BMI's role: 'A blanket approach is not acceptable any longer'
BMI is widely used to classify patients into weight groups and to identify those at higher risk for a variety of conditions, including type 2 diabetes, high blood pressure and certain cancers.
However, according to the Mayo Clinic, the measure has some limitations, including overestimating body fat among patients who are physically fit and muscular, and underestimating body fat percentage in those with lower muscle mass.
“While BMI is flawed, it is still an important measure,” David B. Sarwer, PhD, director of the Center for Obesity Research and Education in the College of Public Health at Temple University, told Healio. “Twenty years ago, many physicians were not calculating the BMI of their patients. As a result, they didn’t have an objective data point to start the conversation about weight and health. Such conversations are critically important if we are going to move the needle around weight management at the individual level.”
Sarwer noted that one issue with BMI is that it “does not readily account for muscle mass.”
“A football player who is 5’10” and 220 lb has a BMI of 31.5 kg/m2 and would be considered to have obesity,” Sarwer said. “That player also could be incredibly muscular and have 6% body fat. As a result, he is less likely to have health problems related to his weight.”
Additionally, he said BMI does not account for the health benefits associated with weight loss.
“For example, the typical patient who undergoes bariatric surgery loses about one-third of their weight and experiences dramatic improvements in their health,” Sarwer said. “However, most of those patients still have a BMI that classifies them as having obesity, even though they are much healthier than they were 1 year ago.”
Research suggests that the use of BMI as an indicator of certain conditions may also be flawed.
A recent study published in The Lancet Diabetes & Endocrinology found that the widely used cutoff of 30 kg/m2 to define obesity would need to be lowered to identify heightened risk for type 2 diabetes among non-white populations.
In the population-based cohort study conducted among patients in England, researchers found that equivalent age- and sex-adjusted incidence of type 2 diabetes to white patients with obesity defined as a BMI of 30 kg/m2 was observed with a BMI of 23.9 kg/m2 among South Asian patients, 28.1 kg/m2 among Black patients, 26.9 kg/m2 among Chinese patients and 26.6 kg/m2 among Arab patients.
“This work highlights that we need evidence for all ethnic groups as they are at risk of diabetes at different levels of BMI,” Paramjit Gill, DM, a study investigator and professor of general practice and head of the division of health sciences at the Warwick Medical School at the University of Warwick, United Kingdom, said in a press release. “A blanket approach is not acceptable any longer.”
According to the CDC, waist circumference can also be used to estimate patients’ disease risk. In this method, patients’ waist circumference is measured above their hip bones just after they exhale. The CDC noted that an elevated risk for obesity-related conditions may be seen in men with a waist circumference more than 40 inches and women who are not pregnant with a waist circumference more than 35 inches.
However, the suggest this method be used as a screening tool rather than a diagnostic test.
“A number of studies have suggested that waist circumference is a more reliable predictor of weight-related health problems,” Sarwer said. “Unfortunately, I don’t think we are at the point in routine clinical care where physicians regularly measure waist circumference. This likely occurs in specialized obesity treatment settings, but probably not in primary care settings.”
Another alternative option to BMI to predict obesity risk is waist-to-height ratio, which may be a more accurate measure to determine risk for obesity than BMI, according to a 2017 study.
The study, published in PLOS One, found that among five different measures of body fat percentage and visceral adipose tissue mass — BMI, waist circumference, waist-to-hip ratio, waist-to-height ratio and waist divided by height0.5 — waist-to-height ratio was the best predictor of both body mass percentage and visceral adipose tissue mass.
“While BMI has its limitations, I think its role in being a place where physicians can have frank, yet respectful, conversations about weight and health should not be overlooked,” Sarwer said. “We need to have more physicians join the battle against obesity, either by offering evidence-based treatments or by making appropriate referrals for treatment.”
- Caleyachetty R, et al. Lancet Diabetes Endocrinol. 2021;doi:10.1016/S2213-8587(21)00088-7.
- CDC. About Adult BMI. Available at: https://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html. Accessed Nov. 12, 2021.
- CDC. Assessing Your Weight. Available at: https://www.cdc.gov/healthyweight/assessing/index.html. Accessed Nov. 12, 2021.
- Mayo Clinic. Mayo Clinic BMI and waist circumference calculator. Available at: https://www.mayoclinic.org/diseases-conditions/obesity/in-depth/bmi-calculator/itt-20084938. Accessed Nov. 12, 2021.
- Study of 1.5 million British people shows BMI cut offs for obesity and diabetes risk do not work for non-white populations. https://www.eurekalert.org/news-releases/862371. Published Nov. 11, 2021. Accessed Nov. 12, 2021.
- Swainson, MG. PLoS One. 2017;doi:10.1371/journal.pone.0177175.