The regulation of food intake is a complex concert of signals from different parts of our body, working together with our biological clock, and of course, our brain. Those signals (or messages) indicate for example whether our stomach is full or empty, if we have energy incoming, or how much energy we have stored in our fat cells. Eating also needs to be pleasurable. But who is coordinating all these different signals?
The Homeostatic System – Eating because we need to
Within the cell-layers that form our stomach, we have a specific cell-type that can sense whether the stomach wall is stretched or not. If it isn’t, and our stomach is empty, those cells start to produce the ‘hunger hormone’ ghrelin. When a certain threshold is reached, our brain integrates all current signals about energy status and hunger and induces eating.
After the meal, the stomach wall is stretched and less ghrelin is produced. It does not depend on the caloric content of the stomach, but the degree to which it is stretched out! This is when all our satiety signals start to kick in. The small intestine produces CCK to stimulate the digestion of proteins and fats. Glucose gets absorbed, the blood glucose rises. The cells of the duodenum and the small intestine starts to produce GLP-1 and insulin is secreted from the pancreas to make sure that all that precious sugar will find its way into the cells. All those messenger molecules signal the brain that we are full and should stop eating.
Besides, the fat cells of our body produce the hormone leptin. The more fat and fat cells we have, the higher the leptin levels or the louder the leptin message in our body will be. The less fat we have, the lower the levels. Leptin is basically updating our brain about the filling level of our energy storage. If our storage is quite full, leptin will lead to the inhibition of our energy intake and instead increase our energy expenditure. If the storage is more empty, food intake will be initiated and we will spend less energy.
A lot of these signals are sent to the brain via the blood and/or the vagus nerve, a looong nerve that emerges from almost right behind your ear and connects to your lungs, heart, liver, spleen, stomach, pancreas, kidneys, and the small and big intestine.
Fun fact: You probably noticed that you use to become hungry around the same time you usually consume a meal. Probably because your energy storage levels are low, right? Nope, not necessarily! Most of the times our body is still in balance with its energy reserves. In general, our body loves balance. However, when a new meal comes in, this balance is disrupted until the nutrients are absorbed and distributed within the body. Our body is sneaky though and prepares for incoming food before the meal actually takes place! It produces insulin and thereby lowers blood glucose to make more ‘space’ for the new incoming sugar. It is likely the low blood glucose that makes us hungry then!
The Hedonic System – Eating because we want to
Looking at it from an evolutionary perspective, eating needs to be fun, it needs to be rewarding. Just imagine: you went on a long hunt (no matter if for berries or meat), walked kilometres in a potentially dangerous surrounding. You are exhausted, but in the end you finally got what you wanted. However, it did not feel rewarding at all. Would you put this much effort again? Probably not.
Different to the homeostatic system which is balancing our nutritional intake and needs, the hedonic system makes sure that we are motivated to work for our food (= wanting), and that the food we are consuming has a positive internal representation leading to sensory pleasure (e.g. in form of endocannabinoid release), especially when the food is sweet or highly calorie dense (= liking). The hedonic system can be influenced by gut input in form of for example ghrelin, insulin, and leptin.
Once crucial for our survival, reward-driven hunger can nowadays lead to overeating (= hedonic hyperphagia) or eating when not even hungry due to the fact that food is widely available and mostly calorie-dense (meaning, having a lot of calories in a small volume like in chocolate). Ultimately, the increased energy intake can lead to weight gain, and eventually obesity.
The Hypothalamus is the Main Conductor of Hunger Signals
The main (but not only) brain area involved in the regulation of food intake is the hypothalamus. It is therefore often referred to as the feeding center of the brain. However, also other brain areas are involved in the regulation of eating. The hypothalamus organizes and interprets signals from the homeostatic (‘We need energy!’), and from the hedonic system (,I want chocolate cause it makes me feel good!‘), eventually resulting in food intake.
Stay safe! Stina. ❤
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