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Fasted exercise has no clear, single definition, but most people accept that it means exercising first thing in the morning, without having had breakfast beforehand.

However, it could just as easily mean exercising in the evening, having skipped lunch and any afternoon stacks. Six to eight hours without food should produce a fasted state – no matter what time of day it is. With this in mind, you can do early morning or evening fasted exercise as you prefer.


Enhanced fat burning – the main reason that most people exercise in a fasted state is that they want to burn more fat. When you are in a fasted state, blood glucose levels and muscle/liver glycogen (stored carbs) tend to be lower, and your body is more likely to use body fat for fuel. This is especially true during aerobic exercise, where fat is the main source of energy anyway.

Better post-exercise glucose tolerance – after exercise, your body is much more sensitive to the action of insulin. Insulin is a hormone that helps ferry glucose into your liver and muscle cells. This can help regulate your blood glucose and enhances recovery from exercise.

Fasted exercise increases insulin sensitivity even more than exercise alone, which means more post-exercise carbs will be directed to your liver and muscles, and away from your blood and fat stores.

Decreased digestive upset – if your stomach has food in it, blood is diverted toward it to aid the digestive process. When you exercise, blood is preferentially diverted away from your stomach and toward your working muscles. If you have food in your stomach as you start to exercise, this can produce nausea. With fasted exercise, there will be little or no food in your stomach and so this ceases to be a problem.

Increased fat burning from “problem” areas – fasted exercise may preferentially burn fat from the abdomen, hips, and thighs. It appears that fasted exercise can increase blood flow to these areas, which helps increase fat loss. While this effect is very small, every little helps when you are trying to lose weight or burn fat.


Reduced workout performance – being in a fasted state will not have much of an effect on low-intensity workouts like cardio, as they tend to favor fat for fuel. In contrast, high-intensity exercise like intervals, sprinting, jumping, and lifting weights are anaerobic in nature, which means they predominately use carbs for fuel. Being fasted means fewer carbs are available for your muscles, and workout performance may suffer as a result. That’s bad news if you are training for increased muscle mass or improved performance.

Increased muscle catabolism – fasted exercise can result in muscle breakdown. Your body may view being in a fasted state and exercising as a serious threat that produces a lot of stress. This stress increases cortisol production, your primary stress hormone. Cortisol increases muscle catabolism or muscle breakdown.

No greater fat-burning – some research suggests that fasted cardio does not increase fat burning at all. The combination of increased cortisol, muscle catabolism, and reduced exercise intensity and/or duration may combine to negate any fat-burning benefit to fasted cardio. At best, you may end up burning the same amount of fat fasted and unfasted. However, other research still suggests fasted exercise enhances fat burning. There may be a genetic element to these different findings.


If you are training for fat loss, and your workouts emphasize low-intensity cardio, exercising in a fasted state may help you burn fatter, providing it doesn’t significantly reduce the duration of your workouts.

In contrast, if you are training to build muscle or improve performance, fasted exercise may reduce workout intensity, volume, or duration, lowering the effectiveness of your training session in the process. If improved performance or muscle building is your primary goal, fasted exercise may not be right for you.


If you exercise first thing in the morning, you may have no choice but to work out in a fasted state. That’s not really a big deal if you are going to walk on a treadmill for an hour, but could present a problem if you have planned an intense workout.

  • Eat a slow-releasing, high carb meal the night before you train.

  • Avoid excess pre-exercise caffeine which can further increase cortisol levels.

  • Consuming muscle-preserving branch chain amino acids before and during your workout.

  • Drink a fast-acting sports drink before exercising.

  • Consume 1-2 servings of whey protein before your workout.

  • Consume fast-acting carbs and protein immediately after your workout.

The bottom line: fasted workouts can work for some people, but are not good for everyone. Some exercisers can operate fine on an empty stomach, and find fasted exercise has no significant impact on exercise performance. Others find that training on empty produces weakness, fatigue, nausea, and light-headedness. The best thing to do is to try fasted exercise for yourself for a month and see if it’s right for you.

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