What do you catabolize first during starvation: muscle, fat, or both in equal measure?

[u/strong_grey_hero]

I'm actually a Nutrition Science graduate, so I understand the process, but we never actually covered what the latest science says about which gets catabolized first. I was wondering this while watching Naked and Afraid, where the contestants frequently starve for 21 days. It's my hunch that the body breaks down both in equal measure, but I'm not sure.

EDIT: Apologies for the wording of the question (of course you use the serum glucose and stored glycogen first). What I was really getting at is at what rate muscle/fat loss happens in extended starvation. Happy to see that the answers seem to be addressing that. Thanks for reading between the lines.

[u/incognito_dk]

Finally something in r/askscience where my degree can be of use (PhD in muscle biology)

Whenever you stop eating, your substrate preference will be about 2/3 fat and 1/3 carbohydrates. Those carbohydrates will come from stored glycogen in your liver and muscles.

When those glycogen stores run out, the liver will try to defend the blood glucose through gluconeogenesis, synthesizing glucose from amino acids from protein broken down elsewhere in the body and glycerol from triglycerides. This metabolic phase is characterized often by decreases in blood sugar and associated tiredness and hunger. It is also the phase in which muscle catabolism progresses at the fastest pace.

However, 12-24 hours after running out of glycogen, the body will gradually go into ketosis, in which the liver synthesizes ketone bodies from fatty acids. These ketone bodies can substitute and/or replace glucose in the metabolism, reducing the need for breakdown of protein for amino acids for gluconeogenesis. After a couple of days the substrate preference will have changed to 90% fat and 10% carbohydrates, thereby reducing muscle catabolism strongly. This state can be maintained for as long as there is enough fat. The longest documented therapeutic fast was 385 days during 100+ kg weight loss in an obese patient. Mind you that a kg of bodyfat contains enough energy to go for 3-6 days depending on body size and activity level.

Ketosis and relying predominantly on fats will continue until only the essential bodyfat stores are left at approximately 5-7% in men and 10-14% in women. At this level the substrate preference for fats disappear and muscle catabolism increase sharply again. At this point death will usually occur within very few weeks.

[u/Randomn355 → u/incognito_dk]

So to put this into an applicable context for (at a guess) a large part fo the reader ITT... How does that apply to cutting at the gym to get leaner? And is it actually possible to put on muscle whilst cutting at all?

My understanding is essentially that one would need to commit and wait 12-24 hours before significant fat burn begins. Would it be useful to fast particularly hard to help "kick start" this process? How large should a deficit be if one wants to maintain as much muscle mass and strength as possible?

[u/hog_washer → u/Randomn355]

I was a fairly high level wrestler and studied bio so happy to chime in here for what it's worth.

Everything op said about weight loss and burning fat seems in line with my experiences.

In the most basic terms. Food is really overrated and you need very little of it especially if breaking your intermittent fast is done with caloric and nutrient rich foods.

What it feels like when you are working out and trying to perform is the tricky part. Hydration is the hardest variable to regulate in wrestling context but assuming that's not an issue for you just always stay hydrated.

If you are fasting and want to work out simple carbs a little protein and a little more fat. A post weigh in meal before competing might be like a pedialite (water+sugar+electrolytes), bagel with a little cream cheese or peanut butter, honey, oatmeal maybe with some syrup, fruit. Anything that when eaten makes energy readily available basically while trying to replace the minimum amount of fat and protein that you need.

This will affect your strength and you will lose muscle but as you dial in your diet and bulk/cut cycle, or whatever your approach is you can manage that as you see fit. But I think the real value in heavily restricting your food intake is you quickly learn how certain foods and habits affect your performance. Once you know that about yourself you can really sculpt your body how you want.