By Gwyn Wright via SWNS
Super-skinny people stay thin not because they do more exercise than the average person but because they eat less food, according to a new study.
Researchers said those who stay thin avoid piling on the pounds because they are less hungry than others who often do a lot more exercise.
A team in China argued people with a very low body mass index are “considerably” less active than those with a normal weight but also eat less food than people with an average BMI.
The thinnest among us do have a higher than normal resting metabolic rate, the scientists found.
The team discovered the thin people ate 12 percent less food and did 23 percent less exercise.
While the lean people may not appear super-fit, their heart health- including cholesterol and blood pressure levels- were very good.
For the study, a team led by researchers in China recruited 173 people with a normal BMI of 21.5 to 25 and 150 people who were classed as “healthy underweight” with a BMI below 18.5.
They used questionnaires to screen out people with eating disorders as well as those infected with HIV.
People who had lost weight in the past six months because of an illness or medication were also not allowed to participate.
The team did not rule out people who said they “exercised in a driven way” but only four out of the 150 super-skinny people said they did.
Participants were followed up for two weeks.
Their food intake was measured using a complex isotope-based technique called the doubly-labelled water method.
This method assesses energy expenditure based on the difference between the turnover rates of hydrogen and oxygen in body water as a function of carbon dioxide production.
Physical activity was measured using a wearable motion detector.
Professor John Speakman, the study’s corresponding author from the University of Aberdeen, said: “We expected to find that these [very thin] people are really active and to have high activity metabolic rates matched by high food intakes.
“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.”
The study’s first author Dr. Sumei Hu from Beijing Technology and Business University said: “Although these very lean people had low levels of activity, their markers of heart health, including cholesterol and blood pressure, were very good.
“This suggests that low body fat may trump physical activity when it comes to downstream consequences.”
The team now want to do similar experiments which explore what participants were eating and their feelings of hunger or fullness, which were not measured in the research.
They also plan to look at genetic differences between people who weigh a normal amount and underweight people.
The findings were published in the journal Cell Metabolism.
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