Cholesterol is a key component in the development of atherosclerosis, the accumulation of fatty deposits on the inner lining of arteries. Mainly as a result of this, cholesterol increases the risks of heart disease, stroke and other vascular diseases.
The scientists from the University of Massachusetts Medical Center, Worcester report that seasonal changes in plasma volume - a component of blood - explained a substantial proportion of the observed increase in cholesterol levels in the winter, particularly in women. They add that there were no statistically significant seasonal changes in dietary and caloric intake.
"Our findings suggest that there is greater amplitude in seasonal variability in women and in people with hypercholesterolemia," writes researcher Ira S. Ockene.
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, they add.
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.
Ockene 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 US 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 per cent more participants had total cholesterol levels of 240 mg/dL or greater (high cholesterol) in the winter than in the summer.
"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," conclude the researchers, adding that further research is necessary to better understand the mechanism.
According to the World Health Organisation, almost one fifth (18 per cent) of global stroke events (mostly nonfatal events) and about 56 per cent of global heart disease are attributable to total cholesterol levels above 3.2 mmol/l. This amounts to about 4.4 million deaths (7.9 per cent of the total) and 2.8 per cent of the global disease burden.
Full findings are published in the April 26 issue of The Archives of Internal Medicine.