Supplement Studies: Sorting Out the Confusion

Bottle with colorful supplements

To consumers, keeping track of the research on vitamin supplements can be an exercise in frustration. Different studies on the same vitamins often present conflicting information. Why the contrary findings?

A closer look at the study design often reveals the cause of these discrepancies. Next time you read a news story about a vitamin trial, keep the following questions in mind, since the answers may help you put conflicting results into context:

1. What vitamin dose did study participants take—and for how long did they take it?

The most obvious source of conflicting findings is the fact that different studies test different doses of vitamins, for different lengths of time. Take vitamin D as an example. Studies have found that it protects against fractures at doses of 700 to 800 IU a day, (1) but that 400 IU a day has less benefit. (2) A short vitamin supplement trial may not show any benefit simply because it takes a long time for a disease to develop or for the vitamin’s protective effects to emerge.

2. Who were the study participants—and how healthy were their lifestyles?

Everyone knows that diet, smoking, exercise, and other lifestyle choices can have a dramatic effect on our health. These lifestyle characteristics can also have an effect on how our bodies respond to vitamins. A supplement is only useful to people whose diets are lacking in that specific nutrient; a randomized trial that gives vitamin pills to well-fed participants may not show any results. Similarly, people who smoke may have greater need for certain vitamins, so a study conducted on smokers could have different results from one conducted on people who never smoked or who have kicked the habit.

3. When did study participants take the supplement?

A supplement may only be beneficial at one stage of a disease or condition and not another, so studies done at different stages may have different results. Folate deficiency in mothers, for example, leads to neural tube defects, but folate supplements only protect against defects if taken in the first few weeks after conception.

4. How did researchers measure the supplement’s effectiveness?

Studies often differ in how they measure their outcomes—that is, how they measure whether a supplement had any benefit. Heart disease, for example, covers a wide range of conditions, including heart attack, stroke, or peripheral vascular disease. If a study measures the effect of a vitamin supplement on heart disease overall, it may miss a supplement’s protective effect against stroke.

For more information on how to understand news stories about nutrition research, see “Nutrition Research and Mass Media—An Introduction.”


1.Holick MF. Vitamin D deficiency. N Engl J Med. 2007; 357:266-81.

2.Jackson RD, LaCroix AZ, Gass M, et al. Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med. 2006; 354:669-83.

Terms of Use

The aim of the Harvard School of Public Health Nutrition Source is to provide timely information on diet and nutrition for clinicians, allied health professionals, and the public. The contents of this Web site are not intended to offer personal medical advice. You should seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this Web site. The information does not mention brand names, nor does it endorse any particular products.