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Hot Bodies Stay Lean

High Metabolic Rates Mean Low BMI

A hot metabolism keeps you lean

By John Salak —

Maybe, just maybe, it’s a person’s internal make-up that determines whether their body mass index is running above or below recommended levels and not entirely how much they eat and exercise.

A joint British and Chinese study found that people with low BMI—think skinnier—aren’t necessarily more active and out there running, biking, lifting and lunging. Instead, these researchers discovered that their internal engines (metabolism) just “run hotter.” They are also not as hungry as others.

“We expected to find that these people are really active and to have high activity metabolic rates matched by high food intakes,” reported the corresponding author John Speakman, a professor at the Shenzhen Institutes of Advanced Technology in China and the University of Aberdeen in the UK. “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 research team uncovered this BMI nugget after screening more than 300 people, about half of which had a normal BMI with the other half having a “healthy underweight” BMI. They screened out people with eating disorders who intentionally restrained from eating, those with HIV and individuals who had lost weight in the past six months due to illness or medication.

These participants were then monitored for two weeks based on their food intake, energy expenditure and their physical activity. Surprisingly, compared to those with normal BMIs, the healthy underweight individuals consumed 12 percent less food, and were 23 percent less active but had elevated energy expenditure and thyroid activity rates while resting. In effect, their bodies run hotter, while eating less.

“Although these very lean people had low levels of activity, their markers of heart health, including cholesterol and blood pressure, were very good,” added first author Sumei Hu, at the Beijing Technology and Business University. “This suggests that low body fat may trump physical activity when it comes to downstream consequences.”

The research focus is now being expanded to examine the genetic differences between normal weight and healthy underweight individuals. It is possible single nucleotide polymorphisms in certain genes might play a role in creating these differences.

What does all this mean for overweight or obese individuals looking for weight-loss solutions? It is unclear, but it is possible the new research may result in positive developments for increasing metabolic rates, which could help control weight issues.

In the meantime, researchers recommend those looking to lose weight should focus on a healthy diet and exercise, while admittedly remaining envious of those whose internal engines help keep them lean.

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