Naturally thin people may not be more active than the rest of us, they just eat less.
It has long been assumed that those who seem to never gain weight are athletes who can eat whatever they want.
But the researchers found the opposite after studying 150 naturally thin people and comparing them to 173 people of normal weight.
For two weeks, naturally thin people did 23% less physical activity than people with normal weight.
Fitness trackers showed they spent more time sitting and lying down.
The study recruited thin people with an average BMI of 17, classified as just below the lower weight threshold, as well as people with a normal weight with a BMI below 25, the threshold for being overweight, and above 21.5.
To determine if naturally thin people could actually eat what they liked, the researchers calculated their calorie intake directly based on the amount of energy they burned.
This energy expenditure suggests that those who stay thin eat 12% less than people with a normal weight.
They were also found to have a faster resting metabolism, which may be a genetic peculiarity.
This has been linked to higher levels of thyroid hormones, which can help make people less hungry and keep them thin.
Unfortunately, less than one in 50 people fall into the naturally slender category.
Professor John Speak-man, who led the study at the University of Aberdeen, said: “Often, when people talk to super-lean people, they say they can eat whatever they want. But our work showed that what they actually eat is much less than people in the normal BMI range. “
The study, published in the journal Cell Metabolism, looked at only Chinese and, of course, other people who are not involved in the study may be more active.