Psycho emotional stress causes the brain to signal the adrenal gland to produce cortisol so that the body can better anticipate stress. If this goes on for too long, we speak of chronic stress and thus there is a long release of cortisol. This raises blood sugar levels for a long time, causing disturbances in energy and insulin balance. Therefore, long-term stress is harmful and contributes to the development of the metabolic system.
Our brain determines much of our behavior but not everything. If too much fatty tissue develops, it can begin to influence the brain to change the distribution of energy throughout the body.
This is because, as in prehistoric times, the brain still wants to maintain energy storage in fatty tissue, for times of possible scarcity. There simply wasn't always food, so when there was, a lot of it was eaten. Excess energy was then stored in subcutaneous adipose tissue. When there was not enough food, the body could break down these fats and the liver converted them to glucose as a source of energy for all organs, including the brain and all cells. Conditions determine the energy distribution in the body so that we can respond flexibly to any threats such as hunger, thirst, heat, cold, injury, etc. This system is still the same. We are very sensitive to scarcity, our brain reacts to it by conserving energy. All processes come to a low level, so to speak, allowing you to survive with little energy. Because this is no longer necessary (think of the supermarket and a well-stocked refrigerator, so food is always available), the body stores the excess energy while this causes complaints rather than benefits.
Motion
We also move super little compared to our distant ancestors. The Netherlands is number 2 in all of Europe in terms of hours of sitting time per day, averaging 8-9 hours per day. Among teenagers this is even 12 hours per day. The "sedentary lifestyle" is probably the biggest risk factor for obesity.
By nature we do not want to waste energy when there is enough available in the body. Also, in nature, look at lions, they only come into action for looking for prey, hunting itself and for reproduction.
The brain responds to hunger and satiety hormones sent from the body through the blood.
Leptin and ghrelin
Over the past decade, it has become clear that adipose tissue is not simply a storehouse of fat as previously thought. Adipose tissue secretes numerous hormones (adipokines) that play important roles in fat and carbohydrate metabolism and the biology of vascular complications. Thus, it is a complex endocrine organ. Through the blood, the hormones of adipose tissue cause other organs in the body to work harder or less hard. Thus, fat cells have many effects on other tissues and so does the brain.
One of the hormones given off by adipocytes (fat cells) is leptin. This hormone signals your brain, in the hypothalamus, that enough has been eaten. It further regulates your energy level and how much body fat may be burned. All of our organs have receptors for leptin. The brain, immune system and reproductive organs are the most sensitive to it.
When leptin is delivered high enough to the blood and arrives in the brain, another signaling substance, ghrelin, which communicates precisely that you want to eat, stops. Ghrelin is also produced by the hypothalamus and prompts the pituitary gland (a hormone gland in the brain) to secrete growth hormone. Ghrelin also plays a role in the hippocampus, where it stimulates learning skills as well as adaptation to changing conditions. It further stimulates fat synthesis and reduces fat breakdown. Sleep deprivation can even lead to obesity due to an increase in ghrelin.
So in a healthy situation, (an empty) stomach releases ghrelin, which reaches the brain via the blood. This makes you feel hungry. Once you have eaten, leptin will stop your hunger signal after a while because enough energy has come in, until you have used up this energy again (through basal metabolism and also your efforts). Then this pattern repeats itself. But when, due to stress, for example, you lie for hours at night brooding, this lack of sleep can thus lead to an increase in ghrelin, which can eventually even lead to obesity.