Fascinating new research from the National Institutes of Health finds that women’s cholesterol levels correspond with cyclic changes in estrogen levels. Total cholesterol levels can vary by as much as 19% over the course of the cycle.

The researchers found that as the level of estrogen rises, high-density lipoprotein (HDL) cholesterol also rises, peaking at the time of ovulation.

In a typical cycle, estrogen levels steadily increase as the egg cell matures, peaking just before ovulation. Previous studies have shown that taking formulations which contain estrogen — oral contraceptives or menopausal hormone therapy — can affect cholesterol levels. However, the results of studies examining the effects of naturally occurring hormone levels on cholesterol have not been conclusive. According to the NIH’s National Heart, Lung and Blood Institute, high blood cholesterol levels raise the risk for heart disease.

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In contrast, total cholesterol and low-density lipoprotein (LDL) cholesterol levels — as well as another form of blood fat known as triglycerides — declined as estrogen levels rose. The decline was not immediate, beginning a couple of days after the estrogen peak at ovulation.

These findings provide another reason for girls and women to learn to track their cycles, so their blood tests can be interpreted more precisely.

It also gives more weight to the frequent assertion of members of the Society for Menstrual Cycle Research that menstruation matters — and is worthy of our study — in part because it is not an event isolated in the uterus and vagina, but a complex part of the endocrine system that has effects on health and well-being throughout a woman’s body.