Terms such as "metabolically healthy obese" do not accurately describe who is at risk of certain diseases, leading to erroneous predictions about who needs monitoring for conditions such as type 2 diabetes, according to research published in Diabetes Care.
Definitions based on weight may be less helpful than has been thought in predicting not only type 2 diabetes, but also cardiovascular disease, high fasting blood sugar or triglyceride levels and high blood pressure, all of which have been linked with obesity.
While these conditions often do co-exist, not all obese individuals have these metabolic risk factors, and conversely, some people with low body mass index (BMI) do have them.
The concept of "metabolically healthy obesity" has gained currency in recent years. The opposite has been referred to as the "metabolically unhealthy lean."
A meta-analysis has been undertaken by scientists at the Medical Research Council (MRC) Epidemiology Unit at the University of Cambridge in the UK, together with colleagues from the European Federation of Pharmaceutical Industries and Associations to examine the validity of the definitions.
'Healthy obese' at higher risk than 'unhealthy lean'
Researchers studied data relating to 140,845 people from 14 different cohort studies assessing the relative risk of developing type 2 diabetes in lean, overweight and obese individuals.
Participants in the studies had been classified as either metabolically healthy or unhealthy.
In follow-up, 5,963 cases of type 2 diabetes were diagnosed. Individuals classified as metabolically unhealthy were found to have a higher relative risk of type 2 diabetes, compared with individuals classified as healthy in terms of BMI and adiposity.
The authors estimated the absolute risk of developing type 2 diabetes at 5 and 10 years in each of the six BMI and metabolic health categories.
The cumulative incidence of type 2 diabetes over 10 years was:
3.1% for "metabolically healthy obese" individuals
2.2% for "metabolically unhealthy lean" individuals
7.6% for "metabolically unhealthy obese" individuals.
In lean individuals, testing metabolic health was not sensitive (40%) but was specific (88%); in obese individuals, sensitivity was satisfactory (81%) but specificity was low (42%).
This would seem to indicate that simplistic definitions of obesity and metabolic health currently in use have limited value in a clinical setting.
For example, while "metabolically healthy obese," people are at lower risk of type 2 diabetes than those who are "metabolically unhealthy obese," they still have a higher absolute risk of developing the disease than people who are lean but "metabolically unhealthy."
Dr. Luca Lotta, of the MRC Epidemiology Unit, who led the analysis, says:
"As the proportion of people who are obese continues to increase, it is important to identify those who are at greatest risk of developing type 2 diabetes and who may therefore benefit most from focused preventive actions. However, there are several different definitions of metabolic health in use, and it has not been shown that these definitions are useful in identifying individuals at a high risk of developing type 2 diabetes in a clinical setting."
Prof. Nick Wareham, director of the MRC Epidemiology Unit, emphasizes the importance of continuing research to identify biological markers that will enable better prediction of the metabolic health consequences of obesity, so that individual-level interventions can be targeted at those at highest risk.
As all the obese categories are at increased risk, population-level interventions are needed to reduce the prevalence of obesity.