- Healthcare professionals use body mass index (BMI) to determine if they should screen people and give them lifestyle advice or drugs to prevent type 2 diabetes.
- However, the cutoff level for obesity was based on studies of BMI and mortality in exclusively white populations in Europe and the United States.
- A new study from the United Kingdom suggests that South Asian, Chinese, and Arab populations have the same risk of diabetes as white people at significantly lower BMIs.
- This means that the health benefits of preventive treatments and advice are not fully realized for these populations.
In 1993, the World Health Organization (WHO) recommended BMI thresholds for diagnosing overweight and obesity. A person’s BMI is their weight in kilograms (kg) divided by the square of their height in meters (m²).
The WHO based these cutoff levels on studies that investigated the association between BMI and mortality in white populations in Europe and the U.S.
Obesity was defined as a BMI of 30 kg/m² or higher, and overweight was defined as a BMI of 25 kg/m² or higher.
Research subsequently found that Asian populations have a higher risk of type 2 diabetes than white populations at lower BMIs.
This is an important distinction because making dietary changes, exercising, and taking medications that lower glucose levels in the blood can delay or even prevent the onset of disease.
Because doctors use the cutoff for obesity to decide whether or not to screen people for diabetes and deploy these interventions, high levels of risk among Asian people may go unrecognized.
As a result, the WHO and the National Institute for Health and Care Excellence in the U.K. recommended lowering the BMI cutoff for obesity to 27.5 kg/m² for people of South Asian and Chinese origin.
However, a study of almost 1.5 million people in the U.K. now suggests that this cutoff remains too high for South Asian, Chinese, and Arab populations.
“This study shows that the one-size-fits-all approach really does not work for BMI and type 2 diabetes risk when we look at the U.K.’s diverse population,” says lead study author Dr. Rishi Caleyachetty, a junior doctor at Warwick Hospital and an epidemiologist at the University of Warwick, both in the U.K.
“As a doctor, I’m extremely concerned that if the current BMI values used for BAME [Black, Asian, and Minority Ethnic] people are not appropriately amended, many BAME people will needlessly slip through the net, leaving them unknowingly at risk of type 2 diabetes.”
The researchers presented the results at this year’s European Congress on Obesity, and they published them in
Ethnicity-specific BMI cutoffs
The team used electronic health records to identify 1,472,819 people aged 18 years or older without an initial diagnosis of type 2 diabetes and who had at least 1 year of follow-up data.
Of these people:
- 90.6% were white
- 5.2% were South Asian
- 3.4% were Black
- 0.7% were Chinese
- 0.2% were Arab
After a median follow-up period of 6.5 years, 6.6% of these people received a diagnosis of type 2 diabetes.
After accounting for age and sex, the researchers calculated ethnicity-specific BMI cutoffs that were associated with the same diabetes risk as that for white people with a BMI of 30 kg/m² or higher.
The BMI cutoffs were:
- 23.9 kg/m² for South Asian populations
- 28.1 kg/m² for Black populations
- 26.9 kg/m² for Chinese populations
- 26.6 kg/m² for Arab populations
As Dr. Caleyachetty concludes:
“A complete revision of ethnic-specific BMI cutoffs to trigger action to prevent type 2 diabetes [is] needed. This will ensure [that] healthcare professionals provide appropriate recommendations for BAME patients regarding lifestyle changes, referrals to weight management services, and investigations for type 2 diabetes.”
The study paper reports that it is unclear whether the higher risk of diabetes among non-white populations at lower BMIs is due to differences in body composition, differences in lifestyle — such as physical activity and diet — genetic factors, or an interaction between genes and lifestyle.
Medical News Today asked Dr. Caleyachetty if he thought that other countries, such as the U.S., should conduct similar studies to determine ethnicity-specific BMI cutoffs for their own populations.
“The U.S. has a different ethnic mix than the U.K., and it would be worthwhile for U.S. investigators to […] replicate this study at scale,” he said.
He said that differences in lifestyle may play a role in any local differences. The original WHO recommendations for BMI were based on the association between mortality and BMI in European and North American populations.
“The statistical models only adjusted for age and sex,” he said. “But they acknowledged the role of different lifestyle[s] on the association.”
Is BMI fatally flawed?
In their paper, the study authors note that their findings only apply to the risk of developing type 2 diabetes and may not apply to other outcomes, such as cardiovascular disease or mortality from all causes.
They also acknowledge that differences in body composition between ethnic groups may help explain the differences in the risk of diabetes.
However, doctors in the U.K. do not routinely record relevant body composition measurements, such as waist-to-hip ratio and total body fat, they write.
BMI has widely recognized limitations. In an accompanying
David E. Cummings, at the University of Washington Medicine Diabetes Institute in Seattle, and Francesco Rubino, from King’s College Hospital in London, U.K., say that the same ethnic differences in BMI cutoffs might apply to other obesity-related conditions.
They write:
“BMI is not a clinically meaningful measure. Looking at obesity as a disease, BMI could not be conceivably used as diagnostic evidence of illness. It might be time to ponder whether we should use BMI at all to define obesity.”
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