To date most research on obesity has focused on studying those with a high body mass index (BMI), but a research group is now taking a different approach. 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.

Source: People with low BMI aren't more active, they are just less hungry and 'run hotter'