The guide

Does losing weight raise your VO2 max?

By GyroscopeUpdated November 15, 20268 sources

Your number goes up, but your engine doesn’t. VO2 max is per kilogram, so losing 10 percent of your weight raises it about 11 percent with the same heart and lungs. Only training raises the liters of oxygen your body can use.

Why does my VO2 max go up when I diet?

Lots of people see it. They start losing weight, they haven’t changed their workouts at all, and their watch says their VO2 max is climbing. It feels like proof that they’re getting fitter.

Part of it is real and part of it is arithmetic. VO2 max is usually given per kilogram of body weight. Your watch also uses the weight you entered in your profile, along with your age, sex and height (Apple says so for Apple Watch). Change the weight and the number changes, even if your heart hasn’t.

Same engine, lighter car

A car’s top speed up a hill depends on its engine and how much it weighs. Take 200 pounds of luggage out of the trunk and it climbs faster, with the same engine.

That’s exactly what happens to VO2 max. Your heart and muscles can use a certain number of liters of oxygen a minute. That’s the engine. Divide by your weight in kilograms and you get ml/kg/min, the number on the chart. Lose weight and you divide by a smaller number.

Here’s how it works out. Take a 200 lb man in his 40s with a VO2 max of 35, about the 49th percentile for his age. His engine is about 3.2 liters a minute. If he drops to 180 lb and his heart doesn’t change at all, the same 3.2 liters divided by his new weight is 38.9, about the 63rd percentile. He went up 14 percentile points without one workout.

The rule of thumb is simple. Lose 10 percent of your weight and the number rises about 11 percent (1 divided by 0.9), with the same engine.

The Dallas men, in reverse

The best real example runs the other way. In 1966, five healthy 20-year-old men in Dallas were tested for a bed rest study. Thirty years later, at 50, researchers tested them again.

Their engine had lost only 11 percent in 30 years. Their per-kilogram number had lost 28 percent, because they were carrying 23 more kilos, most of it fat (their fat-free mass went from 66 to 72 kg). Then six months of training brought the engine all the way back to 3.3 liters a minute, but per kilogram they reached 36, not 43, because most of the extra weight stayed.

Training rebuilt the engine. To get back to 43 with that same engine, they’d have needed to weigh what they did at 20.

Is the gain from weight loss fake, then?

No. You really can do more. Carrying less weight up a flight of stairs or a trail takes less of your engine, so it feels easier. Anyone who’s taken off a heavy hiking pack halfway up a hill knows the feeling, with the same legs and lungs and an easier climb. A higher per-kilogram number is a fair description of that.

It doesn’t mean a stronger heart, though. In healthy people, VO2 max is capped by how much oxygen your heart and blood can deliver, and training raises it mainly by raising the heart’s output (Bassett and Howley, 2000). In the 4x4 trial, stroke volume rose about 10 percent after interval training. Losing weight doesn’t do that. If your liters a minute aren’t rising, your heart isn’t changing.

Both count, for different things. Weight loss raises the number on the chart and makes moving easier. Training raises the engine. Do both and you get both.

What about GLP-1 drugs like Ozempic?

GLP-1 drugs (semaglutide, sold as Ozempic and Wegovy) and the newer GLP-1 and GIP drugs (tirzepatide, sold as Mounjaro and Zepbound) cause large weight loss. On paper, that should push VO2 max up per kilogram.

A 2025 review found two catches. First, about 25 to 40 percent of the weight lost on these drugs comes from fat-free mass, which includes muscle. That’s far faster than people lose fat-free mass with age. Second, clinical studies so far haven’t shown that the drugs improve cardiorespiratory fitness. The authors say more research is needed to know what the drugs do to fitness over the long run.

Muscle is where the oxygen gets used, so losing a lot of it is the wrong direction for your engine, even if the per-kilogram number rises. That’s why we’d pair a GLP-1 drug with cardio most days and lifting twice a week, the muscle-strengthening part of the US guideline, plus enough protein. bodyfat.guide covers how GLP-1 drugs work, whether they make you lose muscle and how much protein you need. Never change a medicine without your doctor. If you’d rather start without a gym, how to increase VO2 max naturally has walks, stairs and hills.

How do you tell the two apart?

Watch your liters a minute, not only ml/kg/min. Multiply your VO2 max by your weight in kilograms and divide by 1,000. If that number rises, your engine grew. If only the per-kilogram number rose, it was the scale.

Two more things help:

• Keep your weight current in your watch’s settings. An old weight gives an old answer.
• Retest with a field test that doesn’t care about your profile. A 12-minute run or a 1-mile walk at your new weight shows what you can actually do. How to measure it has the tests.

Try it

Enter your weight and your VO2 max, then slide the weight. The liters a minute stay flat while the number on the chart moves.