Signals from our belly to the brain
Our body requires energy to function. Every cell needs a certain amount of nutrients to keep working properly and face every day’s stress, so it is normal for our body to ask for those nutrients in the form of appetite and hunger. If you have noticed, we feel hungry several times a day, depending on our energy requirements. So, we are not eating all the time, without a reason; instead of this, our body knows when to request food and when not to. How is food and energy intake controlled, then? As you can imagine, our brain does all the work through chemical and electrical signals that go from our gut to the brain and viceversa.
There are two main signals that our brain relies on to control energy intake, and they are satiation and satiety. The first one tells our body that enough nutrients have been consumed in order to satisfy energetic requirements. Satiation is basically when we feel full and stop eating. Satiety, on the other hand, has to do with how long this feeling of fullness lasts, until we feel hungry again.
Both amount and number of meals affect the total energy intake in one day.
The food intake and energy regulation mechanisms start even before you put anything in your mouth. Here, our senses, including the smell, the sight and even the touch, play an important role. Behavioral changes start occurring when we see food that looks attractive to us, based on its appearance or in previous experiences.
The brain in action
After eating, our stomach distends, or increases in size.
After this, food reaches the small intestine, where nutrients are absorbed. Here, liberation of substances also activates regions of the brain that continue with the satiation and satiety signals. Cholecystokinin (CCK) is a hormone that is secreted as a response to the detection of fats and proteins from food.
Satiety is controlled by other gut hormones that not only act at the same time as CCK, but their effects also tell the brain how much energy is left from the last meal.
Other gut hormones that participate in the promotion of satiety are the glucagon-like peptide, oxyntomodulin, peptide YY and pancreatic polypeptide. All these slow down gastric emptying in order to maintain a constant satiety signal, until it is necessary for us to eat again to fulfil energy requirements. This is why we aren't eating all the time. Our brain knows when to stop and when to eat again thanks to these hormones.
Leptin And Insulin: Weight Control Hormones
Usually, an adult would eat from three to five times a day. So, how do we maintain a constant bodyweight? Our body is so well designed that, also through signalling, it is able to know how much energy we need every day and how much food and which nutrients we must ingest in order to fulfill those requirements. For this, our body requires long term signals that are in charge of two important hormones: insulin and leptin.
Leptin, the weight-lose hormone?
Leptin is produced by our adipose tissue.
For instance, it is known that reduced levels of leptin found in people with mutations that affect leptin production cause obesity, a condition that can be reversed when an exogenous form of the hormone is administered to these patients.
No wonder why there are nowadays so many weight lose products and programs based on increasing leptin levels. But before you get all excited, research has also shown that people with obesity with no leptin genetic alterations actually have high leptin levels, and other studies have shown that the use of exogenous leptin has very small effects on body weight.
There are also several diets that claim to be able to control leptin levels, in order to help you loose weight. If these work or not, I do not know, but what is true is that weight is affected by several factors, not only leptin levels, so it is difficult to determine whether a specific type of diet or food will help you manage leptin levels.
Insulin and glucose control
The pancreas is the one in charge of producing insulin in response to food intake, unlike leptin.
Right after a meal, glucose levels in the blood rise and promote the liberation of insulin, which lets our cells know that glucose is available for them to use. When insulin no longer works, a state known as insulin resistance causes chronic high blood glucose levels, which can alter other body functions and develop into what is known as diabetes.
Insulin also participates in satiety signaling. Overweight and obese people have higher risk of suffering from insulin resistance and eventually, diabetes, than people who have a healthy weight.
In conclusion, both our digestive system and our brain work together to control our food intake and how the energy obtained from food should be used, transformed or stored. These control mechanisms are really precise, but are also very prone to be affected by changes in our diet or schedule. The good thing is that biological systems are flexible and eventually get used to the changing conditions they are exposed to.