In the week before menstruation, the sugar balance is out of whack

The brain’s sensitivity to insulin is higher before ovulation than afterward, in the period before menstruation. Research on a total of 26 women shows this, German researchers write in a study in the scientific journal Nature Metabolism which appeared last week. It could explain the urge to eat that sometimes occurs just before menstruation.

The hormone insulin regulates the absorption of glucose into tissues: after eating, when the amount of sugar in the blood is high, insulin is released and guides glucose into the tissues, where it can serve as fuel. All tissues are sensitive to insulin in healthy people, including the brain. Insulin has an even broader effect in the brain: there the hormone influences eating behavior, fat distribution and metabolism in the body. But previous studies in rats did see a gender difference. Male rats ate less when they were injected with insulin into their brains, but the females did not.

In human men with a healthy weight, insulin in the brain increases the sensitivity of all tissues in the body to the hormone, the same researchers have previously shown. Not so in obese men. But it was not known how this works in women during the course of their menstrual cycle.

The Germans examined eleven young women of a healthy weight to see how sensitive their bodies were to insulin in the week before ovulation and in the week after. The participants came to the lab in the morning on an empty stomach and were administered a high dose of insulin (or placebo) via a nasal spray. Meanwhile, their blood sugar was maintained at a constant, normal level through an IV. Every five minutes, the researchers took a blood sample to measure the concentration of glucose in it. If the glucose administration rate was equal to the uptake rate, this was a measure of the sensitivity of the tissues to insulin. The more sensitive cells are to insulin, the more glucose can be administered.

In the week between the last menstrual period and ovulation, the shot of insulin in the brain increased the body’s sensitivity to insulin. But in the week after ovulation, before the next menstruation, this was not the case.

In fifteen other young women, the Germans examined how the brain responded to the dose of insulin from the nasal spray. They used brain scans made with fMRI (functional magnetic resonance imaging) to look at the hypothalamus, which regulates, among other things, the menstrual cycle and eating behavior. That brain area did respond to insulin in the week before ovulation, but not in the week after.

It therefore appears that even in women with a healthy weight, insulin in the brain improves sensitivity to the hormone in the rest of the body, but only in the period before ovulation. In the period before menstruation, the brain appears to be more insensitive to insulin – and so does the rest of the body, the authors write.

More insensitive to insulin

There are many people whose brains (and sometimes also the body) have become more insensitive to insulin. This often leads to weight gain, obesity or type 2 diabetes, and can also cause memory disorders and mental problems.

The difference in insulin sensitivity before and after ovulation is comparable to the previously found difference between normal-weight and obese men, notes German medical psychologist Nils Kroemer in an accompanying commentary Nature Metabolism. “It is likely that altered signals from the hypothalamus explain the changes in body weight, appetite, and food cravings that many women experience before their period,” he writes. He sees the new findings as a prelude to better treatments, for example for metabolic diseases or eating disorders, which take into account hormonal fluctuations and insulin sensitivity.