*Geek Box: Defining ‘Metabolically Healthy’ in BMI Categories
The primary definition of ‘metabolically healthy overweight/obesity’ [MHO] used to date has utilised the criteria for Metabolic Syndrome [MetS]. The criteria for MetS include: waist circumference of <88cm for women or <102cm for men; triglycerides of <150mg/dL or on drug treatment for elevated triglycerides; HDL <50mg/dL for women or <40mg/dL for men; blood pressure <130/85mmHg systolic/diastolic; fasting plasma glucose <100mg/dL or on drug treatment for elevated glucose levels.
To be defined as having MetS, an individual is required to present with 3 or more of these criteria. Researchers to date have defined MHO as having <3, or sometimes <2 of these. However, there is a clear problem with defining metabolic health in this way, because having just one or two of these may be sufficient to relate to long-term disease risk, e.g., elevated fasting plasma glucose levels or blood pressure may predict progression to type-2 diabetes or cardiovascular disease, respectively.
Other researchers have focused specifically on risk factors like insulin resistance, using measures like HOMA-IR, which is calculated from fasting glucose and fasting insulin levels. However, this defines ‘metabolic health’ in very narrow terms, as it excludes the critical and interactive relationship between lipids and metabolic health.
There is also a very strict definition, which would be defined as having none of the MetS criteria and additionally having a total cholesterol level of <240mg/dL [6.2mmol/L]. One important concept to note here is that all these definitions are a priori, meaning they are decided on before a study and then used in the analysis.
However, it may be more useful to conduct an empirical analysis of risk factors across different BMI categories to determine definitions for metabolic health as they relate to specific health outcomes. A key point here is that many of the studies that purport to classify participants as MHO are not, in fact, often studying healthy participants.