Effects of 24 hour fasting
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Appetite, Food Reward, and Energy Intake After 24-Hour Fasting
A 24-hour fast significantly increases feelings of hunger, the pleasure derived from eating (“liking”), and the desire for food (“wanting”). People who fast for 24 hours tend to eat more at their next meal and find food more rewarding, especially snack foods and sweet items. However, the preference for savory foods may decrease in hedonic value after fasting. These effects are not seen when the same energy deficit is created by exercise instead of fasting, suggesting that fasting uniquely heightens appetite and food reward responses compared to other forms of energy depletion 14.
Metabolic and Hormonal Changes During 24-Hour Fasting
Fasting for 24 hours leads to acute changes in several metabolic markers. There is a notable reduction in trimethylamine N-oxide (TMAO), a compound linked to heart disease risk, although levels return to normal after refeeding. Fasting also alters levels of amino acids and other metabolites, which may contribute to long-term health benefits if fasting is repeated regularly. Additionally, fasting triggers a drop in plasma fibroblast growth factor 21 (FGF21), a hormone involved in metabolism, and this change is linked to individual differences in how the body conserves or spends energy during fasting and cold exposure 26.
The composition of the meal before fasting can influence how quickly the body enters ketosis (a fat-burning state). Starting a fast with a low-carbohydrate, high-fat meal leads to faster ketosis and better glycemic control compared to a high-carbohydrate meal, especially in older, sedentary, overweight adults .
Effects on Blood Pressure, Heart Rate, and Inflammation
A single day of fasting can lower daytime systolic blood pressure, particularly in people with hypertension, without causing dangerous drops in blood pressure. Heart rate responses differ between hypertensive and normotensive individuals, and fasting is generally well-tolerated by those with obesity and hypertension. Inflammatory markers such as high-sensitivity C-reactive protein (hCRP) may also decrease after fasting in hypertensive patients .
Fasting also affects the autonomic nervous system, as shown by increased sympathetic activity and stress index over the fasting period. However, most heart rate variability parameters are not significantly affected by the macronutrient composition of the pre-fast meal, though a low-carbohydrate meal may increase anxiety or stress markers .
Physical Performance and Exercise Capacity
For most people, a 24-hour fast does not impair short-term physical performance or physiological parameters in activities like CrossFit. However, in endurance sports such as cycling, fasting can reduce the time to fatigue, even though it increases the availability of free fatty acids for energy. This suggests that while brief fasting may not harm performance in strength or high-intensity activities, it can limit endurance at higher intensities, likely due to hypoglycemia and muscle glycogen depletion 39.
Immune and Cellular Effects
Preliminary animal studies suggest that a 24-hour fast can increase certain immune cell populations in the liver, which may have implications for metabolic health and obesity-related complications. However, more research is needed to confirm these effects in humans .
Conclusion
A 24-hour fast has clear effects on appetite, food reward, metabolism, blood pressure, and some aspects of physical performance. It increases hunger and the pleasure of eating, alters metabolic and hormonal markers, and can lower blood pressure in people with hypertension. While it does not generally impair short-term physical performance, it may reduce endurance in prolonged activities. The effects of fasting can also be influenced by the composition of the pre-fast meal, especially regarding ketosis and glycemic control. Overall, 24-hour fasting triggers a range of physiological responses that may contribute to its potential health benefits.
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