Effects of prolonged fasting
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Metabolic Health Effects of Prolonged Fasting
Prolonged fasting, defined as abstaining from food or caloric beverages for several days to weeks, leads to significant metabolic changes. It consistently increases circulating ketones and causes mild to moderate weight loss (2–10%), with about two-thirds of the lost weight coming from lean mass and one-third from fat mass. This high proportion of lean mass loss raises concerns about muscle protein breakdown. Blood pressure (both systolic and diastolic) tends to decrease during prolonged fasting. The effects on plasma lipids are mixed, with some studies showing reductions in LDL cholesterol and triglycerides, while others find no significant changes. Glycemic control improves in healthy adults, with reductions in fasting glucose, insulin, insulin resistance, and HbA1c, but these benefits are not seen in people with diabetes. Notably, most metabolic improvements disappear within a few months after refeeding, even if weight loss is maintained. Adverse events such as metabolic acidosis, headaches, insomnia, and hunger have been reported, but overall, prolonged fasting appears moderately safe for short-term use in healthy adults .
Glucose Tolerance and Insulin Response During Prolonged Fasting
The duration of fasting influences glucose metabolism. Longer fasts (e.g., 6 days) increase ketone levels more than shorter fasts (e.g., 2 days). Short-term fasting can impair glucose tolerance and increase stress responses, as indicated by higher epinephrine and core temperature. In contrast, prolonged fasting seems to trigger adaptive mechanisms that improve insulin release and maintain glucose tolerance after refeeding . In obese individuals, prolonged fasting increases insulin resistance and impairs glucose tolerance in nondiabetics, but has little additional effect in those with diabetes .
Energy Expenditure and Substrate Oxidation Shifts
Prolonged fasting causes a rapid and sustained increase in fat oxidation, while carbohydrate oxidation drops and remains low. Total energy expenditure decreases, but the daily rhythm of energy use is preserved. The body’s preference for burning fat over carbohydrates becomes more pronounced the longer the fast continues .
Effects on Oxidative Stress and Antioxidant Capacity
A 6-day fast in healthy men temporarily increases both oxidative stress and antioxidant capacity, but the balance between them remains stable. These changes, including increased ketones and sympathetic activity and decreased testosterone, return to normal after resuming regular eating .
Inflammation and Immune System Changes
Prolonged fasting in humans triggers a shift in adipose tissue metabolism, downregulating pathways for glucose and fat storage and upregulating immune and inflammatory pathways, especially after 10 days. This is reflected in increased markers of inflammation in the blood . In animal studies, repeated prolonged fasting cycles reduce IGF-1 and PKA activity, promoting hematopoietic stem cell regeneration and reversing immunosuppression, suggesting potential benefits for immune system health .
Long-Term and Epigenetic Effects
Animal studies show that prolonged fasting can cause long-lasting metabolic changes, including persistent alterations in body composition, energy expenditure, and hypothalamic gene expression. These changes can lead to increased fat mass and altered responses to future fasting or high-fat diets, indicating that the effects of fasting may extend well beyond the fasting period itself .
Prolonged Fasting in Older Adults
In older adults, habitual prolonged nightly fasting (≥12 hours) is not associated with metabolic benefits. Instead, it is linked to lower HDL cholesterol and some modestly negative changes in electrolyte balance, suggesting that extended fasting may not be beneficial for this population .
Fasting, Caloric Restriction, and Longevity
In animal models, combining prolonged fasting periods with caloric restriction provides greater health and lifespan benefits than time-restricted feeding alone. These regimens improve healthspan metrics and are associated with unique metabolic signatures, especially when started later in life .
Conclusion
Prolonged fasting leads to significant short-term metabolic changes, including weight loss, increased fat oxidation, and improved glycemic control in healthy individuals. However, it also causes loss of lean mass, temporary increases in oxidative stress, and activation of inflammatory pathways. Most benefits are not sustained after refeeding, and there may be long-term metabolic consequences, especially with repeated fasting. In older adults, prolonged fasting may not be beneficial. While animal studies suggest potential immune and longevity benefits, more research is needed to fully understand the long-term safety and efficacy of prolonged fasting in humans.
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