Writing in the Archives of Internal Medicine, the researchers say that a variety of previous studies have suggested that cholesterol levels may be higher in the autumn and winter than in the spring and summer.

To investigate further, the team, led by Dr Ira Ockene, from the University of Massachusetts Medical Center in Worcester, examined seasonal variations in cholesterol levels among 517 "healthy" volunteers.

The researchers collected data on the participants' diets, physical activity, exposure to light, and total cholesterol levels every four months for a period of one year.

They found that average cholesterol levels over the study period were 222 mg/dL (milligrams per decilitre of blood) in men and 213 mg/dL in women. However, these were increased by an average of 3.9 mg/dL in men during the winter months, with a peak in December, and by an average of 5.4 mg/dL in women, with a peak in January.

Increases were greater in participants who had high levels of cholesterol to begin with. Overall, 22% more participants had cholesterol levels of 240 mg/dL or higher - classed as hypercholesterolaemia (high cholesterol) - in the winter months than in the summer.

The authors of the study report that there where no significant seasonal changes in the diets and calorie intake of the participants.

However, they say that seasonal changes in the volume of plasma, a component found in blood, which seemed to rise and fall in conjunction with temperature and/or physical activity levels over the year, could explain a significant proportion of the increase in cholesterol levels in winter.

"The information provided by this study could assist in the continuous development of guidelines for the treatment of hypercholesterolaemia; however, we do not believe that season-specific guidelines would be justified," the researchers conclude.

They add that further research is needed to understand the mechanism through which physical activity and temperature control could help to lower cholesterol levels and thus help in the prevention of heart disease.