Aug. 12, 2010 — Blood cholesterol levels ebb and flow according a woman’s menstrual cycle, and screening tests should be timed accordingly for the most accurate results, National Institutes of Health researchers report online in The Journal of Clinical Endocrinology and Metabolism.

In their study, total cholesterol levels varied by close to 20% over the course of women’s cycles, and this was even more pronounced among obese women over 40.

“Cholesterol levels are the highest in the first half of the cycle and then after ovulation, they start to go down,” says study researcher Sunni L. Mumford, PhD, of the epidemiology branch of the Eunice Kennedy Shriver National Institute of Child Health and Human Development in Rockville, Md. “Cholesterol testing should be done at the same time of your cycle each month.”

Treatment decisions should be based on the most accurate readings.

Estrogen Affects Cholesterol Levels

The researchers measured levels of the female sex hormone estrogen, cholesterol, and other blood fats called triglycerides in 259 healthy women aged 18 and 44. They took as many as 16 measurements from the women over the course of two menstrual cycles. Study participants also charted their own ovulation using at-home fertility tests.

As the level of estrogen rises, HDL “good” cholesterol increases and peaks at ovulation. Total cholesterol, LDL “bad” cholesterol, and triglycerides dropped as estrogen levels rose. Estrogen is thought to stave off heart disease, which is why a woman’s risk rises after menopause when her natural supply of the hormone diminishes.

High cholesterol levels are a known risk factor for heart disease. Total cholesterol of less than 200 milligrams per deciliter of blood (mg/dL) is considered desirable; cholesterol between 200 and 239 mg/dL is borderline high. Total cholesterol of 240 mg/dL or above is considered high, according to the National Heart, Lung, and Blood Institute.

Just 5% of women consistently had total cholesterol levels greater than 200 mg/dL at all study visits, but cholesterol levels among 19.7% of the women reached 200 mg/dL at least once.

Cholesterol Screening for Women

“We used to give cholesterol screens in the morning when people have fasted, and now we  probably need to add another layer and that is knowing where they are in their menstrual cycle,” says Marc Lewis, MD, service chief of the Women’s Health Services at Henry Ford Hospital  in Detroit. “Not timing cholesterol screening tests with the menstrual cycle may mean that we are overtreating women and increasing their risk of drug side effects and costs for no reason.”

The new study is “fascinating and important,” says Marianne J. Legato, MD, a professor of clinical medicine at Columbia University College of Physicians and Surgeons in New York City and the founder and director of the Partnership for Women’s Health at Columbia University.

“If it is true that cholesterol varies with a woman’s menstrual cycle, then treatment decisions should be tailored accordingly,” she says. “This is more evidence that we can’t use men as the norm when developing cut-off points and standards of care,” she says. “We must study women directly.”