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LETTER TO THE EDITOR |
Department of Epidemiology, Statistics and Public Health
Cardiff University
University Hospital of Wales
Heath Park, Cardiff CF14 4XN
United Kingdom
E-mail: pelwood{at}doctors.org.uk
School of Agriculture
Policy and Development
University of Reading
Reading
United Kingdom
Dear Sir:
Several studies have shown that 15:0 is an acceptable marker for the dietary intake of dairy fat (1, 2), and Qi Sun et al (3) have now reported on 15:0 and ischemic heart disease (IHD). Estimations of this fatty acid were made in samples of plasma that had been collected at baseline from 166 women in the Nurses' Health Study cohort who later experienced a nonfatal myocardial infarct or died from IHD and from 327 control women who had been free of IHD at the time of the cases' diagnosis. The adjusted risk for IHD in the third of the women with the highest plasma 15:0 concentrations, relative to that in the third of the women with the lowest levels, was 2.36 (95% CI: 1.16, 3.89).
This result imply a very high vascular risk from dairy consumption, but it is inconsistent with the results for 15:0 made in another cohort. Thomas et al (4) conducted fatty acid analyses of adipose tissue taken from the anterior abdominal wall of 59 men within the Caerphilly cohort (5), who had experienced a silent infarct (echocardiographic evidence of infarction with no relevant symptoms) and from 61 matched control men who had had neither symptoms nor echocardiographic evidence suggestive of infarction. The mean weight of 15:0 as a proportion of total fatty acids in the control samples was 0.49%, and the mean (±SE) difference in 15:0 between cases and controls was –0.01 ± 0.02%.
Wolk et al (2) comment that, in relation to coronary artery disease, "we do not really know whether the biomarker (15:0) or intake estimates are superior.... both are informative." Furthermore, in their article on 15:0, Smedman et al (1) comment on the fact that relations between the intake of fat from milk products and a number of vascular risk factors are all in a favorable direction. These comments indicate that it would be unwise to base conclusions about milk and dairy products and vascular disease on plasma or adipose tissue concentrations of 15:0 alone.
In fact, the coronary heart disease risk of milk consumption within the Nurses' Health Study cohort had already been reported on, using estimates based on food-frequency questionnaires (6). The risk in the fifth of women with the highest consumption of milk, relative to the risk in the fifth with the lowest milk consumption, was 1.67 (95% CI: 1.14, 1.90) for whole milk and 0.78 (95% CI: 0.63, 0.96) for skim milk. Twenty percent of the women in this cohort had been consuming whole milk (3), and an estimate of the relative risk from milk consumption within the total cohort can be assumed to have been about 1.06 (95% CI: 0.90, 1.25). This is markedly different from the estimate by Qi Sun et al (3) based on 15:0 concentrations in selected women within the same cohort.
Estimates of vascular risk associated with milk and or dairy consumption, using a variety of dietary enquiry methods, including 7-d weighed food intake records, have been made in a number of prospective studies (7, 8). These estimates were based on milk intake, dairy food intake, and dairy calcium intake as surrogates for milk consumption. The subjects totaled almost 400 000, and the number of incident vascular events was >8000. The risk estimates reported are all statistically homogeneous, and, for the most part, the studies had been conducted before fat-reduced milks became popular. A meta-analysis of the results gives an overall estimate of vascular risk in the subjects with the highest milk or dairy intake (usually one fifth) of 0.87 (95% CI: 0.74, 1.03) for IHD and 0.83 (95% CI: 0.77, 0.90) for stroke, relative to the risk in the fifth of subjects with the lowest intakes.
On the basis of all of this evidence, it seems inappropriate to accept an estimate of IHD risk based on plasma concentrations of 15:0 to be relevant to the health consequences of milk and dairy food consumption.
ACKNOWLEDGMENTS
All authors stated that they had nothing to declare. All are, and have been, independent research workers, and no funding for the work described in this letter was received.
REFERENCES
This article has been cited by other articles:
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  P. C. Elwood, D. I. Givens, A. D. Beswick, A. M. Fehily, J. E. Pickering, and J. Gallacher The Survival Advantage of Milk and Dairy Consumption: an Overview of Evidence from Cohort Studies of Vascular Diseases, Diabetes and Cancer J. Am. Coll. Nutr., December 1, 2008; 27(6): 723S - 734S. [Abstract] [Full Text] [PDF]
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