[_0w8t]

Fasting is not malnutrition. A typical human even lean one has enough fat to survive without food for 30-40 days without significant loss of lean mass. This sets us quite apart of other animals including apes.

One starts to loose lean mass during prolonged calorie deficit which takes months.

[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.

[sdze]

But in clinical trials they found that even intermittent fasting leads to LBM loss.

> https://www.hawaii.edu/news/2020/10/09/intermittent-fasting-...

Considering Body Builders don’t fast, I tend to believe the study.

[vidarh]

Having done IF while lifting, the issue that stands out to me with this study is that it tested only IF with no advice on exercise or nutrition.

Yes, if you fast without exercise, you will almost certainly lose muscle.

And if you fast without nutritional changes, you likely won't lose much, because IF on its own does not equal a deficit over more than hours at a time.

I gained muscle on IF with ease, and I also lost far more than in their study, with ease.

I'm surprised, given the constraints of the study, that there was an average loss at all.

[raspberry1337]

>Considering Body Builders don’t fast, I tend to believe the study.

There's entire bodybuilding schools (such as LeanGains) that build on intermittent fasting, and its increasingly more common when it comes to cutting fat. Where did you hear this?

[lm28469]

> The study only included recommendations to the timing of food intake and did not provide recommendations for calorie and macronutrient intake or physical activity.

> 116 overweight and obese participants

Yeah I mean IF isn't magic. If you sit in front of netflix and eat three 800cal bags of chips it doesn't matter much if you're doing IF or not.

I'm doing IF for years and go to the gym every other day, most of the time fasted. I made good progress, lost fat, gained muscles, and most importantly I feel amazing.

Eating schedule is only a part of the equation. You still need to eat proper food and exercise.

[_0w8t]

The study run for 3 months and people were under calorie deficit, which is typical time scale when the lean mass starts to shrink.

In general intermittent fast is not a good way to loose fat precisely for this point. In Russia there are fasting resorts where people do not eat for 2-3 weeks under medical supervision. Most people do it not to loose weight but try to improve various medical conditions. Those who go there for weight loss typically gain the weight back since they do not change their eating habits.

As for bodybuilders they do intermittent fasting but not to loose fat but to gain muscles. For some people this works as long as they eat sufficient calories.

[virtual_void]

Body builders don’t fast therefore fasting reduces lean body mass?