The insatiable hunger that often accompanies a period is a very real feeling, but that increased craving for carbs and sweets has not been fully understood by science. Now new research suggests a possible mechanism in the brain that could help explain food cravings related to the menstrual cycle.

A small clinical trial published Thursday in Nature Metabolism has uncovered differences in insulin sensitivity at different phases of the menstrual cycle. While food cravings were not directly studied in the research, the authors suggest that an increased sensitivity to insulin—a hormone that regulates glucose levels, metabolism and food intake—could explain the jump in appetite.

“The brain contributes to metabolic changes and probably changes eating behavior across the menstrual cycle,” says Martin Heni, a professor of endocrinology at Ulm University Hospital in Germany and co-senior author of the study.* “This is nothing bad. It’s physiological, but it may explain what many women report to us about what they feel across the menstrual cycle. This can be one underlying mechanism.”

Insulin is produced in the pancreas, but it can cross the blood-brain barrier and can then target cells in several brain areas to regulate bodily processes. Previous research from Heni’s team found that insulin in the brain can help suppress glucose production and stimulate glucose uptake in muscle tissue—indicators of its important role in managing metabolism and energy for the entire body. This relationship between the brain’s insulin and body metabolism was only tested in young men, however.

The fact that sex hormones fluctuate across the menstrual cycle has held back research on insulin changes in people who menstruate. But bodily processes such as the uterus’ preparation for potential pregnancy could cause differences in how the brain governs glucose metabolism. In one of two experiments, Heni’s team recruited 11 women who reported that they have regular menstrual cycles. The researchers studied the changes that took place when the participants underwent two phases of the menstrual cycle: the follicular phase and the luteal phase. The follicular phase is when an egg is being prepared for ovulation, which usually begins on the first day of a period and ends once ovulation happens. The luteal phase starts after ovulation and is when the egg travels to the uterus, where it waits to potentially be fertilized. The team’s study approach “turned out to be a good idea, because we saw big differences between the two cycle phases,” Heni says.

The researchers attached insulin clamps—considered the gold standard for measuring sensitivity to the hormone—to each of the 11 women, who had a median age of 24. The clamps measured sensitivity after researchers delivered either insulin or a placebo through a nasal spray in each session in addition to giving participants intravenous insulin. This technique helped quickly supply a large amount of insulin directly to the brain. Each participant underwent four sessions: two were conducted in their cycle’s follicular phase, and the other two occurred in the luteal phase.

During the participants’ follicular phase, when a period typically begins, the brain was more sensitive to the effects of insulin. This sensitivity disappeared once women entered the luteal phase, suggesting a switch to insulin resistance.

The new findings suggest that insulin resistance in the brain could make it more difficult to regulate energy production for the rest of the body. It’s also possible that the resistance could interfere with insulin’s other roles, such as controlling appetite. Studies of conditions such as obesity and diabetes have linked increased insulin and heightened insulin sensitivity with the sensation of satiety, or reduced appetite. Insulin resistance, on the other hand, is thought to increase food cravings. Although the team did not formally test subjects’ food intake in the current study, Heni says the findings could help explain why people experience food cravings in the second half of the menstrual cycle. If so, this would align with research reporting an increase in food cravings during the luteal phase.

To confirm their findings, the researchers conducted a second experiment using neuroimaging to get a closer look at brain activity across the menstrual cycle. They worked with a separate group of 15 women with a median age of 23. The imaging revealed a similar pattern of insulin sensitivity, specifically in the hypothalamus, a brain area that acts as a control center for food intake and metabolism.

One of the limitations to the study was its small sample size, which can skew the results of any trial. Maintaining large participant numbers for experimental, observational studies such as these is a challenge, however—particularly because these studies require participants to go through the experiments multiple times, says Alexandra DiFeliceantonio, a Virginia Tech neuroscientist who studies how the brain modulates food preference and who was not involved in the new research. “You will always end up with less participants because it’s expensive and time consuming,” she says.

Still, the changes in brain insulin sensitivity recorded within each participant were “striking,” says Nils Kroemer, a neuroscientist at the University of Bonn in Germany, who was not involved in the research but wrote an accompanying Nature editorial on the study. The results, he says, support a need for a larger-scale study in the future. A larger trial could also consider other factors that may affect energy metabolism, including obesity, birth control use or underlying endocrine diseases, such as polycystic ovary syndrome, that are linked to insulin resistance.

The current findings could suggest a neural mechanism that might explain period cravings, Kroemer suggests. He proposes that hypothalamic circuits involved in energy metabolism could also be intertwined with those coordinating motivation—including the ones involved in food-seeking behavior. “It may explain some changes [in the menstrual cycle] that have been very difficult to pinpoint to a particular mechanism,” Kroemer says.

The hypothalamic circuit could also possibly tie into what is already known about how the brain promotes food cravings, which DiFeliceantonio says has largely to do with the release of the hormone dopamine. The brain chemical is responsible for making a person feel and seek pleasurable experiences, such as indulging in a favorite snack. But DiFeliceantonio says that it’s likely that additional variables and processes in the body, such as those that happen in the menstrual cycle, can influence food cravings as well. Further research is needed to better understand the mechanisms that stimulate hunger and food cravings during periods.

*Editor’s Note (9/22/23): This sentence was edited after posting to clarify Martin Heni’s affiliation.