Should You Exercise While Fasting? Pros and Cons

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There have been heated debates about exercising while fasting. Advocates claim it's the secret to accelerated fat loss and an optimal metabolic state. Critics say it impairs performance and muscle growth. So what’s the truth?

The answer is nuanced. Fasted exercise can offer benefits for certain goals and certain people — but it also has some trade-offs.

The Science Behind Fasted Exercise

When you haven't eaten for several hours, your body enters a fasted state: your insulin levels drop, growth hormone rises, and your body increasingly relies on stored energy (primarily fat and glycogen) rather than nutrients from recently consumed food.

Find out here what happens to your body when you fast, hour by hour.

Your muscles burn proportionally more fat and less carbohydrate compared to exercising after eating. This shift toward fat oxidation continues not just during exercise but for hours afterwards as well [1].

Regular fasted training can enhance “insulin sensitivity” (your cells' responsiveness to insulin) and improve your body’s ability to use fuel (both carbohydrate and fat) for energy.

Does Fasted Exercise Burn More Fat?

Yes, exercising while fasted increases “lipolysis” (breakdown of fat stores) and “fat oxidation” (burning fat) both during and after your workout. So, combining fasting with exercise can help with weight loss and fat loss.

However, just increasing fat burning during exercise doesn't necessarily translate to dramatically greater fat loss over time. If you’re specifically trying to reduce body fat, especially when combined with an overall calorie-controlled diet, fasted exercise can be a useful tool.

The Health Benefits of Fasted Exercise

Beyond fat loss, fasted exercise potentially offers metabolic health improvements that make it attractive even for people who’re not primarily focused on weight loss.

Research in people with non-alcoholic fatty liver disease found that combining alternate-day fasting with aerobic exercise significantly improved liver fat, body weight, and insulin sensitivity [2].

The improvements in insulin sensitivity (meaning your body can handle your blood sugar levels better) will also reduce your risk of metabolic disease (e.g., obesity, metabolic syndrome, dyslipidaemia, fatty liver disease, diabetes), probably beyond what you'd achieve from fed-state exercise or fasting alone.

How Fasted Exercise Affects Performance

So while fasted exercise offers metabolic health and body composition benefits, it may come with a performance trade-off.

Research shows that eating prior to exercise improves exercise performance for sessions that are longer than an hour [3]. For shorter, moderate-intensity aerobic exercise sessions, the performance differences are not usually noticeable.

Fasting high-intensity exercise may be problematic, as your body relies on carbohydrates for this kind of exercise.

So, starting sessions with depleted carbohydrate stores may impair your performance in these types of exercise: examples include high-intensity intervals, tempo training, and hard weightlifting sessions.

However, some studies show that fasted low to moderate intensity training can improve how efficiently you use fuel (and thus potentially exercise performance) [4]. But this still means that pre-exercise feeding will produce better results for competitions and peak performance. It’s a training tool to improve performance — not a performance-enhancing in itself.

What Exercises Work Best While Fasted?

Not all exercise is created equal when it comes to fasted training. The research suggests clear patterns about which activities work well and which don't.

What can work ✅	What to avoid ❌
Steady-state aerobic exercise low to moderate intensity (e.g., walking, light jogging, cycling, swimming)	High-intensity interval training or high volume, high intensity strength or power training
Moderate-intensity resistance training or aerobic exercise	Exercise training requiring peak cognitive function or coordination
Yoga, tai chi, and other mind-body practices	Competition or performance-focused training sessions
Short duration workouts (under 60 minutes)	Long-duration endurance sessions (over 90 minutes)

Match your exercise intensity to your metabolic (fasted or fed) state. Moderate intensity — where you're working but can still hold a conversation — tends to be the sweet spot.

At this intensity, your body can effectively use fat for fuel without requiring large amounts of quick-burning carbohydrates.

Practical Tips for Working Out While Fasted

If you decide to try fasted exercise, these strategies can help you do it safely and effectively:

Start gradually

Don't jump immediately into fasted high-intensity workouts. Begin with shorter, lower-intensity sessions and give your body time to adapt to using fat as fuel. Some people need 2 to 4 weeks to adjust “metabolically” and feel comfortable exercising fasted.

Stay hydrated

Fasting doesn't mean avoiding water. Proper hydration is critical, especially since you're not getting fluid from food. Drink water before, during, and after your fasted workout.

Time it strategically

Many people find morning workouts before breakfast work best for fasted exercise, as you're naturally fasted overnight. Going through a whole day fasted, then working out, may be too much effort for some.

