The enigmatic hunger pangs experienced during menstruation, often characterized by an irresistible urge for carbohydrates and sweets, have long mystified the scientific community. In a breakthrough development, recent research has unveiled a potential neural mechanism in the brain that may elucidate the phenomenon of food cravings during the menstrual cycle.
Published on Thursday in Nature Metabolism, a small-scale clinical trial has unearthed disparities in insulin sensitivity across various phases of the menstrual cycle. Although the investigation did not directly scrutinize food cravings, the researchers postulate that heightened insulin sensitivity, a hormone regulating glucose levels, metabolism, and appetite, could underpin the surge in hunger.
Martin Heni, the co-senior author of the study and head of the clinical research center for diabetes at the Eberhard Karls University of Tübingen in Germany, remarked, “The brain contributes to metabolic changes and probably changes eating behavior across the menstrual cycle. 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.”
While insulin is typically produced in the pancreas, it can traverse the blood-brain barrier, influencing various brain areas to regulate bodily functions. Previous research by Heni’s team demonstrated that brain insulin can suppress glucose production and stimulate glucose uptake in muscle tissue, highlighting its critical role in managing metabolism and energy in the body. Nevertheless, this relationship was initially explored exclusively in young men.
Research into insulin fluctuations in individuals who menstruate has been hindered by the hormonal shifts throughout the menstrual cycle. However, processes such as the preparation of the uterus for potential pregnancy may introduce variations in how the brain oversees glucose metabolism. To investigate this, Heni’s team conducted two experiments, involving 11 women with regular menstrual cycles. The study observed changes during the follicular phase (when the egg prepares for ovulation) and the luteal phase (when the egg travels to the uterus after ovulation).
Insulin clamps, recognized as the gold standard for measuring insulin sensitivity, were applied to each participant. The clamps measured sensitivity after administering either insulin or a placebo via nasal spray, coupled with intravenous insulin injections. Four sessions were conducted, with two in the follicular phase and two in the luteal phase.
During the follicular phase, coinciding with the onset of menstruation, the brain exhibited greater sensitivity to insulin. This sensitivity waned when participants entered the luteal phase, indicating a transition to insulin resistance.
These findings suggest that brain insulin resistance may hinder the regulation of energy production in the body and potentially disrupt insulin’s role in appetite control. Previous studies on conditions like obesity and diabetes have correlated increased insulin sensitivity with reduced appetite, while insulin resistance is thought to intensify food cravings. Although formal food intake assessments were not conducted in the study, Heni believes these findings could elucidate the increased food cravings commonly experienced in the latter half of the menstrual cycle, aligning with existing research indicating heightened cravings during the luteal phase.
To corroborate their findings, the researchers conducted a second experiment using neuroimaging to examine brain activity throughout the menstrual cycle. This study involved 15 women and confirmed a parallel pattern of insulin sensitivity, particularly in the hypothalamus, a brain region governing food intake and metabolism.
One limitation of the study was its small sample size, a challenge inherent in experimental and observational studies that require participants to undergo multiple experiments. Alexandra DiFeliceantonio, a neuroscientist at Virginia Tech who studies brain modulation of food preference, noted, “You will always end up with fewer participants because it’s expensive and time-consuming.”
Nevertheless, the documented changes in brain insulin sensitivity within each participant were described as “striking” by Nils Kroemer, a neuroscientist at the University of Bonn in Germany. While not directly involved in the study, Kroemer emphasized the need for a larger-scale study in the future. Such a study could also consider other factors affecting energy metabolism, including obesity, contraceptive use, or endocrine disorders like polycystic ovary syndrome, associated with insulin resistance.
The current findings offer insights into a potential neural mechanism underlying period cravings, as suggested by Kroemer. He posits that hypothalamic circuits responsible for energy metabolism may intersect with