Cholesterol Levels Fluctuate With The Seasons

According to the article, a variety of studies have suggested that cholesterol levels are higher in the fall and winter than they are in the spring and summer. Although the mechanism for this phenomenon is not clear, such variation could result in larger numbers of people being diagnosed as having high cholesterol in the winter, the article states.

Ira S. Ockene, M.D., of the University of Massachusetts Medical Center, Worcester, and colleagues investigated the seasonal variation in cholesterol among 517 healthy volunteers from a health maintenance organization serving central Massachusetts. Data were collected quarterly over a twelve-month period on diet, physical activity, exposure to light, general behavioral information, and cholesterol levels were also measured.

The researchers found that the average cholesterol level was 222 mg/dL (milligrams per deciliter of blood) in men and 213 mg/dL in women. According to the U.S. National Cholesterol Education Program guidelines, 240 mg/dL is the threshold level for hyperlipidemia (high cholesterol). Cholesterol levels were increased by 3.9 mg/dL in men, with a peak in December, and by 5.4 mg/dL in women, with a peak in January. The researchers found that the increases were greater in participants who had high cholesterol levels to begin with. Overall, 22 percent more participants had total cholesterol levels of 240 mg/dL or greater (high cholesterol) in the winter than in the summer. The researchers write that seasonal changes in plasma volume (a component of blood) explained a substantial proportion of the observed increase in cholesterol levels in the winter. The authors also report that there were no statistically significant seasonal changes in dietary and caloric intake.

"In conclusion, this study demonstrates seasonal variation in blood lipid levels, with a peak in the winter and a trough in the summer. Our findings suggest that there is greater amplitude in seasonal variability in women and in people with hypercholesterolemia [high cholesterol]," the authors write. "However, changes in relative plasma volume seem to explain a substantial proportion of the observed seasonal difference in blood lipid levels. Changes in temperature and/or physical activity in winter and summer seem to be related to concomitant changes in relative plasma volume."

"The information provided by this study could assist in the continuous development of guidelines for the treatment of hypercholesterolemia; however, we do not believe that season-specific guidelines would be justified," the researchers write. "Further research is needed to better understand the mechanism through which physical activity and temperature control systems could aid in the prevention of coronary heart disease morbidity and mortality." (Arch Intern Med. 2004;164:863-870. Available post-embargo at archinternmed.com)

Editor's Note: The SEASONS Study was supported by a grant from the National Heart, Lung, and Blood Institute, Bethesda, Md.