Why muscle is your most underrated metabolic organ

Muscle is where most of the sugar from a meal goes, and it talks to the rest of your body every time you use it.

Muscle is one of the most important metabolic organs you have, because it's where most of the sugar from a meal goes to be used, and because every time it contracts, it sends chemical messages out to the rest of your body. Lifting and walking are the smaller part of its job. In a recent class, I called muscle the most underrated metabolic organ, built around one picture: when you use your muscle, you put up a do-not-demolish sign. This is for anyone who has watched the scale for years and never thought much about muscle.

Why muscle is a metabolic organ, not just a mover

I teach that your body is a city, with neighborhoods like your eyes, heart, kidneys, and liver, all connected by pipes and messengers. Muscle is one of the biggest buildings in that city, and it takes energy just to maintain it.

So the body is practical. When you use your muscle, it reads the building as occupied and needed. When you don't, it reads the quiet as "you don't need this anymore" and sends a crew to take it apart. That's the do-not-demolish sign. Resistance work puts it up. Sitting still takes it down.

Most people let this happen quietly from their 30s onward. And because muscle does so much more than move you, what gets demolished isn't just strength.

Muscle is a furnace for blood sugar.

The best way to remember muscle is as a furnace that burns your blood sugar 24 hours a day. Carbohydrates are digested into glucose, and after every meal,l that glucose is supposed to flow into your muscle cells to be used or stored as fuel.

In class, I asked people to guess how much of that sugar muscle handles. Someone in the chat guessed 80 percent, and she was close. A 2020 review describes skeletal muscle as responsible for about 80 percent of the glucose taken up from the circulation after a meal (Merz & Thurmond, 2020). One honest nuance: the liver also takes a meaningful share right after you eat, and the exact split depends on how it's measured. Either way, muscle is the biggest furnace in the house.

The size of the furnace matters. A bigger furnace burns more. A smaller one lets sugar linger in the bloodstream longer, even when your blood sugar is already within the normal range. Why care, if your numbers are fine? Because sugar that lingers sticks to things. If you've ever spilled soda on a linoleum floor, you know how sticky it gets. In the body, sugar binds to proteins like the collagen in your skin and tendons, and that's one of the quiet contributors to wrinkling and stiffness with age. I wrote more about that in why your blood sugar spikes.

The back door: a second way in for sugar

Normally, insulin works like a key. It opens the front door of the muscle cell so sugar can move in. But muscle has a back door too. When a muscle contracts, it sets off its own signals that move glucose transporters, little doorways called GLUT4, up to the surface of the cell, and sugar comes in through them (Richter & Hargreaves, 2013). The same review describes exercise training as the strongest stimulus for muscle to build more of those transporters over time.

In house terms, every time you train, more sugar-absorbing doors get bolted onto the building.

The dispatch rider: your muscles send messages

Every time a muscle contracts, it releases chemical messengers called myokines that travel through your bloodstream to other organs, carrying instructions. In the city picture, muscle is a dispatch rider heading out to every other neighborhood.

Researchers now describe skeletal muscle as working like an endocrine organ, one that can produce hundreds of myokines, with crosstalk reaching the brain, fat tissue, bone, liver, gut, pancreas, blood vessels, and even the skin (Severinsen & Pedersen, 2020). Only a few of these messengers have been matched to a specific job in people so far, so this science is still young. Some of the signals appear to reach the brain, where they're connected to a growth factor called BDNF that supports brain cells.

I find that remarkable. A muscle that never moves never sends those messages, so the rest of the city stops hearing from one of its most important coordinators.

Why the scale can't see it happening

Muscle and fat weigh roughly the same, so the scale is a poor witness. Picture a piece of bison steak: deep red, almost no marbling. That's roughly what muscle looks like at 30 with average activity. As the years go by without a reason to keep it, the steak starts to marble. Fat threads its way in, and the ratio shifts.

