Aesha Tahir 0:01

Hello everyone. Welcome back to the 925 Wellness Podcast. Today we are talking about a topic that I have covered in much detail on this podcast, but I have realized that I haven’t talked about it as a functional organ. So let’s dive into it. I’m not gonna give it away at this time, but you’ll soon find out what this episode is about. Before we get started, I have a quick question for you guys. What do you think your muscles are for? If you said moving your body, lifting heavy weights, or looking good at the beach, you’re right. Yes, yes, and yes, but you’re only telling about a third of the story because muscles aren’t just like ropes and pulleys. It’s not just meat wrapped around bone. Muscle is one of the most metabolically active and chemically reactive organ in your entire body. It stores fuel, it burns fuel, it holds a reserve of raw building blocks your body can use in an emergency. And when you contract it, when you go for a run, lift a weight, or even just fidget in your chair, it releases a cocktail of signaling molecules that talk to your brain, your fat, your liver, your bones, and your immune system. Today, we are going to walk through muscle’s four other jobs, the ones that have nothing to do with locomotion. By the end of this episode, you’ll understand why doctors and exercise physiologists have started calling skeletal muscle the largest endocrine organ in the human body. Let’s get into it. The first function of muscle as an organ, which I want to talk about, is its use as a glucose sink. Let’s start with sugar, right? Because we all love sugar, we love eating it, and our body loves storing it and using it. Specifically, blood sugar, glucose. Every time you eat a meal with carbohydrates, that food gets broken down into glucose, which enters your bloodstream. Your body now has a decision to make. Where does that glucose go? Some goes to your brain, which is greedy for glucose and burns it constantly. Some goes to your liver, which can store a modest amount as glycogen. But the single largest destination for glucose after a meal by far is skeletal muscle. Here’s the mechanism, and it’s genuinely elegant. Your pancreas release insulin in response to rising blood sugar after you eat a meal. Insulin travels through the blood and knocks on the door of your muscle cells. That knock triggers a protein called GLUT4, think of it as a glucose gate, to move the surface of the muscle cell and let glucose flood in. Muscle tissue accounts for roughly 40% of your total body mass, and it’s estimated to soak up somewhere between 70 to 80% of the glucose you consume after a typical meal. And this has been shown in many different research studies. So there is a lot of evidence supporting this number: 70 to 80% of the glucose you consume after a meal gets stored in your muscles. And I’m gonna link some research studies in the show notes, so take a look there. Now, you might be thinking, oh no, that doesn’t sound right. Well, that’s not a rounding error. That’s the majority of your blood sugar management happening in muscles, not in your pancreas, not in your liver, in your muscle. Once glucose gets inside a muscle cell, it has two possible fates. It can be burned immediately for energy, or it can be stored as glycogen. Basically, glucose linked together in long chains, it’s almost like a paintry stockpile the muscle can dip into later if it’s storing it. An average adult stores somewhere around, I want to say, 400 grams of glycogen and skeletal muscle. That’s roughly four times more than what’s stored in the liver. Now, here’s why this matters beyond trivial questions. Just like when I started this podcast, I asked you. When muscle is small, inactive, or insulin resistant means that it doesn’t respond to insulin, what happens is that glucose has nowhere good to go. So that muscle fiber, which is small, inactive, or isn’t strong enough, and is insulin resistant, isn’t that good at storing glycogen? And you might be wondering why would that happen? So that happens with aging, because as you age, you lose muscle mass and it gets weaker. Second, sedentary behavior, where you’re sitting all day and not moving, not lifting weights, not doing any cardioactivity. And third thing responsible for it could be chronic diseases like prediabetes or type 2 diabetes. Now, when your glucose has nowhere good to go, it lingers in the bloodstream. Blood sugar stays elevated, the pancreas has to work harder, pumping out more and more insulin to try to force the door open. Over years, this is a huge part of the pathway towards insulin resistance and type 2 diabetes. Now, flip that around, and it’s genuinely one of the most encouraging facts in exercise physiology. Building or simply using muscle is one of the most powerful tools we have for blood sugar control. Now, hear this out. A single session of resistance or endurance exercise can improve