The study also found that people with low BMI eat less and “run hotter.”
The majority of obesity research to date has concentrated on examining people with high body mass indexes (BMI), but a Chinese research team is using a different strategy. In a study that was published in the journal Cell Metabolism, the researchers focused on those with very low BMIs. Contrary to the belief that they have a metabolism that makes them inherently more active, their data show that these individuals are really much less active than those with a BMI in the usual range. They also consume less food than those with a normal BMI.
“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 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.”
150 participants who were “healthy underweight” (with a BMI below 18.5) and 173 people with normal BMIs (range 21.5 to 25) were recruited by the researchers. They screened out individuals with eating problems, those who claimed to have purposefully restricted their eating, and those who were HIV-positive. Additionally, they disqualified those who had lost weight recently that may have been caused by an illness or who were using any form of medicine. Only 4 out of 150 people claimed to “exercise in a driven way,” but they did not exclude them.
The participants were observed for two weeks. Their food intake was determined using the doubly-labeled water method, an isotope-based approach that gauges energy expenditure by comparing the turnover rates of hydrogen and oxygen in body water as a function of carbon dioxide production. Their physical activity was tracked using an accelerometry-based motion detector.
The investigators found that compared with a control group that had normal BMIs, the healthy underweight individuals consumed 12% less food. They were also considerably less active, by 23%. At the same time, these individuals had higher resting metabolic rates, including elevated resting energy expenditure and elevated thyroid activity.
“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 investigators acknowledge some limitations of 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, including studies that include 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,” says Speakman.
Reference: “Higher than predicted resting energy expenditure and lower physical activity in healthy underweight Chinese adults” by Sumei Hu, Xueying Zhang, Marina Stamatiou, Catherine Hambly, Yumeng Huang, Jianfang Ma, Yiran Li and John R. Speakman, 14 July 2022, Cell Metabolism.
The study was funded by the National Key Research and Development Program of China and the National Natural Science Foundation of China.