People with low BMI aren't more active, they are just less h
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 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. 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.

Source: https://www.sciencedirect.com/science/article/abs/pii/S1550413122001942?via=ihub
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