Health Options Alan Titchenal  & Joannie Dobbs                   Wednesday, January 21, 1998

Misused terms root of much confusion

If you feel that nutrition recommendations change from one week to the next, don't feel alone. Much of this confusion is caused by reporters and readers not understanding a few key terms.

These are “correlation,” risk,” and “cause and effect.”

“Correlation” implies that as one measured variable goes up, a sec­ond measured variable simultaneously goes up (positive corre­lation) or simultaneously goes down (negative correlation). This is not “cause and effect.”

For example: in the 1970s it was shown that as the number of TV sets in a house increased, the incidence of dying from heart attack increased for men living in the house.

The relationship between TV sets and heart disease was positive. But it would be naïve to suggest that TV sets caused the heart attacks. The man who could afford 3, 4, or 5 TV sets was likely to be in a high stress job, smoked, worked long hours, didn't exercise, and didn't eat properly. In this case, it is fairly obvious that these lifestyle fac­tors were more directly related to the heart attack than the TV sets.

Not all correlations are this obvious. For example, eating lots of fruits and vegetables high in beta-carotene is correlated with a decreased risk of certain cancers. Giving people beta-carotene supplements, however, does not predictably decrease cancer incidence and may even increase some cancers. Therefore, the cancer protection attributed to beta-carotene must come from other compounds in beta-carotene containing foods.

And that might not even be the case. It could be that people who eat more fruits and vegetables eat less high fat foods and/or less total calories. Obviously, a great number of studies are needed to sort out the facts.

Most of the reports about foods and health are from correlation studies.

Rather than showing cause and effect, they do demonstrate the concept of “risk.” The risk of heart disease increases with increased blood cholesterol levels, but not everyone with high blood cholesterol dies of heart disease. Many other factors are involved such as high blood pressure, smoking, and lack of exercise. These are called multiple correlations because they all are related to increased risk of heart disease. Some people with all these risk factors may not develop heart disease, but the statistical chance that they will develop it is very high.

The results of these correlation studies frequently lead to research with animals and humans designed to sort out “cause and effect” relationships. Cause and effect studies on human beings are very costly, often starting at $500,000 for a single study. Even with these carefully controlled studies, it is virtually impossible to control every aspect of the research participants' lives. Therefore, it can be extremely difficult to sort out absolute cause and effect relationships.

So how does a consumer of health information figure out what to believe? Here are a few tips we use:

1. Don't put too much stock in any single study or any single group of researchers.

2. Wait for a body of other stud­ies to confirm or refute the results.

3. Question everything you read

4. Pick your experts carefully.

Conscientious health professionals are cautious about recommen­dations based on only one study, even if it appears to be well-designed. However, someone selling a product or a book based on a single study often will throw all their “eggs in that one basket” to support their belief and promote their money-making scheme. Buyer beware!