Sunday, July 06, 2003

Trans service cut

The BBC reports that Nestlé are to remove hydrogenated vegetable oil and fat from some of their confectionery products. This is due to a link between these trans fatty acids and heart disease.

Here's an example of the method that has been used to obtain such findings, from a study reported in the New England Journal of Medicine in 1997:

We prospectively studied 80,082 women who were 34 to 59 years of age and had no known coronary disease, stroke, cancer, hypercholesterolemia, or diabetes in 1980. Information on diet was obtained at base line and updated during follow-up by means of validated questionnaires. During 14 years of follow-up, we documented 939 cases of nonfatal myocardial infarction or death from coronary heart disease. Multivariate analyses included age, smoking status, total energy intake, dietary cholesterol intake, percentages of energy obtained from protein and specific types of fat, and other risk factors.

So, although the initial sample size sounds good, there are problems straight away. As soon as your research involves questionnaires, it means that the results are likely to be highly variable. Furthermore, the business end of this sample is the one percent who have suffered cardiac problems. Out of this much smaller sample, it is suggested we can learn something about their risks based on diet. Were the questionnaires accurately filled in? Were there any biases in who returned questionnaires and who did not? Were the estimates of heart disease due to other causes (age, smoking etc) correct?

There are a lot of potential compounding factors here. Therefore, to be on safe ground, it is wise to look only for very strong results. So, the link between smoking and lung cancer is very strong and widely accepted. But the results here are much weaker.

It is one thing to find an association in a study. It is quite another to show that the association is actually meaningful and worth acting upon. For example, while the level of trans fat in our diets may be rising, coronary heart disease rates are falling. So, at face value, trans fat is not exactly promoting an epidemic of disease.

Here is an interesting review of the evidence about trans fat by Steve Milloy in the Wall Street Journal from September 2002.

Personally, I don't particularly care what kind of fat Nestlé put in their chocolate bars. Nobody has ever thought chocolate bars were health foods. But I do object to the way the decisions of companies and individuals are distorted by slightly dubious science.

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