Whether you should train before eating (i.e. in a fasted state) for fat loss is a topic often debated. Here’s a summary of the main arguments and evidence.
When you train in a fasted or ‘postabsorptive’ state, your insulin levels or low and your body is using stored energy as the primary fuel source. When you are in a fed or ‘postprandial’ state, your body is digesting the food consumed and your body uses this circulating energy for exercise. So based on this, surely you’re optimizing fat source if you’re training fasted right?
Short-term fat oxidation doesn’t equate to fat loss, being in a caloric deficit equates to fat loss. So if you’re training fasted but still eating in a calorie surplus, you will still gain weight. A study by Schoenfeld et al (2014) had one group of participants doing fasted cardio and one doing fed cardio. Both groups were in a calorie deficit of 500 calories and performed one hour of steady state cardio for one month.
Based on the common theory, the fasted group would lose more fat right? But no, both groups lost equal amounts of weight. Why? Because both groups had the same caloric deficit. Another study that also tested this asked participants to do high intensity interval training (HIIT) over six weeks either fed or fasted. Both groups lost the same amount of weight.
Both of these studies were relatively short term, perhaps a benefit to fasted cardio would be made evident after a longer period of time, but based on the current literature it would be inaccurate to hypothesize that.
However, the literature is somewhat mixed. Training fasted can enhance fat burning which can be beneficial if you are also in a caloric deficit, but if you are in a caloric surplus you will not lose weight. A 2016 systematic review and meta-analysis that included 27 studies suggested that fasted cardio induces higher fat oxidation, but remember: higher fat oxidation does not equate to more fat loss at the end of the day.
If you’re trying to lose fat, focus on being in a caloric deficit. If you can also train fasted then great, but don’t attempt to lose weight by training fasted without altering your energy intake.
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