Genomenon said today it has partnered with Rhythm Pharmaceuticals to create a database of genetic mutations associated with obesity as documented in published studies. Rhythm plans to use the data to better understand rare genetic disorders of obesity. [Visual Mozart/Getty Images]
Credit: Visual Mozart/Getty Images

Some people may be healthy and very lean due to their food consumption and a high metabolism rather than an active lifestyle, a study suggests.

The research found that healthy, underweight adults—who are often thought of being able to “eat what they want”—were nearly a quarter less active than those with a body mass index (BMI) within the normal range.

However, they also ate around an eighth less food than those with a normal BMI and had elevated resting energy expenditure and thyroid hormone activity.

The healthy, underweight group also had a significantly better blood lipid profile than the other group, suggesting low body fat could be more important for cardiovascular health than physical activity.

The findings are published in the journal Cell Metabolism.

“We expected to find that these people are really active and to have high activity metabolic rates matched by high food intakes,” said researcher John Speakman, a professor at the Shenzhen Institutes of Advanced Technology in China and the University of Aberdeen in the U.K.

“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 researchers recruited 173 people with a normal BMI of at least 21.5 but less than 25 and compared them with 150 volunteers who were termed “healthy underweight” and had a BMI below 18.5.

Questionnaires were used to screen out people with eating disorders, those who said they intentionally restrained their eating and those with HIV.

People whose weight loss in the past six months might be related to illness, or those on any kind of medication, were also not included.

The participants were monitored for 2 weeks, during which time food consumption and physical activity were measured.

Results showed that total energy expenditure in healthy, underweight adults was 12.1 per cent less than for people with a normal BMI. Furthermore, physical activity in these lean adults was 23.3 per cent lower than in people with a normal BMI.

However, the lean group also ate around 12 per cent less than those with a normal BMI, albeit more than expected for their weight.

Their resting energy expenditure was significantly higher than expected compared with the other group. This correlated with elevated thyroid activity, with significantly increased free thyroxine, free triiodothyronine and total triiodothyronine.

In addition, levels of triglycerides and low-density lipoprotein cholesterol were significantly lower in the healthy underweight individuals, and high-density lipoprotein cholesterol levels significantly higher.

“Although these very lean people had low levels of activity, their markers of heart health, including cholesterol and blood pressure, were very good,” said researcher 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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