If you are naturally lean, it is not because you are more active but because you are less hungry, and burn more calories at rest than others.

In a study published in the journal Cell Metabolism, researchers looked at people with a very low body mass index (BMI), and therefore thin. Their findings reveal that these people are actually significantly less active than people with a Normal Bmi. They also eat less food than those with a normal BMI.

For this study, the researchers recruited 173 people with a normal BMI (range 21.5 to 25) and 150 whom they classified as “healthy underweight” (with a BMI less than 18.5), i.e., Lean but healthy people.

Participants were followed for two weeks. Their food intake was measured using an isotope-based technique (double-labeled water). Their physical activity was measured using an accelerometry-based motion sensor.

“We expected these people to be really active and have high metabolic rates of activity corresponding to high dietary intakes,” says John Speakman, a professor at the University of Aberdeen in the United Kingdom. “It turns out that something quite different is going on. They had lower food intakes and activity, as well as a high basal metabolic rate, which is a surprisingly higher resting metabolic rate (RMR) than expected, linked to high levels of thyroid hormones.” A high RMR leads to increased calorie expenditure.

The investigators found that compared to a control group with a normal BMI, the healthy lean individuals consumed 12% less food. They were also 23% less active.

“Although these very lean people had low levels of activity, their heart health markers, including cholesterol and blood pressure, were very good,” says first author Sumei Hu, currently at Beijing University of Technology and Commerce. “This suggests that low body fat may outweigh physical activity in terms of health consequences.”

The researchers acknowledge some limitations to this research, including that although they measured food intake, they did not measure what the participants actually ate or how full or full they felt.

“The next step is to better understand the phenotype itself and more clearly understand the mechanisms that generate it,” says Dr. Speakman.