The brain controls body weight and obesity by regulating intestinal fat absorption

Obesity is a state of body adiposity that results from an imbalance between the energy consumed and the energy expended, where the former exceeds the latter. Those who consider it a chronic disease do so paying special attention to its comorbidity — that is, its frequent coincidence and relationship with conditions such as cardiovascular disease, diabetes, osteoarthritis, obstructive sleep apnea and certain forms of cancer, in addition to significant psychological problems that affect above all the self-esteem of the obese person.

The scientific and social study of obesity has shown that it is a complex bodily disorder, the causes of which are multiple and varied, and may include genetic and epigenetic factors, diet and eating habits, socioeconomic status, and personal and social lifestyles. People generally gain weight by eating more than necessary, a behavior whose main roots can be found not only in their immediate environment but also in the homeostatic and incentive systems of the brain that regulate energy balance and body weight.

Although these systems are multiple and complicated, obesity depends largely on the absorption of ingested food, particularly fats, by the intestine. But, contrary to the somewhat intuitive idea that this absorption takes place autonomously by passive diffusion in the digestive system, we now know that it is also controlled by the brain, and a large and coordinated group of researchers from hospitals and universities in Shanghai, China, has just revealed the precise mechanism by which this control takes place in mice. The characteristics of their research, published in the prestigious journal Nature, are explained below, adding context to apply it to humans.

The first signs indicating the presence of nutrients in the digestive tract reach the brain via the vagus nerve, the tenth of the cranial nerve pairs in humans and the longest and most important nerve in the autonomic nervous system for digestion and energy homeostasis. The bodies of its neurons are found in four nuclei of the brain stem, specifically in the medulla oblongata, and its terminals extend throughout the digestive tract, innervating organs such as the stomach, the portal vein and the crypts and valleys of the entire intestine. Its winding and vague course through the interior of the body is what gives it its name. Much of the function of the vagus nerve is parasympathetic, which is why it is part of the autonomic nervous system responsible for anabolism, that is, limiting expenditure and promoting energy increase and saving.

Now, the aforementioned Chinese researchers have shown that, in addition to the vagus nerve's known regulation of gastric motility and digestive processes, chemically inactivating its dorsal motor nucleus in mice reduces intestinal fat absorption, causing them to lose body weight. Likewise, activating this same nucleus causes the opposite effect, increasing intestinal fat absorption and body weight.

But the most novel and surprising thing that researchers have observed is that when a certain group of neurons in that same nucleus is deactivated, specifically those that project to the jejunum, a part of the small intestine, the length of the microvilli in the intestinal wall is shortened, which reduces its surface area and thus the place in whose blood capillaries fat absorption occurs. The brain thus regulates this absorption by controlling the length and surface area of the intestinal spaces in which it takes place.

But that is not all, because the Shanghai researchers have completed their excellent and complex research by showing that puerarin, a substance used to treat vascular diseases of the brain, increases the excretion of fecal fats and weight loss by also inhibiting certain neurons in the dorsal motor nucleus of the vagus nerve, which makes this substance a regulator of intestinal fat absorption and, consequently, a promising treatment to control body weight in humans as well.

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