In a sample of older adults with up to 27 years of follow-up, we first qualitatively described conversions between BMI-MH phenotypes from midlife (age 50–64) to early late-life (age 65–79) and then to late late-life (age 80 and above). In line with previous studies of adult samples [2, 12, 13], many individuals with MHOw or MHOb converted to MetS over time, but around one-fifth remained metabolically healthy from midlife and through early late-life and late late-life. Secondly, we tested for differences in the occurrence of conversions in metabolic health status between the different BMI categories. More than half of those who were metabolically healthy at baseline converted to MetS during the follow-up in all BMI categories. Individuals with MHOb at baseline were more likely to convert to MetS compared to those with MHNw at baseline, while no difference was seen between those with MHNw and MHOw. Interestingly, conversions from MetS to metabolically healthy BMI categories were also common, with no statistically significant difference between BMI categories.

The aging process is linked to major physiological changes, including changes in metabolism [3, 4] and BMI [5]. While transitions between BMI-MH phenotypes have been examined previously, most studies have focused on the stability of MHO in samples with a relatively young age at baseline [2]. Two previous studies have specifically modeled transitions between the six BMI-MH phenotypes, one followed 3512 women aged 50–79 over an average of six years [12] and the other 9742 men and women aged 20–60 over an average of 4 years [13]. In both studies, individuals with overweight or obesity were more likely to convert from metabolic health to MetS and less likely to convert from MetS to metabolic health, compared to those with normal weight. In the current study, we leveraged data from a well-characterized cohort with long follow-up, covering midlife to late late-life, to study conversions between BMI-MH phenotypes in older individuals over substantially longer periods than previously examined.

However, the small sample size limited our opportunities to examine the causes and consequences of conversions in metabolic health status or to apply more advanced models of transitions between BMI-MH phenotypes. It should also be mentioned that no consensus exists for how metabolic health should be defined [14]. While we used the NCEP ATP-III criteria [8] for MetS, the definition means that small changes in one of the constituents over time may lead to artificial changes in metabolic health status. Objective measures of BMI and metabolic health is a strength of the current study, but we also used self-reported information on diagnoses and the use of medications to define metabolic health, which may reduce the validity. In addition, the study sample consists of Swedish twins aged 50 or above, with a comparatively low proportion of MetS and obesity at baseline. As in most countries, the prevalence of obesity has risen substantially in Sweden during the past decades, from 9% in 1995 to 17% in 2017 [15], potentially limiting the generalizability of our findings to older adults today.

In conclusion, conversions between metabolic health and MetS were relatively common in both directions across all BMI categories during aging. Importantly, a substantial proportion of the sample maintained MHOw and MHOb throughout aging, and, in addition, a considerable proportion regained a metabolically healthy status. Taken together, this indicates that conversions between BMI-MH phenotypes seen in younger samples and over shorter follow-up periods [12, 13] continue across late-life. A better understanding of what predicts these conversions and the ability to maintain metabolic health in overweight and obesity could substantially improve obesity care and help promote and maintain metabolic health throughout life.