From the Guidelines
For optimal results, exercise during a fasted state, ideally in the morning before breakfast or toward the end of an intermittent fasting period, as this timing strategy can enhance fat burning and improve insulin sensitivity, as supported by the most recent evidence 1. When considering the optimal timing of fasting and exercise, several factors come into play, including the type and intensity of exercise, the duration of fasting, and individual health status.
- The American Heart Association recommends waiting at least 2 hours after eating before exercising vigorously to avoid depriving organs of blood and resulting in cramps, nausea, or faintness 1.
- Low to moderate intensity exercise like walking, light jogging, or yoga works best during fasting, while high-intensity workouts may be better performed after eating.
- Drinking plenty of water before and during exercise is crucial to stay hydrated, especially when exercising in hot weather or for prolonged periods 1.
- The combination of fasting and exercise can increase production of growth hormone and improve insulin sensitivity, but it's essential to listen to your body and adjust based on your personal response, as some people may need some nutrition before exercising, particularly for intense workouts.
- Recent guidelines suggest that individuals with diabetes who fast have an increased risk for hypoglycemia, dehydration, hyperglycemia, and ketoacidosis, and healthcare professionals should provide education and support to accommodate their choice, including guidance on glucose checking, medication/fluid adjustment, timing and intensity of physical activity, and meal choices pre- and post-fast 1.
- If fasting for 16 hours, try working out in the last 2-4 hours of your fast, and be aware of your body's response to exercise and fasting, stopping and considering a small snack if you feel lightheaded or extremely fatigued.
From the Research
Optimal Timing of Fasting and Exercise
The optimal timing of fasting and exercise is a complex topic, and research suggests that the combination of intermittent fasting and exercise can have various effects on body composition, metabolic health, and physical fitness.
- The efficacy of intermittent fasting appears to be broadly equivalent to that of standard daily energy restriction, although the duration of fasting may play a crucial role in initiating advantageous physiological mechanisms 2.
- Combining intermittent fasting with exercise may lead to decreased fat mass, regardless of weight status, and may improve explosive strength, endurance, and cardiopulmonary measures such as maximal oxygen consumption 3.
- Intermittent fasting protocols, including alternate-day fasting, whole-day fasting, and time-restricted feeding, have been shown to be effective in reducing body weight, body fat, and improving blood lipids in normal-weight, overweight, and obese individuals 4.
Effects of Intermittent Fasting and Exercise on Body Composition
Research suggests that combining intermittent fasting with high-intensity interval training (HIIT) can promote increments in fat-free mass, nitrogen balance equilibrium, and improve physical fitness and strength in women with obesity 5.
- A short-term, combined fasting and exercise protocol has been shown to improve body composition in healthy males, with substantial losses in total body mass and fat mass, and a reduction in android and gynoid fat 6.
Considerations for Future Research
- Higher-powered trials are needed to determine the efficacy of combining exercise and intermittent fasting for benefits on body weight and cardiometabolic risk 3.
- Future studies should consider interventions with increased sample size, longer intervention duration, broadened inclusion criteria, objective measures of diet and exercise adherence, and diversity of sample population 3, 4.