Metabolically healthy obesity, transition to unhealthy phenotypes, and type 2 diabetes in 0.5 million Chinese adults: the China Kadoorie Biobank

OBJECTIVES

To prospectively assess the association of metabolic health status and its transition with incident diabetes risk across BMI categories.

DESIGN

Cohort study based on the China Kadoorie Biobank (CKB).

METHODS

The CKB study enrolled 512 715 adults aged 30-79 years from ten diverse areas in China during 2004-2008. After exclusion, 432 763 participants were cross-classified by BMI categories and the metabolic status was followed up for incident diabetes disease. The changes in BMI and metabolic health status were defined from baseline to the second resurvey.

RESULTS

Type 2 diabetes risk is higher for metabolically healthy obese (MHO) subjects than metabolically healthy normal weight (MHN) individuals (HR: 3.97, 95% CI: 3.64-3.66), and it is highest for those affected by metabolically unhealthy obese (MUO) (HR: 6.47, 95% CI: 6.17-6.79). About 15.26% of participants with MHN converted to metabolically healthy overweight or obesity (MHOO), whereas 48.40% of MHOO remained unconverted throughout the follow-up. In obese or overweight people, the conversion from metabolically healthy to unhealthy might increase the chances of developing diabetes as compared to those with a stable metabolic healthy state (HR: 3.70, 95% CI: 2.99-4.59), while those with persistent metabolic disorders are most likely to have diabetes (HR: 8.32, 95% CI: 7.08-9.78).

CONCLUSIONS

Metabolic healthy is a transient state, and individuals converted from metabolically healthy status to unhealthy phenotypes across all BMI categories might raise the risk of diabetes.

Is metabolically healthy obesity a useful concept?

Obesity poses significant public health concerns, including Type 2 diabetes mellitus, cardiovascular disease and stroke. Although deleterious metabolic profiles typically accompany individuals with obesity, various epidemiological studies have identified a subset of patients with a favourable metabolic phenotype referred to as the metabolically healthy obese. Metabolically healthy obesity is a novel concept that stratifies obese individuals according to their respective metabolic status. It has important implications for healthcare policies, particularly the efficient allocation of resources in the targeted treatment of obesity and prevention of metabolic ill-health. However, conflicting evidence in the literature regarding its risk profile questions its clinical relevance. In addition, the lack of a unified definition of metabolically healthy obesity and agreement on its progression further impede its utility as a stratification strategy. This review aims to describe current concepts of metabolically healthy obesity within the literature, evidence of this metabolic phenotype, the clinical implications of patient stratification and limitations of the concept.

Is the metabolically healthy obesity phenotype an irrelevant artifact for public health?

Some obese persons do not develop (at least in the short term) the metabolic complications of obesity that are thought to be causally linked to cardiovascular events or premature mortality. This phenomenon has been termed "metabolically healthy obesity" (MHO), and it has received much attention recently, to the extent that some authors argue that "new metrics" must be developed to estimate the risk associated with obesity beyond body mass index. In this commentary, we argue that the MHO phenotype is not benign and as such has very limited relevance as a public health target. More efforts must be allocated to reducing the distal and actual causal agents that lead to weight gain, instead of the current disproportionate scientific interest in the biological processes that explain the heterogeneity of obesity.