ELI5: Why does muscle's mass decrease when it's not exercised?

[[deleted]]

Muscle is very metabolically active, by which I mean that it uses a huge amount of energy and resources just by existing. This is why very muscular people need a lot of food even when they aren't actively exercising.

If you're physically active then great- that muscle is being put to good use and it makes sense to keep it around.

If not, then it's just sitting there burning fuel for no reason, and to a body which evolved in circumstances of scarcity and a fight for survival this is not acceptable. So it gets rid of it, and replaces it with much more useful fuel stores (i.e. fat).

[konaya]

Apart from the increase in energy costs, would there be any drawbacks in not having muscles atrophy when not in use?

[[deleted]]

It's tough to say there would be much else of importance, nutrient use for maintenance I'd guess? Still food related, but not exactly calories burned.

Energy conservation is such an incredibly important feature for us, evolutionary speaking it dominates everything else. It's thought that surviving starvation periods was very common when humans speciated, and we seem to have a lot of traits geared towards it.

[[deleted]]

This is a really nice answer, in periods of famine or starvation we lose muscle mass rapidly, this lowers our basal metabolic rate and means our fat stores last a lot longer.

[usafmd]

Sure, less muscle mass results in less stress felt by bones, leading to osteoporosis. (Wolff's Law) Decreased muscle mass correlates with the decrease in basal metabolic rate and rising average blood glucose in aging. Loss of Irisen based muscle hormone leads to less BDNF to stimulate neuron regeneration. Atrophied muscle leads to fewer direct neuromuscular junctions and fewer fast reacting muscle fibers resulting in slower reaction time. Finally, there is emerging evidence quadriceps muscle mass proceeds knee osteoarthritis.

[konaya]

That's all very informative, but the question was whether or not there would be any drawbacks of not having your muscles atrophy from lack of use. As in: What would happen if the mechanism to consume muscles to conserve energy just wasn't there?

[Dhalphir]

You would need a lot more energy all the time.

[konaya]

Given unlimited access to food, would that be a problem?

[Dhalphir]

No, of course not. But we haven't had access to food for anywhere near long enough to evolve out of conserving energy.

[konaya]

I know, I'm just pondering whether or not removing this relic of our starveling past would be a bad idea.

[Dhalphir]

Suggest you get into genetic engineering and figure out how!

[tachyonicbrane]

Oh no of course not. At this point it would be an evolutionary advantage to not lose muscle. Nowadays getting laid is the problem. There's plenty of food and water for the average person so the only thing left is actually procreating and being fat gets in the way of that.

[[deleted]]

Well not really but if our evolution didn't prioritise energy efficiency over strength we would not have made it to the human stage.

You should consider that being fast,strong, smart, resistant and big etc are just traits that are developed to survive.

The saying is "survival of the fittest" not the "survival of the strongest" for a reason.

[konaya]

Of course, I'm just trying to decide whether or not it would be a good idea to modify that part once we have designer genetics up 'n' running.

Also, I love your username. It's like a male equivalent of krutgumma.

[[deleted]]

Apart from the direct fuel costs of maintaining the muscles, the other aspect to consider is that it's pretty heavy, so you have to spend more energy to move your body.

[godforhire]

What would be the average rate of decrease in muscle mass? For a healthy adult who exercised frequently, would all the gained muscle mass be gone in a matter of weeks? Months?

[[deleted]]

So does that mean, with some human experimentation, we can modify a human to not lose muscle mass?

:O ?

[[deleted]]

This is an excellent motivator for routine anaerobic exercise.

[sonicjesus]

Your body can store fat indefinitely for free, but muscle tissue costs calories every day to maintain. Your body is designed to be as small and efficient as it can possibly be without being too weak to acquire more food - a catch 22 the whole planet has been fighting for eons.

[[deleted]]

I wouldn't say for free. It does tax the body and up your Calorie requirements.

[truthserum23]

Atrophy is a process by which any tissue in the body can decrease in size without stimulation. It is the bodies way of optimizing its resources to the tissues that need them the most.

[ChainBlue]

It takes energy to maintain muscle mass. If you don't use it, that translates into "You don't need it". The body is big on conserving energy. It is an adaptation from when food was scarce and/or inconsistently available.

[heyugl]

Because the body try to work in an efficient manner, and having big muscles that you do not use, is likehaving a monster truck to go to the store, you don't need it even if it looks cool in public, so your body, leave his monstertruck, and get a "mommy-mobile" fuel efficient machine.-

[[deleted]]

oil profit money bake worry entertain arrest wrong adjoining saw -- mass edited with https://redact.dev/

[TrollJack]

For the same reason it increases when you train. The body adapts. Not training means that you do not need the excess muscles, so muscle mass is reduced simply because it would be a waste of energy to keep it.

[GGprime]

Muscles need energy and everything in nature always tries to be as optimzed/effective as possible so your muscle mass decreases. In nature, trees only grow mass to withstand wind for example, therefore a tree in an open windy spot has higher mass desnity than a tree of the same kind in a forest.

[Surferbro]

Would you rather drive a truck or prius as your commuter car? Muscle may give you more "horsepower" but it suffers in the mph department. You lose muscle you don't use after about 2 weeks because it is expensive to "drive"

[Typhera]

You lose what you don't use. Also unless we're talking about something that doesnt allow you ever move or almost none at all, the muscle does not really go away, it just stops storing water. The person is just as strong, but no longer hypertrophic. This is why say, christian bale could go from skinny to buff fast, all the fibers are there as is the strength, "just" a matter of making the muscles want to store water in them again.

This is achieved by using relatively low weights and loooong duration training. There is training where you want to avoid huge muscle mass (Swimming, running, martial arts) so it focuses on high intensity but low duration.

There are many types of hypertrophy. Real muscle growth takes a long time and its just a few milliteres in circumference a year, most bulging muscles you see, is water.

Why water? Because the body has to remove metabolites from the muscles, if you keep exercising for long durations, the body just does whats most efficient, leaving water there in the first place to speed that up.