[Lio]

As I understand it, based on books such as Faster[1] by Michael Hutchinson, it depends on the intensity of what you do.

At low intensity exercise you can just burn just fat.

The body can only get so much energy from fat in a given time, so at higher intensity levels it will try to get energy from both fat and stored carbohydrates.

Once you run out of easily available carbs it will try to supplement energy from the next available store, which is protein. i.e. muscle.

So this will happen well before you run out of body fat.

You can apparently increase the amount of base energy you can get from fat by training cell mitochondria with lots of low internet exercise.

Eg lots and lots of zone 1 or 2 on the Coggan zone system.

1. http://www.michaelhutchinson.co.uk/faster.html

[TimSchumann]

This is actually my understanding of it as well, though I kind of arrived at the conclusion on my own pulling from various sources and experience.

People think about fat as if they have access to it all, but it's a surface area per unit time problem. Like all the chemistry in life, on some level, it's diffusion limited.

What you should be thinking about while fasting is not letting your basal rate/time + exercise expenditure/time exceed your total energy available/time. I got pretty good at estimating this, but I wish there were an easy way to measure it.

Everything is stream processing, not discrete units.

[beagle3]

The missing part in your description, to the best of my understanding, is fat to ketone conversion - you don’t use the fat directly, the process takes time, so ketones have to already be circulating when you need them.

It can also explain why ketogenic diets work for many people - if you exercise every few days, your body will maintain sufficient blood ketone level for your exercise days, but unlike glucose, ketones that are not used get peed out eventually (and not stored/converted back to fat).

Couldn’t find anyone knowledgeable enough to confirm I am right, but also no one to tell me I’m obviously wrong - still looking for one of those answers.

[TimSchumann]

I think, to the best of my understanding, most of what you've said is correct. This is also why I always use blood ketone meters rather than urine strips.

Apologies for not including that, I guess I just assumed it was common knowledge? Mea culpa.