Prolonged fasting significantly changes nutrient oxidation and glucose tolerance after a normal mixed meal

Area of Science:

• Metabolic Physiology
• Nutritional Science
• Endocrinology

Background:

• Nutrient partitioning describes how the body utilizes carbohydrates and fats for energy.
• Individual responses to fasting and refeeding can vary, impacting metabolic health.
• Understanding these responses is crucial for managing metabolic disorders and optimizing nutrition.

Purpose of the Study:

• To investigate the effects of prolonged fasting (72-h) versus overnight fasting (13-h) on nutrient partitioning in healthy men.
• To examine the impact of fasting duration on postprandial substrate oxidation and hormonal responses.
• To establish an experimental paradigm for studying individual differences in metabolic adaptation to fasting and refeeding.

Main Methods:

• Eight nonobese men underwent two feeding trials: one after a 13-h fast and another after a 72-h fast.
• Subjects resided in a whole-room indirect calorimeter to measure energy expenditure and substrate oxidation.
• Blood samples were collected periodically to assess glucose, insulin, and free fatty acid levels post-meal.

Main Results:

• Postprandial carbohydrate oxidation significantly decreased, while fat oxidation significantly increased after the 72-h fast compared to the 13-h fast.
• Carbohydrate balance shifted from negative to positive, and fat balance remained negative but less so after prolonged fasting.
• A 72-h fast led to significantly greater glucose and insulin responses to a mixed meal, indicating reduced glucose tolerance.

Conclusions:

• Prolonged fasting (72-h) significantly alters nutrient partitioning, favoring fat oxidation over carbohydrate oxidation in healthy men.
• Extended fasting impairs glucose tolerance and alters hormonal responses to a mixed meal.
• Elevated free fatty acids during prolonged fasting may play a role in mediating these postprandial metabolic changes.