Cholesterol levels may vary with the changing seasons, a study suggests.
High cholesterol increases the risk of heart disease
Doctors in the United States carried out tests on 517 healthy people over the course of a year.
They found that their cholesterol levels changed throughout the year, peaking in winter. They were at their lowest in summer.
Writing in The Archives of Internal Medicine, they said it suggests some people may be wrongly diagnosed with high cholesterol.
High cholesterol can increase the risk of heart disease.
All of those who took part in the study were closely monitored by doctors to see if they changed their diet or lifestyle over the course of the year. None did so.
Men had an average cholesterol level of 222 milligrams per decilitre of blood (mg/dl). The figure for women was 213 mg/dl. Anything over 240mg/dl is regarded as high cholesterol.
Cholesterol levels in men increased over the year by an average of 3.9mg/dl, peaking in December. The average increase for women was 5.4mg/dl, peaking in January.
The variations were highest for those who had high cholesterol levels to begin with.
The doctors found that many people who had healthy cholesterol levels in summer went on to have high cholesterol in winter. Overall, 22% more people had high cholesterol in winter than in summer.
"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," the doctors wrote.
The findings come as scientists at the Rockefeller University report that they have identified a gene that may play a key role in the development of cholesterol.
Tests on mice have shown that the gene, called Pcsk9, is over-expressed in those fed high cholesterol diets.
Writing in the Proceedings of National Academy of Sciences, they said further research is needed to see if the same happens in humans.
If there is a similar mechanism, drugs could be developed to block the gene and reduce cholesterol, they said.
Belinda Linden, Head of Medical Information at the British Heart Foundation, said: "Several factors can influence blood cholesterol measurements, which can vary in range from 4% to 11%.
"More than half of these variations are likely to be related to light and temperature, changes in hormone levels, body weight, diet and exercise.
"The mechanism behind these effects is not fully understood, but does emphasise the need to obtain several measurements of blood cholesterol before diagnosing hyperlipidemia (high cholesterol).
"This study goes some way to explain the mechanism behind these variations but, as the authors suggest, there is no need to change current treatments."