Age and the Impact of Obesity on Mortality

I read with interest the article by Masters et al.¹ Consistent with their previous report² and contradictory to the bulk of existing literature,^3–6 they conclude that “obesity’s effect on mortality risk grows stronger with increasing age.”^1(p1898) Their Figure 2 shows a clear increasing trend of hazard ratios over age for all high body mass index (BMI; defined as weight in kilograms divided by height in meters squared) categories.¹ If valid, their findings could have significant public health implications for developing age-specific weight control guidelines.

In my view, however, their conclusions are not supported by data, specifically the data presented in their online supplemental Tables C and D.¹ The coefficients for interaction terms between age at survey and high BMI categories (overweight, grade 1 and grade 2/3 obese) are all negative, indicating that the hazard ratio estimates decrease with increasing age at survey. Because of the presence of three interaction terms with a high BMI category in each model, estimating mortality hazard ratios for a high BMI category involves four coefficients. For example, the hazard ratio for the grade 2/3 obese in Black men is

where First and Second are assumed to be the derived and orthogonalized cubic splines variables for time in logarithmic scale, ln(t). The mortality hazard ratio estimates for high BMI categories depend on both age and time because of the presence of the interaction terms with age and time variables. In their article, the authors did not provide details on how the age specific hazard ratios were calculated in Figure 2,¹ which shows an increasing trend over age.

I recalculated the hazard ratios for three high BMI categories based on the coefficient values presented in Tables C and D.¹ Because the mean value of orthogonalized First and Second variables are zero, I calculated age-specific hazard ratios assuming both the First and Second variables are zero. Figure 1 shows the recalculated hazard ratios. The hazard ratio estimates decrease with increasing age in an opposite direction to that reported in their study.¹ Therefore, those recalculated hazard ratios actually support that the obesity–mortality association weakens with increasing age, which contradicts their conclusion that the association grows stronger with age. This finding further confirms my view that their conclusions are not supported by their data.

FIGURE 1—

Recalculated mortality hazard ratios by body mass index level, race/ethnicity, and sex for (a) White women, (b) Black women, (c) White men, and (d) Black men.

Note. Overweight defined as body mass index 25.0–29.9 kg/m2, grade 1 obesity as 30–34.9 kg/m², and grade 2/3 obesity as ≥ 35 kg/m².

Source. Adapted from Masters et al.¹