Man watching TV on couch, couch potato

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SHENZHEN, China — Super skinny people are more likely to be couch potatoes than others with a normal or high body mass index (BMI), a new study says. Researchers in China have found that it’s not more activity and less food which keeps many people fitting into a size small, they’re actually just “running hotter” in terms of their metabolism.

“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.”

Scientists have been baffled by “super lean” people who appear to be able to eat what they want while not gaining weight. Defined as having a BMI of less than 18.5, super lean individuals comprise just under two percent of the population in the U.S. Interestingly, the international team reports that these individuals also have a higher rate of sedentary lifestyles than usual. Additionally, they eat slightly less than people with normal BMIs, according to research published in Cell Metabolism.

Super lean individuals had much higher metabolic rates when they were resting, and their cholesterol and triglyceride profiles were indicative of good cardiovascular health.

Researchers used an isotope method to accurately measure energy expenditure and from that could infer their food intake, considered to be more accurate than self-reporting, and accelerometers to objectively measure movement. The team gathered data from more than 150 adults who met the low BMI criteria, and 150 people with normal BMI.

Volunteers were screened to exclude individuals with eating disorders or other illnesses which may affect their diet or mobility. After two weeks, the team found that healthy underweight participants consumed 12 percent less food than heavier individuals — but 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.”

The team is now expanding their research to examine the genetic differences between normal weight and healthy underweight individuals. Preliminary data suggests that single nucleotide polymorphisms in 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.

South West News Service writer Sarah Ward contributed to this report.

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