Should you do fasted cardio?

Fasted cardio is pretty self explanatory: be in a non-fed, fasted state when you exercise. Maybe you’re intermittent fasting for weight loss, or maybe you’re working out before you get hungry, either way is fasted cardio recommended?

Fasted cardio for fat loss

The theory is that when you work out on an empty stomach, you’ll be breaking down fat stores rather than using the energy you’ve just consumed. This is because you don’t have circulating energy, so your blood sugar and insulin levels are low; the ideal combination for optimized fat burning.

When you would then eat post-workout, hypothetically you’d burn those calories quickly to repair muscle damage rather than storing them as fat. Seems logical right? But what does the research say?

A study published in the British Journal of Nutrition in 2013 found that skipping breakfast and doing cardio led to a 20 percent increase in the amount of calories from fat burned during the session. Another similar study tested nine athletes after a fasted workout vs. fed workout to see the difference in calories burnt. After the fasted workout, the athletes burned significantly more.

To conclude on our research for this, a meta-analysis involving more than 27 randomized controlled trials, the gold standard of research, finished by stating that fasted cardio results in higher fat oxidation than being in a fed state. A clear cut answer. But what about if you want to gain muscle?

Fasted cardio for muscle gain

When you’re in a fasted state, your body does not have any circulating nutrients like glucose for energy or endurance during a workout. The benefit of this is that it pushes your body to mobilize fat stores to use as energy, but it also means your body will start to break down muscle mass for energy too, putting you in a catabolic state. If you’re trying to build muscle, this is a bad idea. You will have decreased energy and strength to hit new PRs and push through the workout, and you’ll also not have fuel to repair or retain muscle mass.

Cortisol, a hormone related to stress, is highest in the morning. When you eat, it normally starts to come down as food is metabolized. If you don’t eat it remains elevated, particularly if your adding in an intense workout. Elevated cortisol is related in research to rapid muscle breakdown.

So now you know: fasted cardio can be beneficial, but it depends on your goals. If you’re trying to lose weight, do it! If you’re trying to gain muscle, don’t.