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Letter to the Editor |
Evans Department of Medicine Section of Preventive Medicine & Epidemiology Boston University School of Medicine 715 Albany Street B-612 Boston, MA 02118 E-mail: ldjousse{at}bu.edu
Dear Sir:
Evidence from several prospective studies suggests that n-3 fatty acids play an important role in preventing fatal coronary artery disease (CAD). Specifically, 
-linolenic acid (ALA) has been inversely related with fatal and nonfatal coronary events (1–3). In a recent article published in the Journal, Oomen et al (4) reported a positive association between ALA and CAD; a comparison of the highest with the lowest tertile of ALA intakes showed the adjusted relative risk of CAD to be 1.68 (95% CI: 0.8, 3.29).
Contrary to the findings of that study, Hu et al (5) reported a 45% reduction in incident fatal myocardial infarction when the highest and the lowest quintiles of ALA intake were compared in the Nurses’ Health Study. Other prospective studies showed that a lower risk of CAD was associated with higher ALA intakes (1–3). We reported earlier—in a cross-sectional design—that the prevalence of CAD among men in the highest quintile of total linolenic acid intake was 40% lower than it was among men in the lowest quintile (6).
The discrepancy between the findings of Oomen et al (4) and of other studies merits comment. In the Zutphen Elderly Study (4), tertiles of ALA intake were created based on the percentage of energy intake. With this approach, 2 subjects with the same absolute ALA intakes will belong to the same quantile of ALA intake only if their energy intakes are similar. However, if one subject consumes more energy than the other, the subject with the lower energy intake will be placed in a higher quantile, whereas the subject with the higher energy intake might be classified into a lower quantile. Thus, assessment of the effects of ALA under these circumstances is obscured and could bias the estimate of the effect. Furthermore, intake of more energy implies consumption of other nutrients, which may or may not affect the outcome of interest. Therefore, it is not surprising that in the Zutphen Elderly Study (4), subjects in the highest tertile of ALA intake also had higher intakes of trans fatty acids and total fat. The percentage of energy from ALA does not reflect the absolute amount of ALA consumed. If adjustment for total energy is considered desirable, an unbiased assessment of the effect of ALA would involve a comparison of categories of ALA intakes among subjects with comparable energy intakes.
Furthermore, the rate of conversion of ALA to long-chain fatty acids is dependent on the concentration of linolenic acid (7). A higher ratio of n-6 to n-3 fatty acids may influence the rate of ALA conversion to long-chain fatty acids. Unfortunately, the data reported by Oomen et al (4) do not permit an exact computation of the ratio of n-6 to n-3 fatty acids.
The main results in the Zutphen Elderly Study (4) may have been driven by trans fatty acids among subjects in the highest tertile of ALA intake, because the median difference in the percentage of energy from ALA between the highest and the lowest tertiles of ALA intake was only 0.04% in subjects whose ALA intakes were derived from sources that did not contain trans fatty acids. This finding suggests that the reported increased risk in CAD observed in the highest tertile of ALA intake is probably related to other nutrients.
REFERENCES
-Linolenic acid intake is not beneficially associated with 10-y risk of coronary artery disease incidence: the Zutphen Elderly Study. Am J Clin Nutr 2001;74:457–63.
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