Skinny people are more likely to be less active than normal or even overweight people, according to a new study out of China.
As the new research reveals, it’s not that these people stay skinny by eating less and doing more exercise. Rather, their metabolism is just running at a higher rate.
“We expected to find that these people are really active and to have high activity metabolic rates matched by high food intakes,” says corresponding author John Speakman, a professor at the Shenzhen Institutes of Advanced Technology in China and the University of Aberdeen, in a media release.
“It turns out that something rather different is going on. They had lower food intakes and lower activity, as well as surprisingly higher-than-expected resting metabolic rates linked to elevated levels of their thyroid hormones.”
Skinny people: the real couch potatoes?
Scientists have been long been puzzled by “super lean” people who appear to be able to eat whatever they want without gaining weight. “Super lean” people are defined as having a BMI of less than 18.5 and make up less than two percent of the population in the U.S.
Researchers used an isotope method to measure energy expenditure as accurately as possible and used the resulting data to infer their food intake, which is considered to be more accurate than self-reporting. Accelerometers were used to measure movement. The team gathered data from more than 150 adults who met the low BMI criteria, and 150 people with normal BMI.
The volunteers were screened to exclude individuals with eating disorders or other illnesses which might affect their diet or mobility. After two weeks, the team found that, although healthy underweight participants consumed 12 percent less food than heavier individuals, they were also 23 percent less active.
However, these super lean individuals also had a higher resting metabolic rate and expended more energy while resting.
“Although these very lean people had low levels of activity, their markers of heart health, including cholesterol and blood pressure, were very good,” says 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.”
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The team now wants to examine the genetic differences between normal weight and healthy underweight individuals. Preliminary data suggest that certain genes could be contributing to this phenomenon.
“The next stage is to understand more about the phenotype itself and understand the mechanisms that generate it more clearly,” Speakman concludes.
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