Lean people aren't more active, they are just less hungry and "run hotter"

To date most research on obesity has focused on studying those with a high body mass index (BMI), but a research group in China is taking a different approach. In a study published July 14 in the journal Cell Metabolism, the scientists looked at individuals with a very low BMI.

Their findings reveal that these individuals are actually considerably less active than people with a BMI in the normal range, contrary to speculation that they have a metabolism that makes them naturally more active. Additionally, they eat less food than those with a normal BMI.

The investigators recruited 173 people with a normal BMI (range 21.5 to 25) and 150 who they classified as "healthy underweight" (with a BMI below 18.5). The established questionnaires to screen out people with eating disorders as well as those who said they intentionally restrained their eating and those who were infected with HIV. They also excluded individuals who had lost weight in the past six months potentially related to illness or were on any kind of medication.

The participants were monitored for two weeks. Their food intake was measured with an isotope-based technique called the doubly-labeled water method, which assesses energy expenditure based on the difference between the turnover rates of hydrogen and oxygen in body water as a function of carbon dioxide production. Their physical activity was measured using an accelerometry-based motion detector.

The investigators found that compared with a control group that had normal BMIs, the healthy underweight individuals consumed 12% less food. They were also considerably less active, by 23%. At the same time, these individuals had higher resting metabolic rates, including an elevated resting energy expenditure and elevated thyroid activity.

Researchers ended that though these very lean people had low levels of activity, their markers of heart health, including cholesterol and blood pressure, were very good,This suggests that low body fat may trump physical activity when it comes to downstream consequences.

References: "People with low BMI aren't more active, they are just less hungry and "run hotter"; CELL PRESS; JOURNAL- Cell Metabolism; DOI: 10.1016/j.cmet.2022.05.012.