Up to 40% of obese people may have ‘metabolically healthy obesity’ under a new definition characterised by a person’s blood pressure, diabetes and waist-to-hip ratio.

While obesity is usually assumed to be associated with higher risks of cardiometabolic disease, researchers have previously pinpointed subgroups of relatively ‘healthy’ obese people as defined by an absence of metabolic syndrome or absence of insulin resistance.

But now US researchers are proposing a new and wider  definition for metabolic health in obesity, based on factors that are linked to hard outcomes of cardiovascular and total mortality.

Using data from more than 12,000 adults in the third National Health and Nutrition Examination Survey (NHANES-III),  their research showed that three factors: blood pressure, diabetes and waist-to-hip ration were significantly linked to mortality outcomes.

Their analysis found that for obese individuals ‘metabolic health’ could be defined as systolic BP less than 130 mm Hg, no BP-lowering medication, waist-to-hip ratio less than 0.95 for women and less than 1.03 for men, and no prevalent diabetes.

For obese people with the three factors, the hazard ratio was 0.68 for cardiovascular mortality and 1.03 for total mortality compared to people with normal weight and the same factors.

Dyslipidaemia was not associated with mortality in their analysis.

The metabolic health definition was validated in a larger UK Biobank cohort of more than 374,000 people, in which people with the same factors were again found to have no significant increase in risk of cardiovascular mortality  (Hazard Ratio 1.17) or  total mortality (0.98).

“This definition classified more than 40% of individuals with obesity as [metabolically healthy] within the NHANES-III population, a representative sample of the US adult population, and 20% within the UK Biobank,” the authors noted.

Conversely, individuals classified as metabolically unhealthy had two-four times increased mortality risk compared with metabolically healthy participants, independent of BMI category in both cohorts.

“Thus, our new definition may be important not only to stratify risk of mortality in people with obesity, but also in people with overweight and normal weight,” they said.

An accompanying commentary said the new definition of metabolic health in obesity appeared to be more robust and consistent than previous definitions based on markers of metabolic syndrome such as insulin resistance.

However it cautioned that while the phenotype may be useful for research purposes, any definition of ‘healthy’ obesity may be misleading and only describe a transitional phase for people who are progressing towards a more risky metabolic condition.

“It would seem that the benefits of metabolically healthy obesity are conditional at best on avoiding weight gain, maintaining ideal waist-to-height ratio with good levels of lifestyle factors (ie, diet, physical activity, and smoking), and avoiding development of metabolic syndrome,” the authors wrote.

“Metabolic dysfunction associated with obesity has important clinical and public health implications. As the evidence continues to mount suggesting that BMI classifications alone are insufficient in capturing an individual’s current or future health status, it is clear that a more robust and universally used definition of obesity phenotypes is needed.”

“The present study provides a prototype of how that definition can be derived, but more rigorous tests and evidence using similar techniques are needed, particularly in prospective studies,” they concluded.

The findings are published in JAMA Network Open.