„Fasting“ – Process of Adaptive Organism Mechanisms

The severe restriction of food intake, employed outside therapeutic contexts or as a consequence of chronic conditions, poses a significant threat to the body’s homeostatic equilibrium. This approach, which involves reducing caloric consumption to near-zero levels, is frequently adopted by individuals struggling with eating disorders such as anorexia nervosa or bulimia nervosa, as well as by those seeking rapid and drastic weight loss. Under this nutritional regimen, the body receives only minimal fluids—water, highly diluted vegetable or meat broths, herbal teas, and coffee, occasionally supplemented with small quantities of sugar or dairy. Such extreme caloric deprivation triggers a cascade of defensive responses aimed at preserving essential life functions, albeit at the expense of long-term physical and psychological harm.

Within the initial three-day period of sustained fasting, a marked reduction of approximately thirty percent in peripheral blood glucose concentrations is observed. Concurrently, insulin levels—critical for glucose regulation—decline, while the activity of its antagonistic counterpart, glucagon, increases proportionally. To maintain adequate glucose availability essential for central nervous system function and erythrocyte viability, the organism amplifies gluconeogenesis, the biochemical process of synthesizing glucose from non-carbohydrate precursors. Primary substrates for this metabolic pathway include structural proteins and adipose tissue reserves. As fasting progresses, catabolic degradation of energy reserves intensifies, encompassing muscle and hepatic glycogen stores as well as phosphocreatine. Upon depletion of these deposits, the body initiates mobilization of fatty tissue—first subcutaneous, then visceral fat, which provides mechanical support to internal organs. Lipids subsequently become the dominant energy source, accounting for up to 85% of daily caloric requirements. Enhanced lipolysis within the liver results in elevated production of ketone bodies, byproducts of fatty acid oxidation. Under physiological conditions, their blood concentration remains below 0.2 mmol/L; however, during prolonged starvation, this threshold is frequently exceeded, leading to metabolic acidosis—a disruption of acid-base homeostasis. The normal blood pH range (7.35–7.45) declines, adversely affecting cardiovascular function through tachycardia, arrhythmias, and blood pressure lability. These alterations culminate in respiratory insufficiency and cognitive impairment, reflecting the organism’s adaptive response wherein ketones and protein-derived amino acids are utilized as alternative metabolic fuels.