Those with low BMI are less active and hungry than those with a normal BMI, study says

A recent study found that those with a low body mass index (BMI) are considerably less active than those with a normal BMI.

The study, published in Cell Metabolism, was led by John Speakman, PhD, DSc, a professor at the Shenzhen Institutes of Advanced Technology in China and the University of Aberdeen in the United Kingdom. For his investigation, Speakman and his team sought to better understand obesity through studying those with a low BMI and how they are different from those with a normal BMI.

The study involved 173 people with a normal BMI, ranging between 21.5 and 25 and 150 people who were categorized as “healthy underweight” with a BMI less than 18.5. Those who had an eating disorder, had lost weight because of an illness or medication, or who had intentionally restricted their eating were ruled out of the study. Over the course of two weeks, patients’ food intake was calculated using the doubly labeled water method, which measures energy expenditure through analyzing the difference between turnover rates of hydrogen and oxygen as they relate to carbon dioxide production, and physical activity through an accelerometry-based motion detector.

The study’s results showed that underweight participants consumed 12 percent less food than those with a normal BMI. In addition, researchers found that compared to participants with normal BMIs, participants with low BMIs were 23 percent less active and had a higher resting metabolic rate, including elevated resting energy expenditure and elevated thyroid activity.

"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, PhD, a post doctorate at the Chinese academy of Sciences (CAS) in Speakman’s lab. "This suggests that low body fat may trump physical activity when it comes to downstream consequences."

For future studies, the study’s researchers plan to include additional measures like what participants eat and feelings of satiation in their investigations, as well as take a closer look at the genetic difference between those with varying BMIs.