Results from a large international study test a new definition of obesity that includes waist circumference and waist-to-height ratio and show it better reflects risk of death than body mass index (BMI) alone.
The historical definition classes people of European descent with a BMI at or above 30 kg/m2 as obese, with slightly varying numbers in other countries or regions, and also looks at comorbidities to try and determine health risk. However, this method has long been criticized as inaccurate.
“The absence of a clinically grounded definition has impeded recognition of obesity as a distinct disease, contributing to gaps in care, inconsistent prevention, misallocated resources and persistent stigma,” write the study authors in Nature.
Last year, a Lancet Diabetes & Endocrinology Commission article proposed a new way of defining and diagnosing people with obesity. It suggested splitting the condition into preclinical and clinical groups. For both definitions, individuals had to either have a BMI of 40 kg/m2, a BMI of 30 kg/m2 (28 kg/m2 or above for Chinese populations) plus an elevated waist circumference or waist-to-height ratio, or a BMI of 25–29.9 kg/m2 (24–27.9 kg/m2 for Chinese populations) plus an elevated waist circumference and waist-to-height ratio.
Preclinical obesity was defined as this body size definition, but no organ or daily-function impairment, whereas clinical obesity was defined as having the same body size limits plus at least one of 18 obesity-related organ or functional impairments such as sleep apnea or heart failure.
The current study used data from 844,108 adults in China, the U.K., the U.S., and Spain to test this diagnostic framework. The team examined prevalence, progression and deaths in the different groups. They also statistically estimated what proportion of organ dysfunction was attributable to obesity rather than other causes.
The results showed that BMI alone overestimated obesity in up to 3.1% of people and underestimated it in up to 8.6% of people. Overall, the new definition increased overall prevalence by up to 7.4% versus BMI alone, with clinical obesity accounting for 22.7-31.3% of this group.
The researchers found that people whose BMI put them in the overweight range, but whose waist circumference and waist-to-height ratio were both elevated, had a 6% higher death risk than people who were not overweight or obese. Similarly, people whose BMI put them in the obesity range and who also met the study’s additional criteria had a 7% increase in death risk than people below the BMI overweight threshold.
People classed as obese by the old definition of BMI alone showed no increased mortality, which suggests the additional measurements helped distinguish higher-risk individuals.
Individuals classed as having clinical obesity had a 69% higher death risk than people with neither obesity nor organ dysfunction, whereas those in the preclinical obesity group had a smaller increase of 19% versus those without obesity or organ dysfunction.
“This study shows that up to a third of all cases of obesity are clinical obesity, an active disease that should be treated as such. Most people with preclinical obesity, but not all, remained free of clinical obesity for a decade. Preclinical obesity should therefore be neither treated automatically as active disease nor dismissed as harmless. Some people may need preventive treatment, but its intensity should match their individual risk,” said Francesco Rubino, MD, senior author of the study and chair of Metabolic and Bariatric Surgery at King’s College London, in a press statement.
“Body mass index alone cannot tell us whether someone is already ill or at risk of illness, or what care they need, leaving people vulnerable to both undertreatment and overtreatment. The new definition makes personalized, proportionate care possible whereas a uniform BMI-based definition prevents it.”
