: Cycling burning fat or calories? If I have breakfast and then cycle, say for about an hour at about 80%, will the cycling burn any fat(/muscle) tissue or just the breakfast? If not, would it be advisable to consume no or

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What you're looking for is fasted training vs. unfasted training. It is sort of a religion in the fitness world with regards to whether or not it is beneficial. The question strictly focuses on fat loss, so that's what this answer will be about.

The logic states that when fasted (10+ hours without consuming any calories), the body does not have glycogen to use for the workout so it turns to other sources, fat and protein though (hopefully) mostly fat. There is some research to back that up.

Research using healthy young men has shown that doing aerobic exercise when fasting increases the use of stored fat as an energy supply. The reaction may be related to the low insulin levels that occur during fasting.

There is also some research stating that is does not help, like this one which states:

These findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.

To answer the question:

Would it therefore be possible to reduce body fat while being in an overall daily calorie surplus?

Not likely. The reason is simple. If you burned more fat in the fasted state, then you'll have a big surplus of glycogen that you aren't using in the fed-and-resting state. The caloric surplus would then just go right back in to fat. If you work out in a fed state, you use primarily glycogen, then all your calories are front loaded to the beginning of the day. So you would have less glycogen at the end of the day so you'd use more fat. It all just balances out.

This is of course an oversimplification. There is absolutely no way of knowing exactly what percentages of fat, carbs, protein your body uses during a workout or rest.

If not, would it be advisable to consume no or very little breakfast before the cycling?

Looking at a strictly fat loss perspective, I don't think there'd be any benefit one way or the other.

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@Deev already covered most of this question, I'd just like to add this bit;

If the question is strictly about will you burn fat or your breakfast, the answer is both. Depending on how intense your workout is you will burn both fat and glycogen.

When you're in a lower heart rate, you'll burn relatively more fat than glycogen (around 60% fat and 40% glycogen). When you have a more intense workout (higher heart rate zone), burning fat isn't going to cut it so your body will start burning more glycogen instead.

This doesn't mean that you'll burn more fat while in lower zones, it just means you're burning more fat relative to glycogen in lower heart rate zones. When your workout is more intense you're going to burn even more fat, but you'll also burn a lot more glycogen.

So basically, if you want to burn some fat while minimizing the calories burned that you get from your breakfast, you'd want to train in lower heart rate zones.