Share on PinterestResearchers 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 are not necessarily more active than people with higher BMI numbers.
- Rather, they say, people with a lower BMI tend to eat less and have higher metabolism rates.
- Some experts say that BMI is not an accurate measure of someone’s overall health.
- They say that 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 a lower body mass index (BMI) tend to be less active, but tend to eat less and have a lower resting metabolism than people with a higher BMI.
High BMI has typically been used as a criterion in obesity studies. However, the new study, outside of China, looks at people with a low BMI, which researchers report are considerably less active than people with a BMI in the normal range. This contrasts with the previous thought that people with a low BMI are more active.
The researchers also found that people with a lower BMI tend to eat less food than those with a normal BMI.
“We hoped to find that these people are really active and have high metabolic activity rates matched by a high food intake,” John Speakman, PhD, DSc, study author and professor at the Shenzhen Institute of Advanced Technology at China and the University. from Aberdeen to Scotland, in a statement.
“It simply came to our notice then. They had lower food intake and less activity, as well as resting metabolic rates surprisingly higher than expected related to high levels of their thyroid hormones, ”he added.
The researchers analyzed 173 people with a normal BMI (range 21.5 to 25) and 150 rated them as “healthy underweight,” with a BMI below 18.5. They examined those with eating disorders, people who ate badly intentionally, and people with HIV or who had lost weight in the previous six months related to the disease or were taking medication.
For two weeks, the scientists measured food intake and physical activity. Compared to a control group with a normal BMI, the researchers found that healthy, low-weight people ate 12% less food and were also 23% less active. Subjects also had higher resting metabolic rates, including high resting energy expenditure and high thyroid activity.
Some experts told Healthline that the findings were not surprising, as BMI is an old method of measuring overall health.
“(BMI) was developed in the mid-1800s and has been used ever since with very few changes,” Trista Best, RD, LD, a registered dietitian in Georgia, told Healthline. “BMI does not provide an accurate representation of a person’s health status for many reasons, both men and women.”
“This is mainly because the system uses weight as a factor regardless of the source of the weight or its location in the body,” Best said. “A woman’s weight is mainly carried on her chest and thighs, which does not put her at immediate risk of suffering from chronic diseases.”
“The same goes for men and women who have a lot of muscle, which is heavier than fat in most cases,” Best noted. “It would be better to use body shape along with BMI or other biometric tests such as blood tests, strength and blood pressure measurements. Because of these misconceptions, a person with a low BMI can not being really active or considered healthy. The same goes for someone with a high BMI. They may or may not be physically fit. “
Dr. Mir Ali, bariatric surgeon and medical director of MemorialCare Surgical Weight Center at Orange Coast Medical Center in California, told Healthline that BMI is a reasonable measure of health, with people maintaining a BMI between 18.5 and 24.9 less likely to develop health problems. 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 doesn’t mean they are at risk for health problems.”
“Proper diet and exercise are important components of a healthy lifestyle, so it should never be ignored,” Ali added. “The study in question studies a very select group of people where exercise played a less important role. That said, most of the weight gain or loss is due to diet.”
“Metabolism is determined by a number of factors: age, gender, activity and genetics play a role,” Ali said. “In addition to exercise, certain foods such as protein, green tea and spicy foods can increase metabolic activity for a short period of time. Breakfast and not skipping meals can prevent a decrease in metabolic rate. properly and drinking plenty of water also play a crucial role. “
Some experts say there are already better ways to assess health.
“I would say that maximum VO2 / exercise tolerance, heart numbers, glucose levels and lean mass versus fat mass could 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 get over any exercise we do in a matter of minutes if we really want to. Just eat a cheeseburger, chips and smoothie, and you’ve done more than an hour of exercise,” he said. “So in that sense, what we eat is more important than exercise for our weight / BMI. But for cardiovascular health in general, exercise is very important.”
Jennie Graham, BSc, a nutrition expert and certified personal trainer at Bodycomp Imaging, told Healthline that “BMI only takes into account weight and height, and they don’t tell the whole story.”
“We can have two people with the same BMI, but one is much healthier than the other,” Graham said. “That’s because one of them may 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 to start or improve your healthy lifestyle is to know where you are today. I’m not just referring to the scale number, but you need to know your actual body composition, ”he said.“ By knowing your exact body fat, muscle mass, visceral fat and bone density, a professional will be able to guide you towards right path for your goals “.
Having a lower weight can fool people into believing they are healthy, says Matt Scarfo, personal trainer and NASM-certified running trainer.
“Having a lighter weight means you’re likely to have less muscle mass and less energy reserves to rely on if you want to be active,” Scarfo told Healthline. “When we move our bodies, our muscles first look for immediately available nutrients such as glycogen to feed them. However, if you have little muscle mass or are not yet active, your body may not produce glycogen efficiently to feed. the activity “.
“Instead of relying on weight or BMI, which is a relationship between weight and height, for health, it’s better to look at lifestyle to determine if someone is really healthy,” Scarfo said. “For example, if someone is active several times a week, eats a diet rich in vegetables and lean protein, doesn’t drink and doesn’t smoke, they’re likely to be quite healthy, even if they are overweight.”
“These habits can help them resist getting sick, move their body effectively, and resist injury,” he added. “Focusing on evidence-based behavioral health, rather than physical signs like weight, is often the best way to deduce someone’s well-being.”