While most studies of obesity have looked at those who already have a high BMI, a team of scientists in China has taken a new approach.

In a study that was released today in the journal Cell Metabolism, the researchers focused on people with extremely low BMIs.

Contrary to the belief that they have a metabolism that makes them naturally more active, their data show that these individuals are really significantly less active than those with a BMI in the normal range.

They also consume less food than people with a normal BMI.

According to corresponding author John Speakman, “we expected to find that these people are really active and to have high activity metabolic rates matched by high food intakes.” 

It appears that something quite distinct is taking place. Their resting metabolic rates were unexpectedly greater than expected due to higher thyroid hormone levels, and they also had lower food consumption and activity levels.

The researchers recruited 173 individuals with a normal BMI (range: 21.5 to 25) and 150 “healthy underweight” individuals (with a BMI below 18.5).

They screened out those with eating problems, those who claimed to have purposefully restricted their eating, and those who were HIV-positive using standard questionnaires.

Additionally, they disqualified anyone who had lost weight in the previous six months who might have been due to illness or were using any sort of medicine.

They did not exclude those who indicated they “exercised in a driven way,” although only four out of 150 individuals claimed to do so.

For two weeks, the individuals were observed. The doubly-labeled water approach, which gauges energy expenditure based on the difference between the turnover rates of hydrogen and oxygen in body water as a function of carbon dioxide production, was used to measure the subjects’ dietary intake.

An accelerometer-based motion detector was used to gauge their level of physical activity.

They discovered that the healthy underweight people consumed 12% less food than a control group with normal BMIs.

Additionally, they were 23% less active than usual. These people also exhibited higher resting metabolic rates, which included higher levels of thyroid activity and resting energy expenditure.

Despite having low levels of activity, the very slim individuals had excellent blood pressure and cholesterol readings, according to the study’s first author Sumei Hu.

This shows that when it comes to downstream effects, low body fat may outweigh physical exercise.

The researchers admit that this study has some limitations. For example, even though they measured how much food the participants ate, they didn’t measure what they were actually eating or how full they felt.

The group is currently broadening its investigation to incorporate studies with these indicators.

Additionally, they intend to investigate genetic variations between healthy underweight people and people of normal weight.

Preliminary study indicates that single nucleotide polymorphisms in specific genes may play a role.

When these genetic alterations were duplicated in mice, the rodents exhibited certain characteristics of the human phenotype.

The next step is to have a clearer understanding of the phenotype’s underlying mechanics, according to Speakman.

Image Credit: Getty

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