What Changes When Muscle Shrinks
The short answer
Your arm has grown thinner, but you say, "It has not caused me any trouble in daily life" — is that all right? Muscle is both an organ of movement and the protein reserve the whole body draws on. Strength falls three times faster than mass does, and once the reserve thins out, even a single cold can leave you unusually run down. If your weight drops more than 5% over six months to a year, if the change is one-sided, or if swallowing trouble and changes in speech come with it, getting it checked comes first.
Contents
You tell me your arm has grown thinner, that your sleeve has gone loose. And you immediately add one thing. "But it has not caused me any trouble in daily life."
You are right. Even as muscle shrinks, walking and getting around stay much the same for quite a while. What changes first is out of sight.
Muscle Does More Than Move the Body
If you see muscle only as the organ that moves the body, then muscle loss also looks like a problem of movement alone. But muscle does several things at once in the body.
Movement is indeed its first job. But muscle is also the largest site that takes up the glucose that enters after a meal, and one of the largest sites where the body generates heat. Shivering in the cold is also muscle's work.
And there is one job muscle does more quietly than any other. Muscle is where the largest store of raw material for the body's protein is kept. Even while nothing new is coming in through food, the heart, the liver, and immune cells still have to keep making their own proteins, and that material has to be drawn from somewhere. It is drawn from muscle. The material the liver uses when it manufactures new glucose (gluconeogenesis) also comes from there.
So muscle is at once the strength attached to your arms and legs, and the reserve the whole body draws on.
Strength Goes Before Mass
It is easy to assume that strength declines in step with muscle mass. The two do not move at the same pace.
Leg muscle mass declined by around 1% a year, but leg strength fell about three times as fast. This is what was measured over three years, following 1,880 people over the age of seventy. Over that same period, even among those whose muscle mass actually increased, strength neither held steady nor increased.
Mass and strength are not the same thing. So if hearing that your muscle has shrunk leads you to think only of adding mass back, you miss what actually went first.
Where It Shows Before That
The reserve side.
Ordinarily this does not show. If what is stored up is ample, other organs go on doing their work even through a few days of not eating, or a few days laid up sick, because material is drawn from muscle.
Once what is stored thins out, the same few days becomes a different matter. A single cold, one procedure, a few days of poor appetite eaten through carelessly. What would once have simply passed now leaves you unusually run down. It is because what the body drew on to get through those few days was not generous to begin with.
So a loss of muscle is hard to see before you fall ill, and shows itself only after you have been through an illness.
Disuse Dries Up Muscle First
These numbers show two things. First, the change on the inside was larger than the loss visible on the outside. Second, this happened even though food intake was not reduced. These people ate as they normally did. The only thing that changed was that using that muscle had stopped.
It may seem that pouring more in where it has dried up would fix it, but it is not that simple. Even with the same meal, if the post-meal muscle protein synthesis response is lower, the proportion of incoming material actually used to build muscle protein falls too.
This is not a prescription for what exercise to do or how much. Only that muscle stays only if it is used. Even the capacity to take up incoming nutrition is restored by using the muscle. This runs along the same grain as the idea that supply alone does not heal.
The Pace of Change and When It Shows
To sum up: muscle mass, the post-meal response, and insulin sensitivity do not change at the same pace. Which one goes first is not fixed, either. What tests usually measure is mass, so whatever is changing at a different pace from mass does not show up right away on the same test.
Put the other way around, "it has not caused trouble in daily life yet" does not mean "nothing has happened yet." This is not about losing the ability to do something — it is that where the change is visible and where it is actually happening are not the same place.
Where to Look First
The following should not be written off as age.
When weight loss is unintended. When your weight has dropped more than 5% over six months to a year. When it affects only one side. What age brings comes evenly to both sides. When it affects swallowing and speech together. When choking on food becomes more frequent or your speech no longer sounds the way it used to.
There is a place to check first.
Faced with the news that muscle has shrunk, most people first look for something more to eat.
I look first at what this body can handle right now. And I look at one more thing: what work this muscle is doing over the course of a day. What is being used is what stays.
Written by Dr. Heo Ji-young (PhD in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)
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