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Healthy underweight people are actually less active

With the worldwide increase in levels of obesity since the 1960s, studies have often focused on people who have a higher energy intake than their total energy expenditure. Such people are characterized by having a high body mass index (BMI), calculated by dividing mass (in kg) by height squared (in m). 

Body mass index is a useful indicator of those who are overweight (BMI of 25.0–29.9) or obese (BMI > 30), in comparison with those who have a normal weight for their height (BMI 18.5–24.9). However, there is a population of healthy underweight people who feature at the very low end of the BMI curve and they have seldom been studied.

Little is known about the lifestyles, physiology, metabolic rates or genetics of people who have very low BMI values. It is sometimes said of them that they can “eat what they like” without gaining weight, and this is explained potentially by their high levels of physical activity or food-associated thermogenesis. 

A new study by scientists from academic institutions in China and Scotland has tested the hypothesis that the leanness of healthy underweight people is due to their higher levels of physical activity, higher resting energy expenditure and higher total energy expenditure. The researchers recruited 173 people in the normal BMI range and 150 healthy underweight people with BMI values of less than 18.5, in order to compare their lifestyles, physiology and genetics. 

The experts used established questionnaires to screen out people with eating disorders as well as those who said they intentionally restricted their diets and those who were infected with HIV. They also excluded individuals who had lost weight in the past six months potentially related to illness, or were on any kind of medication. They did not rule out those who said they “exercised in a driven way,” but only 4 of the 150 said that they did. 

The participants’ food intake was monitored for two weeks and their total energy expenditure was calculated using an isotope-based technique called the doubly-labeled water method. This technique 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. Their physical activity was measured using an accelerometry-based motion detector. Other physiological variables, such as cholesterol levels and levels of thyroid hormones were also measured from the participants’ blood.

The results, published today in the journal Cell Metabolism, show that healthy underweight people are not more physically active than people with a normal BMI. In fact, the underweight people were 23 percent less active than the participants with normal range BMIs. Despite the fact that their energy expenditure due to physical activity was lower than expected, their overall total energy expenditure was higher than expected. This was due to the fact that underweight individuals had fast metabolic rates while they rested, and this used up more energy than expected. 

“We expected to find that these people are really active and to have high activity metabolic rates matched by high food intakes,” said study co-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 underweight participants ate 12 percent less food than the normal weight group, although this was not less than they would be expected to consume based on their low body weights.  They also had significantly lower levels of free triglycerides and low-density (bad) lipoprotein cholesterol, while the levels of high-density (good) lipoprotein cholesterol and thyroid hormones were significantly higher. The authors state that the underweight individuals were metabolically healthier than the normal individuals, based on the levels of these substances. 

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

The researchers acknowledge some limitations to this research, including the fact that although they measured food intake, they didn’t measure what the participants were actually eating or their feelings of satiation or satiety.

The team is now expanding its research to include studies of these measures. They also plan to look at genetic differences between normal weight and healthy underweight individuals. Preliminary analysis suggests single nucleotide polymorphisms in certain genes that might play a role. When these genetic changes were replicated in mice, the animals had some aspects of the phenotype that was observed in human subjects.

“The next stage is to understand more about the phenotype itself and understand the mechanisms that generate it more clearly,” said Speakman.

By Alison Bosman, Staff Writer

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