Selection bias can creep into unselected cohorts and produce counterintuitive findings

Obesity measures, metabolic health and their association with 15-year all-cause and cardiovascular mortality in the SAMINOR 1 Survey: a population-based cohort study

Background

The mortality of metabolic-obesity phenotypes has been thoroughly studied, but it is not known if or how the association between mortality and body mass index (BMI), waist circumference or a body shape index (ABSI) differ in strata of cardiometabolic health status.

Methods

We linked data on 12,815 men and women aged 36–79 years from the SAMINOR 1 Survey with mortality data from the Norwegian Cause of Death Registry. We defined metabolically healthy and unhealthy as having zero and ≥ 1, respectively, of the following: MetS, pre-existing diabetes or cardiovascular disease (CVD), or prescribed drugs for high blood pressure, hyperglycaemia or dyslipidaemia. We defined general and abdominal obesity as BMI ≥ 30 kg/m² and waist circumference ≥ 88 cm (women) or 102 cm (men), respectively, and cross-classified these categories with metabolic status to create metabolically healthy non-obese and obese (MHNO and MHO) and metabolically unhealthy non-obese and obese (MUNO and MUO) phenotypes. We used Cox regression to estimate the hazard ratio (HR) for all-cause and CVD mortality for 1) the four phenotypes and 2) BMI, waist circumference and ABSI fitted with restricted cubic splines. We adjusted for age and lifestyle, and tested for interactions with sex and metabolic status (only continuous measures).

Results

The MHO phenotype was present in 7.8% of women and 5.8% of men. During a median follow-up of 15.3/15.2 years, 596/938 women/men had died, respectively. The MUNO and MUO groups had higher mortality than the MHNO group. Sex and phenotypes interacted with respect to CVD mortality: relative to the MHNO group, the MHO group had an adjusted HR (95% confidence interval) for CVD mortality of 1.05 (0.38–2.88) in women and 2.92 (1.71–5.01) in men. We found curvilinear associations between BMI/waist circumference and all-cause mortality irrespective of metabolic status. Corresponding relationships with CVD mortality were linear and the slope differed by sex and metabolic status. ABSI was linearly and positively associated with all-cause and CVD mortality in men.

Conclusion

The relationships between BMI, waist circumference or ABSI and mortality differed by sex, metabolic status and cause of death. Poor metabolic health substantially increases mortality regardless of obesity status.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12872-021-02288-9.

Explaining the obesity paradox in healthcare utilization among people with type 2 diabetes

Background

Several studies of diabetes suggest an obesity paradox: persons without diabetes have an increased risk of death due to obesity, whereas obesity decreases the risk of death for people with diabetes. A recent study finds the same obesity paradox with the number of healthcare visits. Whether empirical biases and confounding lead to this paradox is yet to be determined.

Objective

To examine changes in the relationship between BMI and number of visits in diabetic vs nondiabetic populations, controlling for confounding risk factors.

Methods

Using adults in the nationally representative Medical Expenditure Panel Survey (MEPS) from 2008 to 2016, N = 210,317, we examine the proposed relationship using six measures of healthcare visits with zero-inflated negative binomial regressions controlling for age, gender, race/ethnicity, income, education, region, health insurance, chronic conditions, and smoking. We excluded persons with type 1 diabetes and gestational diabetes.

Results

We find an obesity paradox among people with diabetes for three measures. That is, relative to people without diabetes, normal weight people with diabetes have more emergency room visits, inpatient, and office-based physician visits than do the obese with diabetes. However, we do not find an obesity paradox in any of the six measures once we exclude smokers and persons ever diagnosed with cancer or cardiovascular disease.

Conclusion

The obesity paradox does not exist at the utilization level and is due to the presence of statistical biases such as confounding and reverse causation. Physicians should continue to focus on efforts to prevent obesity in patients with diabetes.