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Researchers say lower metabolism, not activity levels, can determine a person’s BMI. Westend61/Getty Images
  • Researchers say people with lower body mass index (BMI) measurements aren’t necessarily more active than people with higher BMI numbers.
  • Rather, they say, people with lower BMI tend to eat less and have higher metabolism rates.
  • Some experts say BMI is not an accurate measurement of someone’s overall health.
  • They say a person with a low BMI who also has low muscle mass, for example, may not be as healthy as someone with a higher BMI and higher muscle mass.

A new study says people with lower body mass index (BMI) numbers are frequently less active but tend to eat less and have a lower resting metabolism than people with higher BMIs.

High BMI has typically been used as a yardstick in obesity studies. However, the new study, out of China, looks at people with low BMI, who researchers report are considerably less active than people with a BMI in the normal range. That counters previous thinking that people with low BMI are more active.

The researchers also found that people with lower BMI tend to eat less food than those with normal BMI.

“We expected to find that these people are really active and to have high activity metabolic rates matched by high food intakes,” John Speakman, PhD, DSc, a study author and a professor at the Shenzhen Institutes of Advanced Technology in China and the University of Aberdeen in Scotland, in a statement.

“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,” he added.

Researchers looked at 173 people with a normal BMI (range 21.5 to 25) and 150 they classified as “healthy underweight,” with a BMI below 18.5. They screened for those with eating disorders, people who intentionally undereat, and people with HIV or who lost weight the previous six months related to illness or were on medication.

For two weeks scientists measured food intake and physical activity. When compared to a control group with normal BMIs, researchers found that healthy underweight people ate 12 percent less food and were also 23 percent less active. The subjects also had higher resting metabolic rates, including an elevated resting energy expenditure and elevated thyroid activity.

Some experts told Healthline the findings weren’t surprising, given that BMI is a dated method of measuring overall health.

“(BMI) was developed in the mid-1800s and has been used since with very little changes,” Trista Best, RD, LD, a registered dietitian in Georgia, told Healthline. “BMI does not give an accurate representation of one’s health status for many reasons, men and women alike.”

“This is primarily due to the system using weight as a factor without consideration of the source of the weight or its location on the body,” Best said. “A woman’s weight is primarily carried on her chest and thighs, which does not put her at immediate risk for chronic conditions.”

“The same is true for men and women who have a lot of muscle, which is heavier than fat in most instances,” Best noted. “It would be best to utilize body shape along with BMI or other biometric analysis like blood work, strength, and blood pressure measurements. Because of these misconceptions, a person with a low BMI may not actually be active or considered healthy. The same is true for someone with a high BMI. They may or may not be physically fit.”

Dr. Mir Ali, a bariatric surgeon and medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in California, told Healthline BMI is a reasonable measure of health, with people maintaining a BMI between 18.5 and 24.9 less likely to develop health issues due to weight.

“However, BMI does not take into account body composition,” Ali said. “For example, if someone is very muscular, they may have a higher BMI, but that may not mean they are at risk for health problems.”

“Proper diet and exercise are both important components of a healthy lifestyle, so neither should ever be ignored,” Ali added. “The study in question is looking at a very select group of people where exercise played a less important role. That being said, the majority of weight gain or loss is due to diet.”

“Metabolism is determined by a number of factors: age, gender, activity and genetics all play a role,” Ali said. “Besides exercise, certain foods such as proteins, green tea, and spicy foods may increase metabolic activity for a short period. Eating breakfast and not skipping meals may prevent a decrease in the metabolic rate. Proper sleep and drinking plenty of water also play a crucial role.”

Some experts say there are already better ways to assess health.

“I would argue that VO2 max/exercise tolerance, cardiac numbers, glucose levels, and lean mass vs. fat mass might be better indicators than just BMI,” Dana Ellis Hunnes, PhD, RD, a senior clinical dietitian at UCLA Medical Center and an assistant professor at the University of California Los Angeles Fielding School of Public Health, told Healthline.

“We can outeat any exercise we do in a matter of minutes if we really want to. Just eat a cheeseburger, fries, and shake, and you’ve outeaten more than an hour of exercise,” she said. “So, in that sense, what we eat is more important than exercise for our weight/BMI. But, for overall cardiovascular health, exercise is very important.”

Jennie Graham, BSc, a nutrition expert and certified personal trainer at Bodycomp Imaging, told Healthline that “BMI only takes into consideration weight and height – and they don’t tell the whole story.”

“We can have two people with the same BMI, but one is far healthier than the other,” Graham said. “That’s because one of them might have only 15 percent of that weight in body fat, while the other has 35 percent. Or maybe one of them has more visceral fat than the other.”

“The first thing you need to do in order to begin, or improve, your healthy lifestyle is to know where you currently stand. I don’t mean just the number on the scale – you need to know your actual body composition,” she said. “By knowing your exact body fat, muscle mass, visceral fat and bone density, a professional will be able to guide you toward to right path for your goals.”

Being a lower weight may fool people into believing they’re healthy, says Matt Scarfo, a NASM-certified personal trainer and running coach.

“Being lighter weight means that you likely have low muscle mass and fewer energy stores to rely on if you want to be active,” Scarfo told Healthline. “When we move our bodies, our muscles first look to immediately available nutrients like glycogen to fuel them. However, if you have low muscle mass, or aren’t active already, your body may not efficiently produce glycogen to fuel activity.”

“Rather than relying on weight, or BMI – which is a ratio between weight and height – for health, it’s better to look at lifestyle to establish if someone is truly healthy,” Scarfo said. “For example, if someone is active multiple times a week, eats a diet rich in vegetables and lean proteins, doesn’t drink, and doesn’t smoke, they’re likely pretty healthy, even if they’re overweight.”

“Those habits can help them resist getting sick, move their bodies effectively, and resist injury,” he added. “Focusing on behavior-based evidence of health, instead of physical signs like weight, is often the best way to deduce someone’s wellness.”