The brain is highly responsive to insulin, according to this study. Insulin crosses the blood-brain barrier and regulates brain activity in specific regions, influencing eating behaviour, whole-body metabolism and body fat distribution. In particular, it acts on the hypothalamus to increase whole-body insulin sensitivity.
Surprisingly, little attention has been given to how insulin sensitivity in the brain changes during the menstrual cycle, despite the interplay between energy metabolism and reproductive function. The menstrual cycle involves various metabolic adjustments to ensure the body's energy needs are met for processes like ovulation, pregnancy, and lactation. These adaptations are essential for reproductive success and energy balance.
In a recently published study, Hummel and colleagues used intranasal insulin administration and functional MRI (fMRI) scans to assess insulin sensitivity in women during different phases of the menstrual cycle.
Eleven women took part in the study. In four different sessions, they received either nasal insulin spray or a placebo while their blood sugar levels were carefully controlled using a hyperinsulinemic–euglycemic clamp (a gold-standard method for measuring insulin action in the body). The researchers wanted to see how their whole-body insulin sensitivity was influenced by insulin in the brain.
This fascinating study showed that Nasal insulin spray was used since this delivers substantial amounts of insulin to the brain, with only a tiny amount entering the blood stream, enabling selective insulin stimulation of the brain.
The researchers discovered that when the women were in the follicular phase of their menstrual cycle, intranasal insulin increased whole-body insulin sensitivity compared to placebo. However, during the luteal phase of the menstrual cycle, insulin in the brain didn't have the same effect.
The researchers also used fMRI scans to look at how the hypothalamus responds to insulin. They found that during the follicular phase, the hypothalamus was influenced by insulin, but not during the luteal phase.
These findings show that insulin action in the brain increases peripheral insulin sensitivity, but only during the follicular phase of their menstrual cycle. This suggests that brain insulin resistance could contribute to whole-body insulin resistance seen in the luteal phase.
This study shows that insulin in the brain plays a crucial role in regulating whole-body insulin sensitivity during different menstrual phases. It is possible that changes in hypothalamic insulin sensitivity during the menstrual cycle could help explain changes in body weight regulation, mood swings and food cravings premenstrual phase (i.e. PMS). Future research should explore how these findings relate to mood and eating disorders that are more prevalent in women.