Myth or fact

Losing weight makes you fitter

Short answerYes, but

By GyroscopeUpdated November 14, 20266 sources

Yes, but mostly on paper. VO2 max is per kilogram, so losing 10 percent of your weight raises the number about 11 percent with no change in your heart. Your engine only grows with training, and on GLP-1 drugs a quarter or more of the weight lost is lean mass.

Why people believe it

Because the watch says so. Lots of people start losing weight, keep their workouts the same, and watch their VO2 max climb. Your watch uses the weight you’ve entered along with your age, sex and height (Apple lists all four), and the number it shows is per kilogram. It really does go up.

And moving does get easier. Stairs, hills and runs all take less out of you when there’s less of you to carry, which makes it feel like proof.

Same engine, lighter car

Take a car up a steep hill with a trunk full of luggage, then again with the trunk empty. The second run is quicker. The engine was the same and the load was lighter.

VO2 max is the same arithmetic. The engine is the liters of oxygen your heart and muscles can use in a minute. The number on the chart is that divided by your weight. A 200 lb man in his 40s at 35, about the 49th percentile, uses about 3.2 liters a minute. At 180 lb with the same 3.2 liters, he’s at 38.9, about the 63rd percentile. He moved up 14 percentile points without one workout.

The Dallas men, run backward

The clearest real case runs the other way. Five healthy men tested in Dallas at 20, in 1966, were tested again at 50. Their weight had gone from 77 to 100 kg, and their bodyfat from 14 to 28 percent.

Their engine had lost only 11 percent, from 3.30 to 2.90 liters a minute. Their per-kilogram number fell 28 percent, from 43 to 31. More than half of what the chart showed as lost fitness was weight.

Then they trained for six months. The engine came all the way back to 3.3 liters a minute. Per kilogram they reached 36, not 43, because most of the weight stayed on. Training fixed the engine, and the extra weight was the rest of the gap.

What about GLP-1 drugs?

GLP-1 drugs like Ozempic and Wegovy (semaglutide) and Mounjaro and Zepbound (tirzepatide) cause big weight loss, so the per-kilogram number should rise. A 2025 review raised two problems.

1. About 25 to 40 percent of the weight lost comes from fat-free mass, which includes muscle, far faster than people lose it with age. Muscle is where oxygen gets used.
2. Clinical studies haven’t shown an improvement in cardiorespiratory fitness on these drugs. The authors said the long-term effects of that lean mass loss are unknown.

The number on your watch can rise on a GLP-1 while your engine shrinks. Pair the drug with cardio and lifting twice a week, and enough protein. bodyfat.guide covers whether GLP-1 drugs make you lose muscle and how they work. Any change to the medicine itself is for your doctor.

The best case for “yes”

“VO2 max per kilogram is what predicts health in the big studies, and per kilogram is what weight loss raises. So it counts.”

There’s something to that. The big studies of fitness and survival mostly use per-kilogram numbers, and being lighter really does make daily life easier. We’d count it. But those studies measured people who got their numbers mostly from their heart and muscles, and we haven’t found a study showing that a number raised only by losing weight carries the same benefit. In healthy people, VO2 max is capped by how much oxygen the heart and blood can deliver, and training raises it by raising the heart’s output (Bassett and Howley, 2000). Weight loss leaves that untouched.

How to tell the difference

Multiply your VO2 max by your weight in kilograms and divide by 1,000. That’s your engine in liters a minute. If it’s rising, you’re getting fitter. If only the per-kilogram number is rising, it’s the scale. Weight and your VO2 max does it for you, and weight and VO2 max has the long version.