In the Paleo world, we talk a lot about insulin and insulin resistance, and how diet affects metabolic health, diabetes, and other related diseases. But all that talk tends to be really focused on insulin in the bloodstream, and it ignores insulin in the brain. Insulin is one of the few chemicals that can cross the blood-brain barrier, and it’s important for appetite regulation, learning, and memory. Your brain can get insulin resistant just like your muscles or fat tissue, and insulin resistance in the brain is associated with weight gain and also degenerative brain diseases (like Alzheimer’s Disease).
Insulin and the Brain
Insulin is best-known as a carbohydrate storage hormone. If you eat something containing digestible carbohydrates, you’ll end up with higher blood sugar: more carbohydrates (glucose) in your bloodstream. In the long term, high blood sugar is dangerous. That glucose needs to get out of the bloodstream and preferably sooner rather than later. Enter insulin, which shuttles it off to muscle and/or fat cells as needed.
It shouldn’t be surprising that insulin is important for your brain - the brain is the biggest glucose hog in your whole body. It’s the only organ that actually requires glucose. Everything else can run on fat if it has to, but for the brain, it’s glucose or bust. Even if you don’t eat any carbohydrate, your liver will turn protein into glucose (through a process called gluconeogenesis) to make sure your brain gets enough. Even if you don’t eat anything at all, your liver will break down your own muscle tissue to get protein to make glucose for your brain.
This review goes over some of the major points about insulin and the brain. Unlike most substances, insulin can pass fairly easily between the bloodstream and the brain. If blood insulin spikes, insulin levels in the nervous system also rise. That suggests that insulin can cross the blood-brain barrier.
Insulin in your body does a lot more than just ferrying glucose around, and insulin in your brain is the same way. In the brain, insulin affects two very important things:
- Hunger hormones and appetite, .
- Memory formation and cognitive function
That makes it important for two major chronic problems: diabetes and metabolic disease, and neurodegenerative diseases like Parkinson’s Disease or Alzheimer’s.
Brain Insulin and Weight
In general, healthy insulin action in the brain reduces hunger and increases metabolic rate. In animals with normal insulin receptors, infusing insulin directly into their brains reduces their appetites by affecting the levels of various different peptides and hormones that control hunger. Mice with no insulin receptors in their brain (meaning that their brain can’t “hear” the insulin signal) get fat because they’re always eating.
Insulin resistance in the brain also decreases thermogenesis (burning energy to keep your body temperature up), which lowers metabolic rate. Mice with insulin receptors in their brain knocked out can’t stay warm in a cold room. This may work through reducing the activity of brown fat.
The authors of this review did a series of experiments suggesting that insulin resistance in the brain contributes to obesity in humans (not just mice). That’s not exactly a huge leap of logic - it’s very reasonable that something that scrambles up normal hunger signals and reduces metabolic rate would contribute to obesity.
It also seems to work the other way around. In humans, insulin resistance in the body changes levels of insulin in the brain. The data from this study suggested that insulin resistance in the body could prevent insulin from getting into the brain, which would prevent it from having its anti-hunger, thermogenic effects.
Insulin and Brain Health
Insulin doesn’t just affect weight and metabolic issues. It’s also very important for memory and cognitive processing. People with Type 2 Diabetes are much more likely than the general population to have memory and cognition issues as they age, and there’s a reason why Alzheimer’s Disease has been referred to as “Type 3 Diabetes.”
Insulin is important for forming memories. Just like people with body insulin resistance, the elderly might be less efficient at getting insulin from the blood into the brain. People with Alzheimer’s Disease and other neurological diseases may also have insulin resistance in the brain (meaning that regardless of how much insulin is or isn’t there, their receptors aren’t hearing the insulin signal). The lower transport of insulin across the blood-brain barrier means that there’s a reduced ability to temporarily compensate the way people with Type 2 Diabetes do, by simply increasing insulin levels.
The consequences could be serious. Insulin resistance in the brain may reduce cholesterol levels in the brain, which makes people more vulnerable to Alzheimer’s Disease. (No, that wasn’t a typo: lower cholesterol levels in the brain are bad, not good). Giving patients with Alzheimer’s Disease insulin through a nasal tube (which delivers it straight to the brain) or increasing the level of insulin in their brain by feeding them glucose temporarily improves their symptoms, just like giving a person with diabetes insulin injections temporarily improves their blood sugar.
Summing it Up
All this research suggests that healthy insulin action in the brain is just as important for weight loss as healthy insulin function in the body. Insulin in the brain helps to control hunger signals and regulate appetite, so insulin resistance here is a real problem. Brain insulin might be one critical link between metabolic diseases like diabetes and neurodegenerative diseases like Alzheimer’s. So in that sense, it's important for much more than weight, particularly in people who are getting older.
Given the link between body insulin resistance and brain insulin levels, the importance of insulin resistance in the brain is one more reason to pay attention to metabolic health in general. It’s not clear if any specific foods particularly affect insulin sensitivity in the brain, but the research is pretty consistent about the fact that body insulin resistance is bad news for your brain. Even if you’re not overweight - and some of the studies found problems with insulin resistance completely independent of weight, so this is not a “overweight people’s problem” - paying attention to metabolic health really pays off.