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Physiological regulation of weight and diet

 


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The normal individual maintains body weight at a predetermined level of stability very remarkable, given the wide variation in daily caloric intake and activity level. Due to the physiological importance is keeping energy reserves, it is difficult to lose weight and keep it off voluntarily.

The appetite and metabolism are regulated by a complex network of neural and hormonal factors. The hypothalamic centers of hunger and satiety are essential in these phenomena. Some neuropeptides such as corticotropin-releasing hormone ( corticotropin-releasing hormone , CRH), the melanocyte-stimulating hormone alpha ( α-melanocyte stimulating hormone , α-MSH), and the transcript related to cocaine and amphetamines ( cocaine and amphetamine-related transcript , CART) produce anorexia by acting on the central nervous system (CNS) in the centers of satiety.

The gastrointestinal peptide ghrelin, glucagon, somatostatin and cholecystokinin are originating satiety and decrease food intake. The hypoglycemia suppresses the action of insulin, which reduces glucose utilization and inhibits the action of the satiety center.

Adipose tissue produces leptin and contributes largely to the conservation of weight homeostasis (long term) to act on the hypothalamus to decrease food intake and increase energy consumption. Leptin suppresses the expression of neuropeptide Y in the hypothalamus, which is a potent appetite stimulant, and intensifies the expression of alpha-melanocyte-stimulating hormone, that through the melanocortin receptor (MC4R) exerts the effect of mitigating the appetite. Thus, leptin activates several descending nerve pathways that alter the foraging behavior and metabolism.

However, leptin deficiency, which occurs together with loss of adipose tissue, stimulates appetite and induces other adaptive responses, such as inhibition of the hypothalamic thyrotropin releasing hormone ( thyrotropin-releasing hormone , TRH) and gonadotropin-releasing hormone ( gonadotropin-releasing hormone , GnRH).
There are several cytokines, including tumor necrosis factor alpha ( tumor necrosis factor α , TNF-α), interleukin (IL) 6 (IL-6), IL-1, interferon gamma (IFN-γ), the factor ciliary neurotrophic ( ciliary neurotrophic factor , CNTF) and leukemia inhibitory factor ( leukemia inhibitory factor , LIF), which can induce cachexia. In addition to such action, these factors can cause fever and depress myocardial function, modulate inflammatory and immune responses, and stimulate various metabolic disorders specific.

The TNF-α for example, mobilizes fat preferentially, but respects the striated muscle. Concentrations of one or more of these cytokines may be elevated in people with cancer, sepsis, chronic inflammatory disorders, AIDS and congestive heart failure.
Weight loss occurs when energy consumption exceeds the calories available for this purpose. In most individuals, about half of the energy used in foods such basic functions such as maintenance of body temperature. In a 70 kg weight, basal metabolism consumes about 1 800 kcal / day. About 40% of calories consumed in exercise, although athletes may use more than 50% when performed in intense degree.

About 10% of the caloric intake is used for food thermogenesis, a process in which energy is consumed in the digestion, absorption and metabolism of foods.
The mechanisms of weight loss are the reduced food intake, malabsorption, the loss of calories and increasing energy requirements ( fig. 1 ). Changes in body weight may be due to the changes experienced by the tissue mass or volume of liquid contained in the body. In general, a shortfall of 3 500 kcal corresponds to a loss of 0.45 kg of body fat, but must also take into account the water (1 kg / L) to be won or lost. Weight loss that takes weeks to months almost always involves a loss of tissue mass.

Energy balance and pathophysiological aspects of weight reduction

In food intake is influenced by a variety of visual, olfactory and gustatory, as well as genetic, psychological and social. Absorption may be reduced if there is pancreatic insufficiency, cholestasis, celiac sprue, intestinal tumors, radiation injury, inflammatory bowel disease, infections or drug effect. The abnormal expression of these pictures may be suspected if there are alterations in the frequency of defecation and stool consistency. They can lose calories through vomiting or diarrhea, with glucosuria of diabetes mellitus or the presence of fistulas. The resting energy expenditure decreases with age and can change the status of thyroid function. Body weight begins to decrease after 60 years at a rate of 0.5% per year. Body composition also changes with aging and adipose tissue increases lean muscle mass decreases with age.

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