insulin sensitivity in muscle for up to 24 to 48 hours. Independent of weight loss, independent of diet changes, your muscle quite literally becomes a better sponge. So the first point I want you to remember about muscle’s functionality is muscle isn’t just a passenger in blood sugar regulation, it’s the main character. Now let’s talk about muscle’s second hidden job, which is muscle acts as an energy furnace, burning fuel constantly, even when you’re not moving at all. You’ve probably heard people say muscle burns more calories than fat. That’s true. But the why behind it is more interesting than the soundbite. Muscle tissue is packed with mitochondria, the cellular structures that generate ATP, the energy currency your cells run on. Compare that to fat tissue, which is largely inert from a calorie burning standpoint, its main job is storage, not combustion. Gram for gram, resting muscle burns several times more energy than resting fat tissue. This matters for something called your basal metabolic rate. The number of calories your body burns just existing. Before you’ve taken a single step. Compare that to fat tissue, which is largely inert from a calorie burning standpoint, its main job is storage, not combustion or burning of the energy. Now, if you look at it gram for gram, resting muscle burns several times more energy than resting fat tissue. This matters because that increases your resting metabolic rate. Or sometimes we call it basal metabolic rate, the number of calories your body burns just existing, just resting, like while you’re sitting on your couch watching Netflix, you’re still burning some calories, and that is your basal metabolic rate. So it doesn’t matter if you’re taking you’ve taken a single step or you just woke up in the morning, your body was still burning calories at rest. And if you have more muscle mass, it just increases that basal metabolic rate. Skeletal muscle typically accounts for somewhere around 20% of resting energy expenditure in an average adult. And that share climbs substantially once you’re actually active. During vigorous exercise, muscle’s energy demands can increase more than tenfold compared to the body at rest. There’s also a fascinating cold-related version of this. When your body needs to generate heat, say you’re out in the cold without a jacket, or if you’re like me, a runner, and you’re just cold because you’re not wearing a lot of clothes, you don’t want to throw your jacket out while you’re on your run. Guess what? Muscles contract rapidly and involuntarily, which we call shivering. We are all familiar with that, right? Now that’s your muscular furnace being deliberately overworked to produce warmth. Even without visible shivering, muscle has some capacity for what’s called non-shivering thermogenesis, contributing to keeping your core temperature stable. Here’s the part that tends to surprise people. This furnace effect isn’t just about the muscle you have right now. It’s shaped by the muscle you build, a few pounds of lean muscle mass through resistance training. And you can meaningfully raise your resting metabolic rate. Because you’ve added metabolically expensive tissue that costs energy to maintain, repair, and fuel 24 hours a day, seven days a week, even while you sleep. Even while you sleep. Not just a strength problem. As muscle mass declines, so does resting energy expenditure. Fewer calories are being burned at rest, which combined with typically decreasing activity, is a major contributor to the weight gain and metabolic slowdown many people associate with getting older. It’s not anevitable aging chemistry so much as it’s a muscle mass problem. And muscle mass is something you have more control over than you might think. Let’s talk about muscle’s third job. Its third job, and this one is less well known, is muscle acting as an amino acid reserve or your body’s emergency protein bank, or your body’s emergency protein bank. About 40% of your total body protein lives in skeletal muscle. That protein isn’t inert, it exists in a constant state of turnover. Your body is always breaking down some muscle protein and rebuilding it. Breaking it down and rebuilding it in a continuous cycle is what helps it exist. That’s why we call it the muscle protein synthesis, or where the amino acids are being rebuilt and they’re creating muscle fibers in your body. Under normal conditions, this cycle is roughly balanced and it’s fueled by adequate nutrition, particularly dietary protein. But here’s the key insight: your body treats muscle protein as a reserve pool of amino acids that can be broken down and then redirected when something more urgent comes up. So you might be thinking, what could be urgent for my body? Or what counts as urgency, right? So here are a few scenarios during prolonged fasting. A lot of people are big fans of fasting. Um, so like your 48 or 72-hour fasts, yes, you