We read with interest the study by Ockene et al1 evaluating the seasonal variation in serum lipid levels. This study is important in that it highlights a significant variation in lipid levels that occurs seasonally, and this variation does not appear to be associated with changes in diet or activity. However, there are several limitations in study design that restrict their conclusions. The authors attribute the change in lipid levels to changes in plasma volume. This assumption is fraught with difficulties and is not consistent with either their data or other published data. The measure of plasma volume using hemoglobin levels is not only imprecise,2 but relationships between estimates of change in hemoglobin level and plasma volume and serum lipid levels are never examined. Moreover, assessment of the data fails to incorporate potential seasonal changes in other hormones such as erythropoietin that could also explain the changes seen in hemoglobin level. Other hormones have been reported to vary seasonally, and hemoglobin A1c level is 0.4% lower in the summer than in the winter, a measure not related to hemodilution.3 In addition, in our small study,4 we found significant correlations between changes in lipoprotein lipase activity and changes in high-density lipoprotein cholesterol and triglyceride levels. The lipoprotein lipase activity is a bioassay done ex vivo, and the activity is expressed per gram of tissue or adipocyte cell size. This measure is not altered by changes in hemodilution and yet can explain some of the seasonal changes in serum lipid levels in a physiological way.
Donahoo WT, Eckel RH. Physiological Mechanisms for the Seasonal Changes in Serum Cholesterol Level. Arch Intern Med. 2004;164(22):2500–2509. doi:10.1001/archinte.164.22.2506-a
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