Metabolic Testing
Content from our newest sponsor, Grassroots Medicine
Why Two Runners Can Train the Same and Feel Completely Different
You’ve seen it before — maybe you’ve lived it.
Two people follow the same training plan. Same mileage, same effort, same commitment. One of them is improving, recovering well, and feeling strong. The other is tired, stuck, and wondering what they’re doing wrong.
Whether you’re chasing a PR or simply trying to enjoy Sunday’s long run without feeling wrecked on Monday, that gap can be genuinely frustrating. And it almost never comes down to willpower.
More often, the answer is metabolism — and metabolism is far more personal than most people realize.
Your metabolism is not a number on a scale
When most people hear the word metabolism, they think about how fast or slow the body burns calories. But metabolism is really the entire system your body uses to convert food and oxygen into energy — and to decide where that energy goes.
It governs how efficiently you fuel a long run. How quickly you recover after a hard effort. Whether your body burns fat, carbohydrates, or a mix of both at any given intensity. How your heart and lungs respond under load.
And here’s the thing: none of that works the same way from one person to the next.
Two runners at the same pace can be doing completely different things on the inside. One might be comfortably burning fat as a primary fuel source — an efficient, sustainable energy system that keeps them going mile after mile. The other might be pushed into carbohydrate-dominant metabolism much earlier, burning through glycogen faster, accumulating fatigue more quickly, and hitting that wall long before the finish line.
Neither runner is doing anything wrong. Their bodies are simply operating differently.
The zone that changes everything
Here’s something most runners have never been told: the intensity at which your body shifts from burning primarily fat to burning primarily carbohydrates is not the same for everyone. That crossover point — sometimes called your aerobic threshold — varies significantly from person to person, and it has enormous implications for how you train, how you fuel, and how you feel at mile 8, mile 13, or mile 20.
For a competitive runner, knowing exactly where that threshold sits can unlock smarter interval training, better race-day pacing, and more efficient fueling strategies. For someone running for fitness and enjoyment, it can mean finally understanding why you feel great on easy days and completely depleted after certain efforts — and how to fix it.
Training in the wrong zone for your metabolism isn’t just inefficient. It can leave you chronically fatigued, stalled in your progress, and more susceptible to injury — regardless of how many miles you’re logging.
Recovery is metabolic too
The same principle applies to how your body bounces back. Some runners feel fresh the day after a long effort. Others need two or three days before they feel human again. That’s not a fitness issue — it’s physiology.
How efficiently your body clears metabolic byproducts, rebuilds muscle tissue, and restores energy stores is deeply individual. And it’s something that can be measured, understood, and improved — once you know where you’re starting from.
Training plans are general. You are not.
Most training programs are built around averages. They work reasonably well for a broad range of people. But if you’ve ever felt like you were doing everything right and still not getting the results you expected — or if you recover more slowly than your training partners, feel like you’re always running on empty, or hit a wall that others don’t seem to hit — it’s worth asking whether the program is actually matched to how your body works.
Understanding your own metabolism doesn’t mean overhauling everything. Often it means making small, targeted adjustments — to pacing, fueling, intensity, or recovery — that are grounded in how your body actually responds, rather than how the average body is supposed to.
What if you could actually see it?
Here’s the question worth sitting with: do you actually know how your body is using energy when you run?
Not a rough estimate. Not a generic heart rate zone from a chart. But a precise, individualized picture of your fat-burning capacity, your carbohydrate dependence, your aerobic threshold, your VO₂ max, and how efficiently your metabolism is working right now.
That data exists. It’s measurable. And for competitive athletes and recreational runners alike, it tends to be one of the most eye-opening things they’ve ever learned about themselves.
That’s precisely what a PNOÉ metabolic assessment does. PNOÉ is a clinical-grade test that measures your exact training zones, fat-burning efficiency, VO₂ max, aerobic threshold, and biological age — giving you a precise, individualized picture of how your metabolism is actually working. It’s not a generic fitness test. It’s a detailed look at the engine behind your running, so you can stop guessing and start making adjustments that are grounded in your biology.
The runners who make the most consistent progress over time — at every level — aren’t always the ones who train the hardest. They’re often the ones who’ve learned to train in a way that works with their biology, not against it.
Dr. Seth Osgood is the founder of GrassRoots Functional Medicine in West Lebanon, NH, specializing in functional and longevity medicine. GrassRoots is pleased to provide UVRC members 25% off PNOÉ metabolic assessments at GrassRoots.
