People With Low BMIs Aren’t Necessarily More Physically Active, Shows Study


Jul 19, 2022


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There are a lot of untruths, scientific and cultural, about health and fitness, that we have come to embrace. Chief among them is that of the body mass index (BMI), a value calculated using people’s weight and height. For decades, efforts to address obesity have thus found a high BMI indicative of poor health, of “fatness” that is then shamed for being a direct result of lack of exercise. Conversely, it also fuels the false notion that people with low BMI have a metabolism that makes them naturally more active; they can “eat what they like” because of perceived high levels of physical activity.

The myth of the BMI is tireless. But a recent study pierces this idea — finding that people with a low BMI, who are “underweight,” often exercise less than their normal-weight counterparts. This contradicts the belief that low BMI automatically guarantees a healthy metabolism which makes people more active; rather higher metabolism may be the reason why some people have lower BMI without being more physically active. The research, thus, not only points to further evidence on how BMI can be an incredibly misleading index of health due to its association with physical fitness, but also holds implications on how we present exercise as a solution to obesity or a higher BMI. It could then improve healthcare by taking away the inadvertent focus on weight in medical assessments of people’s health.

The study, published in the journal Cell Metabolism, looked at 173 people with a normal BMI (range 21.5 to 25) and 150 who they classified as “healthy underweight,” with a BMI below 18.5. The formula to calculate BMI is to divide a person’s weight in kilograms by their height measured in meters squared. A BMI of 25.0 or more is termed as overweight. The participants were in China, and researchers screened the participants for any eating disorders or HIV infection, and asked if they restrained their eating in any way. The participants’ eating habits and physical activity were then monitored over two weeks, to understand the degree of credibility that links metabolism with BMI.

“We expected to find that these people [with low BMI] 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.”

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They ate 12% less food than people in the normal weight group and were found to have relatively better metabolic health — which is gauged by indicators such as lower levels of free triglycerides and low-density (bad) lipoprotein cholesterol.

What this means is people’s BMI had little to do with their physical exercise levels; they still had healthy cholesterol and blood pressure, but just ate less. “This suggests that low body fat may trump physical activity when it comes to downstream consequences,” said study first author Sumei Hu.

There are some limitations to the study. It doesn’t account for what the participants were actually eating or if they felt full or satisfied afterward; they measured the food intake levels only. Arguably, there is a lot of merit in looking at the components of meals as well as levels of satiety — something that experts plan to study eventually.

But the research is instructive in identifying the gaps in how we study and understand obesity. Obesity is a growing concern in India too; recent reports found an increase in obesity levels among adults. Experts have attributed the rise to sedentary lifestyles and lack of exercise.

Since obesity is intertwined with a higher BMI, health and welfare policy have scrutinized individuals with a high BMI, looking specifically at the total energy consumption. The link between low BMI and physical activity has also meant that interventions like weight loss and exercise have been upheld as the more effective, sustaining ways of being healthy. Only recently did an Indian workplace encourage people to reduce their BMI as a “healthy” intervention, even offering them bonuses in exchange for reducing their BMIs.

A growing body of research has over the years debunked BMI. Since it bases itself only on the height and weight of a person, is remains “an inaccurate measure of body fat content and does not take into account muscle mass, bone density, overall body composition, and racial and sex differences,” said researchers from the Perelman School of Medicine, University of Pennsylvania. So, to use this index as the end-all and be-all of all fitness and obesity policies is not only misleading and unscientific, but detrimental in the long run too. BMI remains a poor proxy of fat mass in the body. “Obesity needs to be redefined… Other factors should be taken into account too: blood parameters, cholesterol, sugars, insulin, and lifestyle all need to be accounted for [not just appearance, height, or weight],” Dr. Ankita Gharge, a gynecologist, also told The Swaddle.

Instead, if we understand health indicators of people with low BMI, those who are underweight and have traditionally been linked with physical activity, we may be better equipped to understand and address obesity as a health challenge. What the current study tells us is exercise is not always the key to losing weight, leanness, or low BMI in general. There are better ways to approach fitness, ones that don’t inherently fat-shame people by clutching on to ideas that have long been proven untrue.


Written By Saumya Kalia

Saumya Kalia is an Associate Editor at The Swaddle. Her journalism and writing explore issues of social justice, digital sub-cultures, media ecosystem, literature, and memory as they cut across socio-cultural periods. You can reach her at @Saumya_Kalia.


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