Fasted Exercise

Fasted Exercise and Fat Metabolism

A 2019 University of Bath study showed that exercising before eating breakfast burns more fat, improves insulin sensitivity, and increases glucose uptake into muscle tissue compared to exercising after eating breakfast.

• 6 weeks study of 30 obese men
• Those who exercised before breakfast burned double the amount of fat than the group who exercised after breakfast
• Increased fat use is mainly due to lower insulin levels during exercise when people have fasted overnight, which means that they can use more of the fat from their fat tissue and the fat within their muscles as a fuel
• The muscles from the group who exercised before breakfast were more responsive to insulin

In a video interview with Mike Maser of Zero Fasting (27:44), Dr Rhonda Patrick cited studies that shown that training fasted result in robust enhancements in glucose sensitivity and mitochondrial adaptation for fatty acid metabolism. When you train fed, those adaptations are blunted.

Pre-feeding only improves long-duration aerobic exercise and anaerobic exercise, but not for aerobic exercise less than 60 minutes or HIIT.

A study looking at muscle fibers before and after training in a fasted state found the combination of low insulin and high adrenalin levels created by the fasted state stimulates the breakdown of fat and the burning of fat for energy.

Other studies ​have shown that breakdown of intramyocellular lipids (fat inside the muscle) is increased by training fasted.

My thoughts:

• With an overnight fast, glycogen in the liver depleted. If one does long duration, low impact Zone 2 exercise, the body will be forced to oxidise fat for energy
• In my experience, if I do a long fasted walk (60 minutes) followed by 50 minutes of Crucycle (Zone 4/5 spinning), I will tend to see a drop in weight the next day.
• It takes about two weeks to become fat adapted – so any loss in performance is short term.
• However, if doing longer duration high intensity or anaerobic exercise, there may be a risk that your body starts breaking down protein to make glucose that the body requires.
• You may hear some people clam that studies show that one will not lose any more weight doing fasted exercise vs fed exercise. However, the way I interpret this is that fasted exercise does improve your metabolic flexibility and your ability to metabolise fat. During the duration of the study however, the participants doing fasted exercise may not show any more weight loss than the participants doing fed exercise but in the longer run (beyond the term of the study), their metabolic ability to burn fat will be improved if they continue doing fasted exercise, which means they may lose more fat over the long-run.

Other Resources

Fasting and Exercise – IDM Programme