Fasting

Should You Exercise While Fasting?

Exercising while fasted can improve fat loss and metabolic health, but it may impair performance.

Fasted exercise burns more fat both during and after training and can improve metabolic health beyond what fed-state exercise achieves alone.
Pre-exercise feeding improves performance for sessions longer than 60 minutes. But moderate-intensity sessions are not affected.
High-intensity intervals, competition training, and sessions over 90 minutes are better performed in a fed state.

'Fasted exercise' is a practice that has gained popularity with people who follow intermittent fasting or time-restricted eating patterns regularly. This article explains the science behind training while fasting, examining the potential benefits for fat loss, health, and balancing the performance trade-offs and safety concerns.

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 not straightforward. Fasted exercise can offer benefits for certain goals and certain people — but it also has some trade-offs.

What happens in your body when you exercise fasted?

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.

Your muscles burn proportionally more fat and less carbohydrate compared to exercising after eating. This shift toward burning more fat continues, not just during exercise, but for hours afterwards as well. Regular fasted training can enhance 'insulin sensitivity' (your cells' responsiveness to insulin) and improve your body’s ability to utilise fuel (both carbohydrates and fats) for energy.^[1]

Does fasted exercise burn more fat?

Yes, exercising while fasted increases 'lipolysis' (breakdown of your 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 the amount of fat you burn 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.

What are the health benefits of fasted exercise beyond fat loss?

Beyond fat loss, fasted exercise potentially offers health benefits that make it attractive even for people who don't want to lose weight. 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 improvement in insulin sensitivity (meaning your body can handle your blood sugar levels better) will also reduce your risk of metabolic diseases (like obesity, fatty liver disease and diabetes), probably beyond what you'd achieve from fed-state exercise or fasting alone.

Does fasted exercise hurt performance?

So while fasted exercise offers metabolic health and body composition benefits, it may come with a performance trade-off. Research shows that eating before exercise improves exercise performance for sessions that are longer than an hour. For shorter, moderate-intensity aerobic exercise sessions, the performance differences are not usually noticeable.^[3]

Fasted high-intensity exercise may be problematic, as your body relies on carbohydrates for this sort 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 energy (and thus potentially your exercise performance). But this still means that eating a few hours before exercise will produce better results for competitions and peak performance. It’s a training tool to improve performance — not a performance-enhancing tool in itself.^[4]

Which exercises work best in a fasted state?

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 Works

Steady-state aerobic exercise, low to moderate intensity (e.g., walking, light jogging, cycling, swimming).
Moderate-intensity resistance training or aerobic exercise.
Yoga, tai chi, and other mind-body practices.
Short-duration workouts (under 60 minutes).

What Doesn't Work

High-intensity interval training or high-volume, high-intensity strength or power training.
Exercise training requires peak cognitive function or coordination.
Competition or performance-focused training sessions.
Long-duration endurance sessions (over 90 minutes).

Match your exercise intensity to your 'metabolic state' (fasted or fed). 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.