Fasting has become an increasingly popular way to lose weight, improve health, and simplify eating habits. Intermittent fasting (IF) involves alternating between periods of fasting and eating. The most common approach is time-restricted fasting, where eating is restricted to set hours each day. Many people pair IF with exercise to maximize weight loss and health benefits. But is it safe and beneficial to exercise while fasting?
Can you exercise while fasting?
Yes, you can exercise during fasting periods. In fact, research suggests that exercising while fasted may provide unique benefits compared to exercising in a fed state.
During fasting, your body is in a catabolic state, meaning it’s breaking down stored energy sources for fuel. Your body becomes increasingly reliant on fat stores for energy as fasting progresses. Exercising while fasted forces your body to derive more energy from fat, rather than recently consumed carbohydrates or food still in your system (1).
This can boost fat burning and make the exercise more beneficial for weight loss. One study found that fasting before aerobic exercise resulted in higher fat utilization and greater loss of body fat (2).
The benefits of exercising while fasting
Here are some of the top evidence-based benefits of exercising in a fasted state:
Increases fat burning
As mentioned, exercising while fasted prompts your body to burn more fat for fuel.
One study had participants exercise either before or after eating. Those who exercised before eating burned significantly more fat, while those who exercised after eating burned more carbs (3).
Research also shows that exercising in a fasted state lowers insulin levels more than exercising after eating. Lower insulin allows for greater release of fat from your fat cells (4).
Enhances metabolic health
Exercising while fasted also appears to benefit your metabolic health and blood sugar control.
One study found that fasting before aerobic exercise reduced blood sugar more than exercising after eating. Exercising fasted also resulted in lower insulin levels (5).
Other research shows that exercising fasted increases adiponectin levels. Adiponectin is a protein involved in blood sugar regulation. Higher levels may reduce risk factors for diabetes (6).
Helps retain muscle mass
Exercising while fasting may help you retain more muscle during weight loss.
Higher levels of adiponectin while exercising fasted correlate with increased markers of muscle retention. This indicates that fasting exercise may prevent muscle breakdown (7).
May aid workout recovery
Research on Muslim athletes during Ramadan suggests that exercising while fasted doesn’t negatively impact performance or workout recovery (8).
One study found that fasting before aerobic exercise aided workout recovery just as well as carbohydrate supplementation. Fasted training may support quicker recovery between workouts (9).
However, the effects on recovery after resistance training while fasted are less researched. More studies are needed on recovery from weight training during fasting.
Improves exercise tolerance
Interestingly, some studies indicate that doing aerobic training in a fasted state can increase your endurance.
One study had cyclists perform two hours of cycling before or after eating. The researchers found that the fasted training increased fat utilization and allowed the cyclists to ride significantly longer before exhaustion (10).
Another study found similar improvements in performance after four weeks of fasted aerobic training (11).
The reasons for this are still unclear. Exercising in a fasted state may improve metabolic adaptations and make workouts feel less difficult.
Is exercising while fasting safe?
For most people, exercising while fasting is safe and well tolerated. However, there are some precautions to keep in mind:
– Stay hydrated. Dehydration is a risk with any type of exercise. Be sure to drink plenty of non-caloric fluids, especially around your workout.
– Mind your electrolytes. Prolonged fasting can result in low electrolyte levels. Consider adding a pinch of salt to your water or taking an electrolyte supplement.
– Avoid overtraining. Exercise recovery may be impaired with very frequent or long fasting periods. Allow proper rest between workouts.
– Beware of feeling lightheaded or weak during exercise. This may indicate low blood sugar. Have a small carb-based snack beforehand if needed.
– Break your fast soon after exercising. Refuel with a balanced meal rich in carbs and protein to optimize recovery.
– Start slow if new to fasted training. Allow your body time to adapt by gradually building up duration and intensity.
Pay attention to your body’s signals when exercising fasted. Overall, research indicates fasted workouts are safe for most people, but individuals with certain medical conditions may need to exercise with caution.
Tips for exercising while fasting
Here are some tips to get optimal results from your fasted workouts:
– Do low- to moderate-intensity cardio. This could include brisk walking, jogging, cycling, swimming, etc. High-intensity interval training (HIIT) can be done fasted but may require pre-workout carbs if you feel weak.
– Lift weights/resistance train before cardio. Weight training requires more energy and muscle glycogen. Do this first in your fasted state before moving on to cardio.
– Exercise first thing in the morning, or 3-4 hours after your last meal. This ensures you’re exercising in a fully fasted state after your body has used up food energy sources.
– Drink coffee or another caffeine source beforehand. Caffeine boosts energy levels and fat burning. Black coffee is fine to have while fasting.
– Take 5-10 grams of BCAAs before exercise. These branched-chain amino acids may help preserve muscle mass when training fasted.
– Eat 0.3-0.5 grams of carbs per pound (0.7-1.1 g/kg) post-workout. The fast-digesting carbs restock muscle glycogen after training.
Should you exercise while fasting? The bottom line
Here’s a quick summary of the benefits and drawbacks:
Potential benefits:
– Burns more fat for fuel
– Improves metabolic health and blood sugar control
– May help retain muscle during weight loss
– Can extend endurance while exercising
– Safe for most people
Potential drawbacks:
– Increased risk of dehydration, hypoglycemia, and electrolyte imbalance
– Possibly impaired recovery from hard training sessions
– Requires habit adjustment if new to fasted exercise
In general, exercising while fasting can be safe, well tolerated, and provide unique fat loss and endurance benefits. But it requires smart preparation and monitoring of your body.
It’s best to start with low- to moderate-intensity exercise in a fully fasted state. Pay attention to hydration, fueling, recovery, and how you feel. Allow your body time to adapt to exercising in the fasted state.
With patience and prudence, exercising while fasting can be an effective way to boost fat burning, endurance, and overall health. But it may not suit everyone’s needs or preferences. As with any dietary change, it’s smart to consult your healthcare provider before beginning an exercise routine while fasting.
Frequently Asked Questions
Here are answers to some common questions about exercising while fasting:
Should I take BCAAs before a fasted workout?
BCAA supplements are unnecessary for most people doing fasted exercise. But taking 5-10 grams of BCAAs pre-workout may help minimize muscle protein breakdown during prolonged fasting or chronic training.
What’s the best time of day to exercise while fasting?
Morning workouts before eating are best for exercising in a full fasted state. For intermittent fasting, working out 3-4 hours after your last meal ensures your body has used up food energy and shifts into fat-burning mode.
Can I do HIIT training while fasting?
Yes, you can perform high-intensity interval training (HIIT) while fasted. However, some people report feeling weak or fatigued when doing intense exercise with no pre-workout meal. Taking caffeine or a small carbohydrate snack beforehand can help boost energy.
Should I break my fast immediately after exercising?
It’s generally recommended to break your fast within one hour after exercising. Consuming protein and carbohydrates soon after a workout helps supply amino acids for muscle repair and restores glycogen to energize your next training session.
Can I build muscle effectively while training fasted?
Some research indicates you can build muscle during intermittent fasting if your overall protein intake and training volume are sufficient. However, trying to build muscle while training fasted on a severe calorie deficit may impair your results.
The bottom line
Exercising while fasting can be safe and beneficial for many people. Research suggests it may boost fat burning, improve metabolic health, retain muscle mass, and increase endurance.
However, there are potential downsides like increased dehydration and impaired workout recovery. Start slowly, fuel properly, and listen to your body when trying fasted workouts.
For most, the pros seem to outweigh the cons. With smart preparation and awareness, exercising while fasting can be an effective way to augment your fitness and fat loss goals.