are breaking down muscle as fuel at that point. Or maybe it’s a case where you don’t have food available and you go into starvation mode. Again, your body is going to break down those amino acids and use it as a fuel. Once liver glycogen is depleted, usually within 24 hours or first 24 hours without food, your body starts breaking down muscle protein and converting some of those amino acids into glucose through a liver process called gluconeogenesis. This is how your brain, which needs a steady glucose supply, stays fed even when you haven’t eaten, even when you haven’t had a meal. This is how your brain, which needs a steady glucose supply, stays fed, even when you haven’t had a meal for a long time. During serious illness, trauma, burns, or major surgery, the body’s demands for amino acids spike dramatically for immune cell production and for wound healing, and for making acute face proteins that manage inflammation. Muscle is the fastest, most abundant source your body can use to meet that demand. This is a major reason why people who are critically ill in hospital, especially in intensive care units, can lose striking amounts of muscle mass in a matter of days, sometimes losing measurable muscle within the first week of bed rest and illness combined. And in chronic diseases, cancer being the best studied example here, a wasting syndrome called cachexia involves the body’s aggressively breaking down muscle protein even when the person is eating adequately, driven by inflammatory signals from the tumor. This muscle loss isn’t just cosmetic, it’s strongly associated with worse outcomes and reduced ability to tolerate treatment. So the amino acid reserve function of muscle is a bit of a double-edged sword. It’s a brilliant survival adaptation, a body that can canabolize its own muscle to keep the brain fed and the immune system running, which is built to survive short-term crisis. But it also means that having more muscle mass, going into an illness and a surgery or an injury, or maybe you were in a car accident unexpectedly, gives you a bigger buffer, a bigger reserve to draw from before you start losing function. This is part of why physicians increasingly look at patients’ muscle mass as a predictor of how well they’ll tolerate surgery, chemotherapy, or a hospital stay. Now, the fourth reason why muscle is so critical for our bodies is muscle as an endocrine organ. Let’s talk about that part. This is my favorite part by far. Now, this is the reason that has genuinely reshaped how exercise physiologists and scientists think about exercise. This discovery is only about 20 years old, so it’s not that long ago that we didn’t even know that muscle has this function, but it’s a big one. For most of the medical history, we thought of endocrine organs as specific dedicated glands like your thyroid, your adrenal gland, your pancreas. Muscle wasn’t on the list. It was considered a purely mechanical tissue. Then in the early 2000s, researchers discovered that contracting muscle releases signaling proteins directly into the bloodstream, molecules that travel to distant organs and change how those organs behave. These molecules were named myokines. From myo for muscle and kine for the messenger molecules called cytokines. That discovery essentially added a new endocrine gland to the textbook, one that just happens to make up close to half your body weight. Let’s talk about a few of the most well-studied myokines and what they actually do. The first one I want to talk about is interleukin 6 or IL6. This one’s a great example of context mattering enormously in biology. Chronically elevated IL6 from inflammation is associated with disease and poor health outcomes. But the IL-6 released acutely from contracting muscles during exercise behaves completely different. It’s anti-inflammatory in that context. It stimulates the liver to release more glucose to fuel the working muscle, and it appears to help the body break down fat for fuel. Same molecule, opposite behavior, depending on where and why it’s released. The second myocine that I want to share with you about is irisin. I love the name. It’s like a superhero name. This one made headlines when it was discovered because of what it does to fat tissue. Now, hear this out. Irisin is released from contracting muscle and appears to signal to the white fat cells. So we have white fat cells and brown fat cells, but it signals to white fat cells, the ordinary fat storing kind, and it encourages some of them to take on the characteristics of brown fat, the metabolically active kind of fat, which burns energy to produce heat. So that means that it converts those white dormant fat cells into brown active fat cells. Researchers nicknamed this process browning of fat. The excitement about irisin is as a potential obesity treatment has been tempered by most, has been tempered by more recent research. And