CHICAGO -- Cholesterol levels tend to rise in the winter and fall in the summer - variations that in some cases could affect treatment decisions, researchers say.
Their study found the biggest seasonal changes occurred in women and in people with already elevated cholesterol, whose levels fluctuated as much as 18 points.
"It's important for physicians to know this because they need to factor it into how they treat patients," said lead author Dr. Ira Ockene, a University of Massachusetts cardiologist.
Ockene said the findings could be useful to patients who want to try lowering their cholesterol through lifestyle changes instead of drugs. He said many patients could end up getting discouraged if they started diets in the summer and did not see much change or even an increase by wintertime.
The study did not examine whether seasonal changes in cholesterol affected participants' overall health. Without that evidence, there is no way of knowing whether the fluctuations are medically important or "just an isolated chemical observation," said Cleveland Clinic cardiologist Dr. Steven Nissen, who was not involved in the research.
The study appears in Monday's Archives of Internal Medicine.
Ockene said the changes are probably due to an increase in blood volume that naturally occurs during warm months. Cholesterol measurements reflect how much cholesterol there is in the bloodstream; greater blood volume results in a relative decrease in cholesterol.
Physical activity and weight loss can reduce cholesterol levels, and people often gain weight in the winter as they become more sedentary. But Ockene found minimal seasonal changes in weight and said the study participants were too sedentary year-round for him to gauge the role of physical activity on cholesterol counts.
The study involved 517 men and women ages 20 to 70 from Worcester County, Mass., who started out with an average total cholesterol level of 222 milligrams per deciliter of blood for men and 213 for women. Levels of 240 and above are considered high.
Participants' levels were measured every three months for a year.
Average levels were about 4 points higher in winter than in summer in men and 5.4 points higher in winter in women - relatively modest changes, Ockene said.
But seasonal variations of up to almost 11 points and 18 points were found in men and women respectively with initial cholesterol levels above 240 - substantial changes that would probably affect treatment decisions, he said.
Blood tests showed evidence that participants' blood volume also increased in the summer.
Ockene said more research is needed to determine why women had greater seasonal fluctuations.
The results bolster evidence from smaller studies also showing seasonal variations in cholesterol levels, said Dr. Ronald Krauss of the American Heart Association. They also underscore recommendations that doctors measure patients' cholesterol levels more than once before prescribing medication, Krauss said.
Krauss called the study "a reminder that a single cholesterol reading at any given point in time is only a snapshot in a moving picture that can vary over time."
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