Keep intensity moderate

If you're breathing so hard you can't talk, your workout is probably going to be too intense for optimising the benefits of fasted training.

Listen to your body (“biofeedback”)

Dizziness, extreme fatigue, shakiness, confusion, or feeling faint are warning signs to stop and eat/drink something. These symptoms may indicate “hypoglycaemia” (low blood sugar), which can be dangerous if ignored.

Eat after your workout

While exercising fasted can offer benefits, proper post-workout nutrition supports recovery and muscle maintenance. Plan to eat within a few hours after finishing your fasted workout.

Consider electrolytes

If you're doing longer fasted sessions (over an hour), adding electrolytes to your water can help maintain performance and prevent cramping, especially if you're also restricting salt intake while fasting.

Who Should Avoid Exercise While Fasting

Fasted exercise isn't appropriate for everyone. Certain groups should avoid it entirely or only attempt it under close medical supervision:

People with diabetes or blood sugar issues

Exercise powerfully affects blood sugar levels, and combining it with fasting (which also affects your blood sugar levels) increases the risk of hypoglycaemia. If you have diabetes, other metabolic conditions, or are taking medication that affects your blood sugar levels, you’ll need oversight from a healthcare professional.

Pregnant or breastfeeding women

The energy and nutrient demands of pregnancy and lactation make fasted exercise generally inadvisable. Exercise during pregnancy has health benefits, but adequate nutrition is essential.

People with a history of eating disorders

Fasted exercise can become intertwined with disordered eating patterns. If you have current or past eating disorders, discuss it with healthcare professionals.

Athletes training for performance

If your primary goal is maximising strength, power, or endurance performance, pre-exercise fuelling usually produces better results than fasted exercise.

Beginners to Fasting or Exercise

If you're just starting an exercise programme, master the basics of consistent training with proper fuelling before adding the complexity of fasted workouts.

Frequently Asked Questions

Is fasted exercise safe?

For healthy adults doing moderate-intensity exercise, yes. However, it's not appropriate for a number of people or situations (see above).

What's the best time to exercise while fasting?

There is no “best time”, but some people find morning workouts after an overnight fast work well. This provides a natural 8-12 hour fasted period and allows you to eat soon after exercising as well.

Can you build muscle while exercising fasted?

Fasting in and around resistance training can probably build muscle, but it won’t be optimal for muscle-building purposes.

You won’t lose muscle if you train fasted (in fact, it may reduce your chances of losing muscle), but you’ll be able to train and grow muscle more efficiently in a fed state.

Does fasted exercise increase autophagy?

While fasting itself may promote “autophagy” (the cell’s clean-up processes), there’s little convincing evidence that exercise boosts autophagy any further while in a fasted state.

Does fasted exercise spike your cortisol?

Fasted exercise can produce a modest rise in cortisol (especially at moderate-to-high intensities relative to a fed state), but in healthy people it typically normalises quickly and isn’t linked to any negative side effects [5].

What is the best food to break your fast with?

Evidence supports a mixed meal containing 20–40 g of high-quality protein plus carbohydrates to accelerate muscle growth and repair, and restore carbohydrate stores and energy levels.

The Bottom Line

Fasted exercise isn't inherently good or bad — it's best viewed as a tool with specific applications; it has some benefits, and some limitations. The research shows it can support fat loss, improve metabolic health, and some health and performance adaptations.

However, it has also been shown to impair high-intensity performance and increase adverse effects while exercising in some people.

For people focused primarily on performance, strength/power output, or muscle growth, feeding in and around exercise typically produces better outcomes.

The decision about whether to do fasted workouts (or not) comes down to individual goals and priorities.

Sources

1. Vieira AF et al. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. Br J Nutr. 2016 Oct;116(7):1153-1164. PMID: 27609363.

2. Ezpeleta M et al. Effect of alternate day fasting combined with aerobic exercise on non-alcoholic fatty liver disease: A randomized controlled trial. Cell Metab. 2023 Jan 3;35(1):56-70.e3. PMID: 36549296

3. Aird TP et al Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018 May;28(5):1476-1493. PMID: 29315892

4. Van Proeyen K et al. Beneficial metabolic adaptations due to endurance exercise training in the fasted state. J Appl Physiol (1985). 2011 Jan;110(1):236-45. PMID: 21051570

5. Kim TW et al. Comparison of the effects of acute exercise after overnight fasting and breakfast on energy substrate and hormone levels in obese men. J Phys Ther Sci. 2015 Jun;27(6):1929-32. PMID: 26180350