Insulin and leptin, the push and pull of what we eat

Eat or not eat, that is the question. Insulin and leptin are the hormones responsible for regulating how much food we need to eat. So crucial is its role that resistance to these hormones can lead to obesity, diabetes or even Alzheimer’s.

When hearing the word insulin, practically all of us think of those elderly people who cannot eat a piece of a delicious cake in the typical family snack and who, unfortunately, have to inject themselves every two to three because their sugar is high. But what is insulin? And more importantly, why do so many people need it?

The traffic lights of intake

Sugar from the diet (glucose) is the main source of energy for human beings. This glucose is responsible for moving our muscles, making our organs work and, above all, feeding our brain.

Being so important, how do we make sure that our organs and our brains get that energy? The answer is simple: thanks to insulin. It is a pancreatic hormone that opens a portal in the cells that make up our organs. This allows glucose from the blood to pass into it and be used for energy.

What happens when we eat? The sugar – or glucose – from the food we eat passes into the blood and, when the levels are high, an alert is generated in our body that quickly tells the pancreas to produce insulin. Insulin travels through the blood, reaches the cells and opens a door in them for glucose to enter. As a consequence, blood glucose levels drop, glucose is transformed into energy within the cells and we, after a while, feel active.

Another hormone that is also produced when we eat is leptin. This substance is considered an adipokine because, as its name indicates, it is produced in adipose tissue, the place where we accumulate fat. Yes, those love handles that some people hate so much have a first and last name: adipose tissue.

When we eat, adipose tissue produces leptin, which travels to our brain and says, “Come on, stop eating, you’ve had enough.” That is, it is a satiating hormone that regulates fat reserves.

Insulin and leptin, as you can guess, are not independent. In fact, when we eat, the insulin that is secreted favors the production of leptin. Leptin makes us less hungry (because we are filling up our fat stores) and prevents more insulin from being secreted. They make a good team.

The consequences of excesses

These intake control systems and glucose levels can be altered in various situations: with aging, smoking, sedentary lifestyle, genetic problems, when we overeat (and not just when we go home from grandmother), etc.

In all these cases, our body becomes exhausted and desensitized to the effects of these hormones. It happens like when we put on a strong cologne: at first it smells a lot, but after 10 minutes it seems to us that it doesn’t smell like anything.

If we do not maintain a healthy rhythm of life, the pancreas can stop producing insulin correctly. It can also happen that our cells stop being sensitive to insulin, even though the levels of the hormone are normal or even high. We are then said to be insulin resistant.

Therefore, people with insulin resistance, and also people who do not produce it properly, have very high blood glucose levels and an increased risk of diabetes. In fact, some people with diabetes need to take insulin to keep blood sugar levels stable.

Leptin resistance also exists. For example, it occurs in conditions of obesity. Obese people have elevated levels of leptin in their blood. Curious, right? What happens is that overeating in the long term produces damage to the brain that prevents leptin from giving the signal to stop. This prevents us from feeling full and stop eating, which becomes a vicious circle. In fact, animal models genetically modified so that they do not produce leptin or its receptor develop severe obesity.

Insulin and leptin resistance are increasingly serious health problems due to the lifestyle that has been promoted in our society for decades. Many times they are steps prior to the development of chronic diseases such as obesity or diabetes. They can also accompany unhealthy aging.

Healthy men in healthy body

Insulin and leptin resistance have been reported to be related to Alzheimer’s disease. What will they have to do?

As we said before, glucose is the brain’s favorite food. In animal models of Alzheimer’s, failures in insulin signaling have been detected in the brain, specifically in the hippocampus area. This region, responsible for memory, is the first to begin to deteriorate at the beginning of the disease.

As we said, insulin and leptin go hand in hand and, as expected, leptin signaling also fails. Leptin resistance has been described in patients with Alzheimer’s. Fortunately, this has made it possible to define both insulin and leptin as crucial components for the proper functioning of the brain.

Ok, it has become clear to us, insulin and leptin are responsible for regulating what we eat so that we have energy and our body and brain work well. But in addition, the direct relationship with Alzheimer’s disease makes them potential therapeutic targets, targets to direct therapies that can help treat or slow down this type of neurodegenerative disease. And to achieve in this way, as our Roman ancestors used to say, a healthy men in healthy body.


This article was a finalist in the II edition of the youth disclosure contest organized by the Lilly Foundation and The Conversation Spain.


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