Research has uncovered a surprising fact: the brain produces insulin, a hormone typically associated with the pancreas. This same insulin is absent in individuals with type 1 diabetes and dysfunctional in those with type 2 diabetes.
For over a century, scientists have understood that the pancreas houses insulin-producing cells known as islets, which contain beta cells dedicated to insulin production. However, the brain’s ability to produce insulin remained largely overlooked until recent rediscoveries.
The notion of insulin in the brain was initially proposed in the late 1970s, where a 1978 study found insulin concentrations in the rat brain were ‘at least 10 times higher than that found in plasma … and in some regions … 100 times higher.’ Despite this groundbreaking finding, skepticism arose after researchers demonstrated that the insulin observed in the brain was derived from pancreatic sources. This led to a long-held belief that the brain was reliant on the pancreas for insulin, an assumption that persisted for nearly three decades.
While it’s established that insulin can be transported from the blood to the brain, emerging evidence indicates that insulin is also produced locally in specific areas for distinct purposes.
Scientists have identified at least six types of brain cells capable of synthesizing insulin, some of which have been validated in both rodent and human studies. Among them are neurogliaform cells, located in a brain region crucial for learning and memory, tasked with insulin production based on glucose levels—much like pancreatic beta cells. The specific functions of this insulin are still under investigation, although its presence in such areas suggests a role in cognitive function.
Additionally, insulin-producing neural progenitor cells, responsible for generating new neurons, were found in this region. Other discoveries include insulin-producing cells located in the olfactory bulb, which processes scents, although their functions remain to be fully understood.
A 2020 study revealed that neurons in the mouse hypothalamus, which governs growth and metabolism, also produce insulin. Remarkably, when subjected to stress, the hypothalamus showed diminished insulin production, correlating with reduced growth in the mice, suggesting a link to growth hormone regulation in the pituitary gland.
The choroid plexus, a brain area that produces cerebrospinal fluid, has also been identified as a source of insulin. This area secretes growth factors and nutrients into the brain, potentially facilitating a broader distribution of insulin throughout the brain’s ventricles. A recent 2023 study indicated that changes in insulin production from the choroid plexus could directly influence appetite by affecting the hypothalamus.
Furthermore, insulin-producing neurons in the hindbrain have been shown to decrease food intake, indicating that brain insulin may have a role in appetite regulation. However, despite these findings, current evidence does not support the idea that brain insulin regulates blood glucose levels similarly to pancreatic insulin. Instead, it may contribute to maintaining brain health as individuals age.
The concept of Alzheimer’s disease being linked to insulin resistance has emerged, with some referring to it as type 3 diabetes. This condition could hinder the brain’s ability to utilize glucose properly, leading to cognitive deficits. Some studies suggest that administering insulin intranasally may improve cognitive performance in Alzheimer’s patients, although results vary.
Interestingly, the relationship between cerebrospinal fluid insulin levels and cognitive performance appears complex, particularly in women, as higher insulin levels are correlated with worse outcomes.
While research continues to unearth the intricacies of insulin production in the brain, questions remain about whether the brain’s insulin-producing capabilities evolved before pancreatic beta cells. It is hoped that further studies will not take another three decades to clarify.
With the mounting evidence of brain insulin production, it is likely that educational resources will soon reflect this important discovery.