guess what? Scientists have even tried to use irisin as a potential obesity treatment. But that has been kind of like that excitement has been tempered by more recent research. We don’t have enough data. And enough proof that that can work as an obesity medication. However, it would be very cool if it could. That being said, irisin is one of the myokines that is released when we do exercise, when we perform exercise, when we lift weights, when we run, and that converts your dormant fat, white fat cells, to brown fat cells. Let’s talk about the third one, which is BDNF. That’s one of my favorite myokines. BDNF, brain deride, neurotrophic factor. Muscle contraction increases the release of BDNF, a protein that supports the growth and survival of neurons and strengthens connections between them. This is one of the most compelling biological threads connecting exercise to brain health, mood, and memory. It’s a big part of the explanation for why regular exercise is associated with better cognitive function and lower rates of depression. Let’s talk about interleukin-15. This appears to act on fat tissues as well, and it helps to reduce fat mass. And it also seems to play a big role in muscle tissue maintaining itself so that it’s not broken down. And the last one that I want to talk about is myostatin. Now, myostatin as a myokine flips the script a little bit. Myostatin is a myokine, but it works in the opposite direction from most of the others. You’re wondering how? It puts the brakes on muscle growth. It’s actually released by resting muscle to keep muscle mass from growing indefinitely. Like you don’t want to be someone who has like, who’s just carrying like loads of muscle too, because it’s an expensive tissue, right? One of the most striking findings, though, in genetics came from cattle. Um, and later, a small number of human studies also told us about myostatin gene. The muscle mass in those cases is dramatically visibly larger than typical people. You might know someone who has this gene. Um, regular resistance exercise tends to reduce myostatin activity, which is part of the biological like green light that allows train muscle to grow. So that is another example of myocines in action when you exercise. And they so they also put some brakes on that you don’t have like an indefinite amount of muscle mass. There are dozens more of these myocines which are still being studied, they’re being characterized. For example, decorin, which also seems to inhibit myostatin, SVARC linked to reduced colon cancer cell growth in some studies. Then we also are looking at FGF21, which is involved in fat metabolism. This is an active, fast-moving area of research, and the list keeps growing. What ties all of this together is a bigger idea. Exercise isn’t just calories burned or muscles worked. Every time you contract a muscle with any real effort, you’re triggering a chemical broadcast that reaches your brain, your fat tissue, your liver, your bones, your blood vessels, and your immune system. You’re not just training your legs when you go for a run. You’re sending signals to organs that never touch the ground. Okay, so let’s bring it all in. Let’s pull this together. Muscle is a glucose sink, the primary destination for the sugar in your bloodstream, and one of your best tools against insulin resistance. Muscle is an energy furnace, metabolically expensive tissue that raises the number of calories you burn even while at rest. Muscle is an amino acid reserve, a bank of protein that your body can drop on during starvation, illness, trauma, an accident, a car accident, or a surgery, which is part of why maintaining muscle mass is linked to better outcomes in serious illness and surgery and is a big, big, big biomarker of longevity and health span. And muscle is an endocrine organ releasing so many different kinds of myocines with every contraction that talked to your fat, your brain, your liver, and your immune system. Now you might be wondering, okay, let me hit the gym like tomorrow. But here’s the thing none of this requires you to become a bodybuilder. The research on myokine release and glucose uptake shows meaningful effects from moderate consistent activity, like walking, risk walking, resistance training a couple of times a week, simply not just sitting for 10 hours straight. The muscle you carry around every day isn’t just the thing that lets you climb stairs. It’s arguably one of the most active chemical labs in your entire body. This is what I wanted to share with you about muscle mass and muscle health. That’s it for today’s episode. I hope you enjoyed it as much as I enjoyed recording it and sharing it with you guys. Next time you’re deciding whether to skip a workout, maybe think of it less as should I go move my body? And more as should I go run my internal pharmacy for a while. Thanks for listening. I’ll see you next time. Bye for now.