywhere, and the claims are loud. The evidence is quieter. For most healthy people, food does most of the work. Some people do need a supplement, and a doctor can help find out. This page sums up what careful research says about seven common supplements. It names no brands, gives no doses, and sells nothing.
Vitamin D
What it does: helps your body absorb calcium for strong bones, and helps muscles, nerves, and the immune system.
What the evidence shows: your skin makes vitamin D in sunlight, and a few foods contain it. Breastfed babies, older adults, people with darker skin, and people who rarely go outside are more likely to run low. In clinical trials, vitamin D pills did not lower the risk of heart disease, did not prevent or ease depression, and did not help weight loss.
Caution: too much vitamin D can be harmful, and it almost always comes from supplements, not sunshine.
Vitamin B12
What it does: keeps blood and nerve cells healthy and helps make DNA.
What the evidence shows: B12 is found naturally only in animal foods. People who eat little or no animal food, many people over 50, and people who have had some stomach or bowel surgery may not get or absorb enough. Some acid-reducing medicines and metformin can lower B12. Extra B12 does not boost energy or athletic performance in people who already get enough.
Omega-3 (fish oil)
What it does: omega-3 fats are found in seafood and some plant oils.
What the evidence shows: a large review found omega-3s slightly lowered the risk of some heart events, and another found they lower triglycerides (a blood fat) by about 15%. For several health questions, the evidence for eating seafood is stronger than for taking pills.
Magnesium
What it does: helps muscles, nerves, blood sugar, blood pressure, and bones.
What the evidence shows: beans, nuts, seeds, whole grains, and leafy greens are good sources. Magnesium supplements might lower blood pressure, but only by a small amount.
Caution: high amounts from supplements can cause diarrhea and cramps, and extremely high amounts are dangerous.
Creatine
What it does: helps muscles make quick energy for short, hard efforts.
What the evidence shows: a sports nutrition society's review says creatine consistently improves performance in short, high-intensity exercise and was well tolerated in studies of healthy people. Much of the research is in athletes and active adults. It is not a substitute for food, sleep, or training.
Iron
What it does: your body uses iron to make hemoglobin, which carries oxygen in the blood.
What the evidence shows: most people get enough from food. Teen girls and women with heavy periods, pregnant women, infants, and frequent blood donors are more likely to run low. A doctor can check with a blood test.
Caution: too much iron is harmful. Accidental overdose of iron products is a leading cause of fatal poisoning in children under 6. Always keep them out of reach.
Multivitamins
What the evidence shows: a U.S. expert panel found too little evidence to say whether multivitamins help prevent heart disease or cancer. It advises against beta-carotene and vitamin E pills for that purpose. See Do you need a multivitamin?
A note on quality
In the United States, the FDA does not approve dietary supplements before they are sold. A label is not proof that a product works or is safe. That is one more reason to talk with a doctor or pharmacist first. See also Nutrition and Medicines from nature.
This is general information. For your own health, talk to a doctor.