Study finds people with low BMIs are not more active, they just eat less

People with lower body mass index (BMI) are considerably less active than people in the normal range, a study has shown. 

The research group, based in China, found that such people also ate less food than those with a normal BMI. The index is a measure of body fat based on height and weight that applies to adult men and women.

Earlier, it was believed that people with low BMIs have a metabolism that makes them naturally more active. 

The researchers expected such people to have high activity metabolic rates matched by high food intakes.

However, they found something much different. “It turns out people with a very low BMI had lower food intakes and lower activity levels,” said corresponding author John Speakman. 

Speakman is a professor at the Shenzhen Institutes of Advanced Technology in China and the University of Aberdeen in the UK.

“Although these very lean people had low levels of activity, their markers of heart health, including cholesterol and blood pressure, were very good,” said first author Sumei Hu, currently at the Beijing Technology and Business University.

This suggests that low body fat may trump physical activity when it comes to downstream consequences, said Hu. 

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 study was published in the journal Cell Metabolism.

They used 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. 

The study also excluded individuals who had lost weight in the past six months potentially related to illness or were on any medication. They did not rule out those who said they “exercised in a driven way,” but only 4 of 150 participants said they did. 

For two weeks, the scientists measured their food intake and physical activity.

Compared with the control group with normal BMIs, the “healthy underweight” individuals consumed 12 per cent less food. They were also less active by 23 per cent. 

The study found the subjects had an elevated resting energy expenditure and thyroid activity.

The investigators acknowledge some limitations of the research, including the fact that although they measured food intake, they didn’t measure what the participants were actually eating or their feelings of satiation.

The team is now expanding its research that will include these measures. They also plan to examine genetic differences between normal weight and healthy underweight individuals.