Let’s say it’s been at least 8 hours before you last ate. Will you burn more fat? Well, it depends. If you ate a carbohydrate-rich meal the night before and then went straight to bed, you will still have plenty of glycogen left to fuel your workout, and are going to burn very little fat. Say you didn’t do this and exercised in a glycogen-depleted state. In this case, yes, your body will oxidize more fat, but does this matter? No. There is a difference between burning fat and losing fat. Fat-burning is something that is occurring all the time, but the amount of fat utilized, as compared to other substrates, can be manipulated by things like fasting and the type of activity. However, if you are in a sustained calorie deficit, your body is eventually going to pull from your fat stores anyway, and if you are in a calorie surplus, whatever doesn’t get used will be stored as fat. So what ultimately matters for fat loss isn’t what type of fuel you are using during exercise, but your overall energy balance.
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