I’ve seen it, and so have you. We coach the same workout all day. Some athletes thrive on the challenge, and others don’t look as good. We second-guess ourselves: “Did I scale them right?” “Was that the right workout today?” But sometimes the answer is simpler. It’s not the workout that needs a closer look, it’s the athlete who is struggling. This is where mitochondria enter the conversation.

MetFix is the only commercial entity concerned with the care and feeding of the mitochondria.

I’ll admit that line didn’t land at first. Mito-what? It sounded like textbook science, not a coach’s job. But experience has taught me that when Coach Glassman makes a claim that doesn’t make sense right away, it’s worth listening to. If I don’t understand it yet, it usually means there’s something big I’m missing. That’s why, when I heard Pete Shaw’s lecture at the first Foundations Course, the line finally clicked: capacity lives or dies inside the mitochondria. What we call fitness is really mitochondrial function expressed on the gym floor.

If we’re going to talk about fitness and health, we have to care about mitochondria, because talking about performance without them is like talking about a drag race without talking about the engines.

So let’s look under the hood. Nearly every cell in your body is built around these engines. Mature red blood cells and lens cells of the eye are the exception. Red blood cells carry oxygen, but they do not have mitochondria. Most tissues do. The mitochondria you “see” in performance live mainly in skeletal muscle, the heart, and the nervous system. Those are the engines that move the load and keep output steady. But mitochondria in organs like the liver, pancreas, and gut set the fuel mix and metabolic signals that determine whether those performance engines run clean or overheat. At the cellular level, each mitochondrion fires like a cylinder, turning the fuel you eat into ATP, the cell’s universal energy currency. Mitochondria turn fuel into ATP. That’s where we get the horsepower. Nutrition sets the fuel mix, and training controls how hard the engine has to work. This tiny engine explains what coaches see every day: elbows drop, midlines soften, footwork gets sloppy, while someone right beside them stays crisp and in control. Mitochondria determine who has the power to do the work, who adapts, who fades, and who can show up again tomorrow. Performance and health share the same mechanism: when these engines adapt, athletes get fitter; when they run on the wrong fuel for too long, dysfunction isn’t far behind.

The mitochondria don’t care about food brands or nutrition fads; they care about substrates. Everything—fat, carbohydrate, and protein—gets broken down into acetyl-CoA, the molecule they burn to produce ATP. Fat enters through beta-oxidation. Carbohydrate runs through glycolysis and pyruvate oxidation, and amino acids contribute during fasting or excess intake. In metabolic terms, fat is the long, clean burn; carbohydrate is the fast, hot flame. With oxygen, a molecule of fat yields roughly three times more ATP than a molecule of glucose. That difference shows up on the gym floor, where fat supports steady output and repeat efforts, while glucose is powerful but limited—one feels sustainable, the other urgent.

Metabolic flexibility is the capacity to burn the right fuel at the right time. When mitochondria are efficient and abundant, fat supports the easy gears and glucose is reserved for speed. When they’re limited, so too is the capacity to burn fat. The engine runs too hot, too soon. More glucose is burned without oxygen through glycolysis. Coaches see the result: athletes who look in distress instead of control, faces flushed, breathing panicked, pace collapsing long before the clock demands it, recovery slowing to a crawl. Without metabolic flexibility, training becomes futile suffering instead of driving adaptation.

What we see as poor performance is often early metabolic disease. The same engines that decide who holds form in a workout also decide who drifts toward chronic disease. When mitochondrial oxidative capacity is limited, athletes rely more heavily on glucose and fatigue sooner. Chronic overexposure to refined carbohydrate and industrial seed oils drives the overload, increasing oxidative stress and eroding the ability to adapt. First, performance breaks. Then health breaks. Diabetes, fatty liver, cardiovascular disease, and cancer—many of them begin with the same failing engine. Research in cancer metabolism, insulin resistance, and cardiometabolic disease increasingly identifies mitochondrial dysfunction as an upstream driver of metabolic disease. Workout performance is the first test of metabolic health, and coaches administer it every day. Doctors use a similar principle with exercise stress tests, unmasking cardiovascular problems that are silent at rest. Long before fasting insulin climbs above 5 µIU/mL or the triglyceride-to-HDL ratio rises past 2.0, coaches can see the engine struggling on the gym floor.

Back to what we see all the time—one athlete keeps mechanics sharp and breathing steady, while the other fades early: red face, slumped posture, confusion in the eyes. What you’re noticing is the engine beginning to fail. One engine is responsive; the other is overheating and struggling to convert fuel into power. Scaling won’t fix a failing engine; you have to change what goes into the tank.

When adaptation stalls, it’s usually a fuel problem, not a programming problem. Fasting can help restore insulin sensitivity and reset energy signaling. If you want mitochondria that make power and recover fast, you need adequate protein, sufficient total energy, and food that supports stable blood sugar so training can stay consistent. The principle is the same moving from sick to well as it is from well to fit, but the dose changes: total intake, macros, training load, and recovery have to be fitted for the athlete in front of you.

This is the care and feeding of mitochondria. In MetFix terms, mitochondria are the biological engine behind work capacity across broad time and modal domains. Mitochondria drive metabolism and control capacity—and they’re trainable. Give them a consistent signal and they expand, giving you more capacity to make energy, hold output longer, and recover faster between bouts. Remove the signal and they diminish. The same workout feels harder, breathing spikes sooner, and recovery slows. Coaches measure it with a stopwatch and record it with a whiteboard.

Coach the mitochondria, and you coach both performance and health.