A while ago someone asked me about training before breakfast and whether that would contribute to faster weight loss. I’m not a fan of the many diet fads that are out there, and generally just stick to the idea of balancing your energy in with your energy out. But, I found some interesting research on the effects of training while fasted. Fasted is generally defined as more than six hours since consuming food. Fat oxidation, which is a term used below, is a process where the body breaks down fat to access electrons which can be used to make ATP. ATP is the short term energy molecule that is used by your muscles to do work.
Iwayama, et al (2015) found that exercise before breakfast did increase the rate of fat oxidation over 24 hours compared to exercising with in the afternoon or the evening. All three of the study groups, and the control group, were eating an energy balanced diet, so equal energy in compared to out.
Achten and Jeukendrup (2004) also report increased fat oxidation when exercise is done after fasting for 6+ hours, as well as during moderate, compared to high, intensity work outs.
In a review of recent research Deyhle, Mermier, and Kravitz describe how trained individuals are better able to use fat molecules for energy during exercise because of cellular adaptations. I include this, because even though it doesn’t address fasting itself it may have an impact on how successful your body is at using fat for energy and therefore how good an early morning workout might be.
So what does all this mean? Your body can burn more fat if you exercise more than six hours after eating. However, this will only impact weight loss if you are consuming less energy than you are spending. And if you are in an energy deficit than exercise anytime is good. However, it may be slightly more beneficial to reducing body fat levels to exercise in a fasted state. This then leaves your meals, and particularly the carbohydrate in your meals to fuel the rest of your day.