"Healthy Underweight" Individuals Topple Research Expectations
Obesity is a major global health problem, and most research to date has focused on studying those with the highest body mass index (BMI). However, a research group in China is taking a different approach to understanding human metabolism and its effect on BMI.
These scientists focused on individuals with very low BMI and obtained results that contradicted their assumptions about metabolism in low BMI individuals. Their findings were published in Cell Metabolism on July 14.
"We expected to find that these people are really active and have high activity metabolic rates matched by high food intakes," said corresponding author John Speakman, professor at the Shenzhen Institute of Advanced Technology (SIAT) of the Chinese Academy of Sciences 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 researchers recruited 173 people with normal BMI (ranging from 21.5 to 25) and 150 people who they classified as "healthy underweight" (with BMI below 18.5).
They used established questionnaires to screen out people with eating disorders as well as those who said they intentionally restrained their eating and those who were infected with HIV. They also excluded individuals who had lost weight in the past six months potentially related to illness and people who were taking any kind of medication. They did not rule out those who said they "exercised in a driven way," but only four out of 150 said they did.
Body composition was determined using a bioimpedance method that measures levels of water and body fat. BMI was verified by measuring the study subjects' height and weight.
The participants were monitored for two weeks. Their food intake was measured with an isotope-based technique called the doubly labeled water method, which 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.
The researchers found that the healthy underweight individuals consumed 12% less food compared with the control group with normal BMI. They were also 23% less active. At the same time, these individuals had higher resting metabolic rates, including an 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," said first author HU Sumei, currently at Beijing Technology and Business University. "This suggests that low body fat may trump physical activity when it comes to downstream consequences."
The researchers acknowledge some limitations in this research. For example, they measured total food intake but didn't record what foods the participants ate or their feelings of satiation or satiety.
The team is now expanding its research to include studies that measure these factors. They also plan to look at genetic differences between normal weight and healthy underweight individuals. Preliminary analysis suggests that single nucleotide polymorphisms in certain genes 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 its mechanisms more clearly," said Prof. Speakman.