To investigate cohort effects in arthritis prevalence across 4 birth cohorts: World War II (born 1935-1944), older and younger baby boomers (born 1945-1954 and 1955-1964, respectively), and Generation X (born 1965-1974), and to determine whether birth cohort effects in arthritis prevalence were associated with differences in risk factors over time or period effects.
Analysis of biannually collected data from the longitudinal Canadian National Population Health Survey, 1994-2011 (n = 8,817 at baseline). Data included self-reported arthritis diagnosed by a health professional, risk factors (years of education, household income, smoking, physical activity, sedentary behavior, body mass index [BMI]), and survey year as an indicator of period. We used hierarchical age-period-cohort analyses to compare the age trajectory of arthritis by birth cohort and to examine the contribution of changes in risk factors and period to cohort differences.
More recent cohorts had successively a greater prevalence of arthritis. Risk factors were significantly associated with arthritis prevalence independently of cohort differences. The effects of increasing education and income over time on potentially reducing the arthritis prevalence were almost counter-balanced by effects of increasing BMI. Significant cohort-BMI and age-BMI interactions indicated an earlier age of arthritis onset for obese individuals than those of normal weight.
Projections that only take into account the changing age structure of the population may underestimate future trends. Our understanding of the impact of BMI on arthritis is likely an underestimate. Cohort differences focus attention on the need to target arthritis management education to young and middle-aged adults.