Jul

15

Researching Dietary Supplements

Filed Under Dietary Supplements | Comments Off

Despite their natural connotation, dietary supplements are biologically active substances. Evidence for their safety and efficacy should therefore be based on accepted principles of science.
What Is Good Science?
The Agency for Healthcare Research and Quality ranked scientific studies by the value of their results. Below they are ranked from most powerful to least powerful:
• prospective, randomized, double-blind, placebo- controlled clinical trial with crossover;
• prospective, randomized, double-blind clinical trial;
• single-blind clinical trial;
• open-label clinical trial;
• retrospective epidemiological study;
• other types of consumer or patient-based, interview-type studies (including meta-analyses).
Randomized, controlled trials are considered the gold standard of scientific research. Such trials include a control group, people who are observed but receive no treatment of any kind, and an intervention group, people who receive a certain treatment, such as a dietary supplement. Members of the control and intervention groups are similar (matched) in age, sex, ethnicity, marital status, socioeconomic status, health status, and diet and are randomly assigned to the respective groups. The power of such a trial is that it controls for any variation between the two groups, so that the only relevant difference is that the intervention group receives the specific treatment. Thus, any difference in the outcomes between the control and intervention groups is likely attributable to the intervention.
To determine whether the results of a study are due to the intervention or to chance, scientists conduct statistical analyses. A Pvalue (probability value) of 0.05 is generally used to indicate statistical significance. Pvalues greater than 0.05 indicate that the likelihood the results were a product of chance is greater than 5 percent. Conversely, the smaller the Pvalue, the more significant the results. Thus, a Pvalue lower than 0.001 indicates that it is very unlikely the results were due to chance; that is, the intervention had a highly significant effect on the treatment group.
In case-control studies, cases who have a particular outcome (for example, a disease) are identified and their past exposure to various components (such as a dietary supplement) is compared with that of control subjects, who do not have the particular outcome. By matching case and control subjects for sex, age, and other variables, there is less chance that the results are due to anything but the difference in exposure.
Cross-sectional studies measure the prevalence of a health outcome (for example, a disease) or determinants of health (such as ethnicity) in a population at any one time. For example, a cross-sectional study could measure the relationship between osteoporosis and calcium intake. This type of study is vulnerable to confounding, however, as a result of selection bias, which distorts statistical analysis by including a samplethat is not representative of the population of interest. In addition, cross-sectional studies are not good for determining cause and effect.
Cohort studies are long-term studies that compare subjects who have a particular outcome (for example, a disease) and/or who receive a particular intervention (such as a dietary supplement) with those who do not have that outcome or exposure. Cohort studies tend to be less reliable than randomized, controlled trials because there is less control over the differences between the two groups. For more accurate results, cohort studies may have to last for several years, which allows for additional differences between the groups to enter into the study and confound the results.
The validity of a scientific study should determine whether it is published in a prestigious scientific journal, such as the Journal of the American Medical Associationand the New England Journal of Medicine. Most scientific journals require that published material is reviewed by peers, other experts who can evaluate the strength of the evidence and identify shortcomings of studies. Published studies can be retrieved online at PubMed.
Many dietary supplement manufacturers promote their products through testimonials and anecdotes from satisfied customers. While persuasive, testimonials cannot replace scientific evidence and are often fabricated, paid for, or provided by people who are emotionally bound to a product. Testimonials should never be substituted for rigorous scientific data.


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