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Comment by debacle | Hacker News Reader

Not a biologist, but I would wonder why only this muscle would be capable of this. Metabolizing fats is a complex process.

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All muscles are capable of metabolizing fat. In cycling (and other endurance sports), one of the adaptations observed in top athletes is that their muscles become highly efficient at metabolizing fat during medium-intensity exercise. A professional endurance athlete will metabolize about 70% fat, 30% carbs for the majority of a multi-hour event. This preserves their muscle glycogen for the high-intensity bits where they need to push 400+ watts for 20-30 minutes up a final climb, or do a 1200w sprint to the finish line. When the intensity level exceeds a threshold, the muscle will begin switching to nearly 100% glycogen. Once that glycogen is depleted, muscles lose their top-end peak power output.

Sedentary overweight people tend to become very inefficient at metabolizing fat. At anything higher than a slow walking pace, for example, they will begin the cutover to glycogen and turn down fat metabolism.

I believe the press release is saying that the soleus muscle is unique in that it does not have a readily accessible store of glycogen. So even in sedentary people that are normally extremely inefficient at metabolizing fat, exercising the soleus will force their body to metabolize blood glucose and fat. Normally it takes months or even years of slow and steady exercise to make a sedentary overweight person effectively metabolize fat while exercising at an intensity high enough to trigger serious metabolic improvements. So if true, the soleus muscle would be a magic shortcut to this process.

In this case, is the body releasing actual fat into the bloodstream for use by the muscles, rather than the fat stores burning fat directly for ATP?

Yes, fatty acids are bound up in TriAcylGlycerol (TAG). Exercise triggers the breakdown of TAG in fat reserves, sending fatty acids into the blood. These fatty acids go through a pretty complicated process to be delivered into a muscle cell, and then into the muscle cell mitochondria. This transport process cannot keep up with energy expenditure during intense exercise, thus the cutover to stored muscle glycogen (and at even higher peak loads under about 10 seconds, creatine phosphate).

Sedentary people lose the ability to rapidly deliver fat into muscle cell mitochondria.

This is a good summary of the current state of the research.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5766985/

What is meant by intense exercise? For example, vigorous cardio (age dependent but about hr 150 bpm) is one familiar benchmark, but is this considered intense exercise?

Is there a place I can read about exercise metabolism like this?

"Sedentary overweight people tend to become very inefficient at metabolizing fat. At anything higher than a slow walking pace, for example, they will begin the cutover to glycogen and turn down fat metabolism."

Hm...very interesting. So is there an ideal protocol for fat mobilization in sedentary people? Asking for a friend.

From what I read, lot and lot of zone 2 training.

Ahh, thanks. Now off to research. For my friend.

Yes, muscles can use fat but only when forced to do so bc the primary fuel source is no longer available. This article is showing that even while the muscles and liver have glycogen and glucose is available, the soleus chooses to use fat. This is huge if it’s true!

Also not a biologist - just an enthusiastic layman.

The Soleus muscle (Soleal pump) is partially responsible for helping to return blood from your legs back up to your heart while upright. [1] This is a fairly critical process so it would make sense that it would be able to metabolize multiple energy sources.

[1] https://www.physio-pedia.com/Soleus

Interesting. And the logical second conclusion is that this is an evolutionarily costly process, otherwise it would be common across our muscles. Maybe the muscle has a higher risk of injury, degeneration, cancer than othe muscles...?

If anything, the above article says that the soleus can’t utilize multiple energy sources, it relies mostly on the blood stream (which makes sense for better endurance at running/walking).

Also, pumping the blood back is a purely mechanical process, the same is true for your arm muscles, I don’t think it has a relevance here — its a very important process that circulates lymph and helps circulate blood.

Might have something to do with the "need for speed". Running further than your glycogen alone can take you - increases your survival odds.

It has more to do with the ratio of “red” vs “white” muscle fibers. Some muscles trade off some power for endurance and vice versa.

If someone ever tried to train for their biceps they will know how even at the end of the training, the first few repetitions of a set will be “easy”, while afterwards it feels like you can’t move it anymore. It’s because biceps typically operates on its local energy storage, and it is not good at endurance.

If you do aerobic exercise, you metabolize fat.

But in a muscle? That seems to be the argument here, unless it's just bad journalism.

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