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Health benefits of dietary supplements

Evaluating the Research
January 1999

In evaluating scientific research (both surveys and double-blind trials), it's important to know two things:

  1. How many people were involved (how large was the sample size)? The larger the groups of people, the less likely it is that the results are due to chance.

  2. What is the probability (P) that the difference between the groups is due to chance or other irrelevant factors? The smaller the P value, the stronger the evidence. A value of P<.05 is good, P<.01 is better, and P<.001 means an incredibly tiny possibility that the result was due to chance.

Also, is it possible that the differences or similarities between the groups are due to some other factor entirely? This question can be quite subtle and difficult to assess, especially if many potential causes are involved.

Making sense out of conflicting evidence

Double-blind studies with small P value results are the strongest kind of scientific evidence available, yet different groups of researchers doing double-blind studies sometimes get different results. One study will say the substance is effective; another will say it's not. Whom do we believe?

If twenty studies were done, and seventeen conclude that a supplement produces a health benefit where only three conclude it does not, the chances are good that the supplement does in fact provide a health benefit.

Funding source

Another factor in judging research is the source of funding. When the results of a study work to the advantage of the sponsor, and the study's results are different from those obtained by independent researchers, it suggests that the methods used in the study may have been biased.

Peer review

Disagreements and the potential for flawed research are the reasons why responsible journals use the peer-review process to evaluate a research study before they decide whether to publish it. If an article was published in a peer-reviewed journal, you can be sure that other experts have examined and critiqued the researchers' methods and findings.

What kind of evidence is strongest?

In examining the evidence, how does a well-informed consumer decide whether to take a particular supplement? Each individual must decide whether the likelihood and magnitude of a benefit are worth the cost.

  • If a friend says, "It worked for me," then the supplement might be worth trying.

  • If a culture's accumulated botanical knowledge indicates that many people have benefited in a particular way, this is stronger evidence that the supplement provides specific health benefits.

  • If a survey of thousands of people shows more than 60 percent got good results, the chances are good that you might too.

  • If several double-blind studies show twice as many of the experimental group benefited than the control (placebo) group, with a P value of .01 or less, you can be quite sure that the supplement performs as claimed for most people.

See also SQ's links to information about specific supplements.


previous page What kinds of scientific evidence are useful for substantiating a health benefit claim? When advertising says a health benefit is "proven by clinical research" -- what does this mean?




More about health benefits:

Will dietary supplements benefit your health?

Standard terminology: an important first step in evaluating health benefits

What kinds of scientific evidence are useful for substantiating a health benefit claim?

Evaluating the research

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(c) Copyright 1999-2003 Dietary Supplement Quality Initiative. For permission to reprint, please contact our editor.