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It is well known that body mass index (BMI) is a major determinant of mortality and research suggests that the mortality risk of a high BMI is also attenuated by age. However, it is less clear how cardio-metabolic factors (CMFs) such as hyperglycemia and dyslipidemia affect the relationship between BMI and mortality.

Dr Mark Wahlqvist and colleagues at the National Health Research Institutes in Taiwan set out to study the mortality risks of CMFs as compared to those of BMI in a large population cohort of Taiwanese for a mean follow-up time of 8.1 years. The researchers followed 377,929 participants, aged 20 years or older, registered with MJ Health Screening Center in Taiwan and were able to consider the role played by traditional cardiovascular risk factors on the health effects of BMI across the entire adult life course, which is missing in most studies of BMI and mortality.

Our findings shed new light on how the known association between BMI and mortality may be altered by cardio-metabolic factors, and thus could help improve strategies for a longer, healthier life

They found that high blood pressure and blood glucose were associated with an increased risk of mortality in both younger and older individuals, irrespective of gender or body composition. Compared to a 1.04% increased risk in obese individuals, high blood pressure was associated with an 8.57% risk of mortality and high blood sugar was associated with a 6.49% risk of mortality.

The authors were surprised to find that underweight was a more important predictor of reduced survival than overweight or obesity, except in young adults. Furthermore, older age was protective against the risk posed by higher BMIs, suggesting that weight-management programs may need to be tailored according to age.

Dr Wahlqvist et. al. also found that taking metabolic syndrome into account meant that mortality risk of being underweight was greater than that of obesity for all age groups, but especially for people aged 20-39 years old. This may be due to the incidence of high blood pressure and high blood sugar in this group.

Although the observational nature of the study does not allow for conclusions about cause and effect, it provides strong support of the notion that, in addition to weight management, life-extension policies warrant attention among all age groups to cardio-metabolic disorders such as hypertension, pre-diabetes, and diabetes.