What are the effects of physical exercise and fasting on the body?

From the Guidelines

Exercising while fasting can be beneficial for improving insulin sensitivity and glucose uptake, but it requires careful planning to avoid hypoglycemia and dehydration, as evidenced by studies such as 1 and 1. When considering the effects of physical exercise and fasting on the body, it's essential to prioritize morbidity, mortality, and quality of life as outcomes. The available evidence suggests that a combination of aerobic and resistance training may be more effective for blood glucose management than either type of exercise alone, as stated in 1.

Key Considerations

• Intensity and duration: Perform low to moderate intensity workouts during fasting periods, such as walking, light jogging, or gentle yoga, and keep sessions under 60 minutes to avoid excessive stress on the fasting body, as recommended in the example answer.
• Timing: Morning workouts before breaking the fast can enhance fat burning, but listen to the body's signals, and consider modifying the exercise program to avoid hypoglycemic episodes, as suggested in 1.
• Hydration: Stay hydrated by drinking plenty of water before, during, and after exercise, aiming for at least 16-20 ounces per hour of activity, and consider adding electrolytes (sodium, potassium, magnesium) if fasting for extended periods.
• Post-workout nutrition: Break the fast with a balanced meal containing protein (20-30g) and complex carbohydrates within 1-2 hours after intense exercise to maintain muscle and support recovery, as advised in the example answer.

Benefits of Physical Activity

• Regular physical activity improves cardiovascular risk factors and lowers the risk of developing other chronic diseases, as stated in 1.
• Physical activity may reduce the risk of several types of cancer, including cancers of the breast, colon, prostate, and endometrium, as reported in 1.
• A physically active lifestyle provides other important health benefits, including associations with reduced risk of other chronic diseases, such as heart disease, diabetes, osteoporosis, and hypertension.

By carefully planning and executing a physical exercise and fasting regimen, individuals can potentially improve their insulin sensitivity, glucose uptake, and overall health outcomes, while minimizing the risks associated with hypoglycemia and dehydration, as supported by the evidence from studies such as 1, 1, and 1.

From the Research

Effects of Physical Exercise and Fasting on the Body

• Combining intermittent fasting with exercise leads to decreased fat mass regardless of weight status 2
• Evidence is equivocal for the influence on other aspects of weight loss and body composition, fat-free mass, and cardiometabolic risk factors and may be dependent on weight status or exercise doses 2
• Fasting and exercise may induce synergistic effects and further improve cancer-related outcomes, however, its safety and efficacy in the oncology setting remain unknown 3
• Skeletal muscle plays a key role in the control of blood glucose levels, and the metabolic changes and related signaling pathways in skeletal muscle induced by fasting overlap with those induced by exercise 4
• Combining fasting/food withdrawal with exercise causes changes unlike those seen during fasting/food withdrawal or exercise alone, which favor repair of muscle over autophagy 4

Metabolic Effects

• Exercising during a fasting state increases lipolysis in adipose tissue while also stimulating peripheral fat oxidation, resulting in increased fat utilization and weight loss 5
• Fasting decreases body weight, lean body and fat content in both trained and untrained individuals 5
• Fasting has a broader impact on metabolism, with effects on protein and glucose metabolism in sedentary and untrained subjects 5

Body Composition

• Short-term, combined fasting and exercise improves body composition in healthy males, with substantial loss of total body mass and fat mass 6
• Lean mass as a percentage of total body mass was not reduced, and android/gynoid ratios were decreased 6
• A loss of android fat over total fat mass and gynoid over total fat mass was observed 6