Do birth cohorts make a difference for deficit accumulation trajectories in older age? First results from the KORA-Age study

Introduction Higher age is related to higher numbers of accumulated health deficits. However, to date it remains unclear how much of this association is due to age effects, how much is contributed by birth cohort effects and if a general trend of compression or expansion of morbidity can be assumed. The objective of this study was to investigate the role of cohort effects with regard to health deficit accumulation (DA) trajectories in people aged 65 and older. Methods Data originates from the baseline assessment of participants aged 65–71 years in 2008 (i.e. born 1937 to 1943) from the KORA (Cooperative Health Research in the Region of Augsburg)-Age study in Southern Germany as well as from an independent assessment in 2015 consisting of a younger birth cohort (aged 65–71 years in 2015, i.e. born 1944 to 1950). All participants were former participants of the population representative MONICA/KORA surveys conducted between 1984 and 2001. DA was measured with a Frailty Index (FI). This KORA-Age FI includes in total 33 items, covering 10 diseases, 13 measures of functioning and 10 signs and symptoms. All deficit items were coded as values between 0 (deficit absent) and 1 (deficit fully present). The FI for a person results as the number of the person-specific deficits divided by the total number of listed deficits. Thus, FI scores range from 0 (= no deficits present) to 1 (= all deficits present). If a participant scored missing on one or more of the deficit items, the denominator of the FI was reduced accordingly. Age-specific mean FI scores were graphically presented separately for the younger (born 1944 to 1950) and the older birth cohort (born 1937 to 1943), stratified by sex. The effects of birth cohort, age and sex (all coded as factor variables) on FI levels were analyzed using a generalized linear model (GLM) with the number of present health deficits as negative binomially distributed outcome and the number of possible health deficits as offset term. Significance level was set to 0.05. Results FI values were available for 1919 participants aged 65–71 years in 2008 (51% female) and for 1456 participants aged 65–71 years in 2015 (53% female). Participants from the younger birth cohort aged 65–71 years in 2015 had average FI values (FI = 0.141) comparable to participants from the older birth cohort aged 65–71 years in 2008 (FI = 0.136, P  = 0.1071). Plotting birth cohort and sex-specific FI averages by age revealed that both birth cohorts started at the same FI levels, but thereafter, age-specific FI values were higher for the younger birth cohort and for women, specifically for those born in 1944 and 1945. These results were partly confirmed by the GLM. Higher age, especially for those aged 70 (RR: 1.23, KI [1.13; 1.34]) and those aged 71 years (RR: 1.23, KI [1.14; 1.34]) as compared to those aged 65 years, was associated with higher FI values. Also female sex (RR: 1.14, KI [1.09; 1.19]) was independently, significantly and positively associated with higher FI values. Having been born 1944–50 (RR: 1.04, KI [1.00; 1.09]) was positively associated with higher FI values, but the effect was not significant ( P  = 0.059926). Conclusion Based on our comparison of adults of the same age from different birth cohorts, an expansion of morbidity cannot fully be excluded. This seems to be largely due to the comparatively high levels of deficit accumulation for women born in 1944 and 1945, which coincides with the last years of World War II. Further research is warranted to investigate how specific life course experiences including deprivation in critical developmental periods for these cohorts may have contributed to these effects.