Fasting and Exercise

There’s intermittent fasting, and there’s exercising. IMO, both most likely have benefits. The occasional energy-deprivation and signaling cascades (i.e. autophagy) achieved from fasting have their place in human physiology. Exercise is definitely something nobody should skip out on, for similar reasons and more.

Then there’s the combination of the two: Exercising while fasted.

I think there’s some hope that exercising while fasting will create some synergistic effect or something. Maybe some people hope that the exercise will increase the caloric deficit and be a double whammy to that pesky muffin top (hey, I have one).

I don’t know. For one, the mechanisms behind the benefits of fasting and exercise are very different — and can conflict with each other. Prolonged fasting, especially, really puts the body into a preservation state aimed at long-term survival. Exercise, on the other hand, engages that fight-or-flight response for the short-term do-or-die situation. The context matters.

De Bock et al., 2008: No changes in fat oxidation after 6 weeks of fed vs fasted endurance training. Fatty acid binding protein and UCP3 were increased in the fasted athletes, but I’m not sure this actually means much. Glycogen was also spared in the fasted athletes (performance benefit?).

Knapik et al., 1991: Exercise while fasting increases leucine oxidation. Probably not good if your goal is to maintain/increase muscle.

Although…

Exercise performance may not be impaired.

There may be some short-term effects on intramuscular fat. (Again, this may not impact much in the long term)

I don’t think the evidence is very suggestive of extra benefits of exercise during a fast (and may be counterproductive if your goal is to gain muscle). Fast some, and exercise some, but don’t expect any synergy.