I also describe it as a house renovation you never approved. The solid oak beams are quietly swapped for styrofoam at the same weight. You look the same size, and the scale reads the same, but the structure has changed. The technical word for this age-related loss of muscle is sarcopenia. When it happens, four things tend to follow:

• A smaller furnace. Less muscle to take in the sugar from the same meal.
• A slower idle. Muscle uses more energy at rest than fat, so losing it lowers your energy needs. Portions that were fine at 30 can add up later.
• Fewer messages. Less contraction means fewer myokines going out to the rest of the city.
• A thinner reserve. In a hard stretch, the body draws on muscle, its insurance layer. Less muscle means less to draw from.

If you want to see what the scale can't, a DEXA scan measures body composition directly. And this isn't a one-way street. In a small, well-known study, ten frail nursing-home residents averaging 90 years old did eight weeks of high-intensity resistance training, and the nine who finished raised their strength by an average of 174 percent, with larger thigh muscles and faster walking (Fiatarone et al., 1990). It was a tiny group, but the point has held up since: muscle answers the signal at any age. The best time to start was years ago. The second-best time is now.

Repainting versus reinforcing: why cardio alone isn't enough

Remember the aerobics and running craze of the 1980s? Cardio is good for you, and I'm not putting it down. It improves circulation and mood. But it does a different job. In the house picture, cardio is repainting. It makes the place look and feel better. Resistance training is reinforcing the foundation and replacing the load-bearing walls. Both matter, but only one rebuilds the structure.

The reason is stress, the good kind. When you push or pull against a force that challenges a muscle, it reads that as a small, beneficial stress- what's called a hormetic stress- and it answers by building more capacity: more furnace, more doors. A daily walk is wonderful, but it isn't enough load to send that building signal. You don't need to lift heavy to start. You need to lift consistently, at a truly challenging level.

This matters even more when weight is coming off quickly, including for people using GLP-1 medications. Think of rapid weight loss as a demolition crew hired to take down a building that got too big. The crew doesn't always know which walls are load-bearing, and part of what comes off can be muscle rather than fat. Resistance training is the preservation order you hand the crew: don't touch this.

A building crew also needs material. When you lift, the crew shows up motivated and ready to build, but if the lumber yard delivers half the order, the job stalls. That material is protein. When appetite is small, protein is usually the first thing to fall short, which is why I teach protein first at every meal, and why protein needs tend to rise after about 65. For how to pace protein across your day, see my class on the protein and fasting rhythm. If you want an easy base for that first bite, I've written about egg white protein too.

And don't judge the gym by some of the characters in it. You're there to signal to your body that you want to stick around as long as you possibly can.

Questions I get about this

Will lifting weights make me bulky?

No. That's a myth that has kept a lot of women away from weights, and it isn't a real concern. As you get older, you may not even see the muscle you gain, but the furnace still grows, and so does your strength.

Are resistance bands and light weights enough?

They're a good start. Heavier weights are better when you're ready, because the goal is a point where the last couple of reps are nearly impossible. Workouts that mix cardio between sets can dilute that effort.

Do vibration plates count?

They give your muscles an interesting kind of workout and help with core stability, so they're a nice added benefit if you have one. You still need resistance work to put up the do-not-demolish sign.

Is it too late to start in my 60s or 70s?

Not at all, as the study of 90-year-olds above shows. Start where you are, get help with form, and if you have a medical condition or haven't exercised in a while, check with your doctor first.

References

1. Merz KE, Thurmond DC. Role of skeletal muscle in insulin resistance and glucose uptake. Compr Physiol. 2020. PubMed
2. Richter EA, Hargreaves M. Exercise, GLUT4, and skeletal muscle glucose uptake. Physiol Rev. 2013. PubMed
3. Severinsen MCK, Pedersen BK. Muscle-organ crosstalk: the emerging roles of myokines. Endocr Rev. 2020. PubMed
4. Fiatarone MA, et al. High-intensity strength training in nonagenarians: effects on skeletal muscle. JAMA. 1990. PubMed