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Long term age-related trajectories of cardiovascular risk factors are affected by cohort effects: the
Whitehall II study
D.R. Witte1, A. Hulmán2, T.A. Nyári2, M. Kivimäki3, A.G. Tabák3,4, E.J. Brunner3;
1
Centre d'Etudes en Santé, Centre de Recherche Public de la Santé, Strassen, Luxembourg, 2Department
of Medical Physics and Medical Informatics, University of Szeged, Hungary, 3Department of
Epidemiology and Public Health, University College London, UK, 4Department of Medicine,
Semmelweis University, Budapest, Hungary.
Background and aims: Analysis of long-term changes in cardiovascular risk factors based on sequential
measurements can give insight into the process of ageing, but longitudinal analyses may be influenced by
cohort effects and secular trends. Our aim was to investigate how age-related trajectories of obesity, blood
pressure and lipid levels were affected by year of birth.
Materials and methods: We fitted quadratic age-related trajectories for BMI, waist circumference,
systolic and diastolic blood pressure as well as total and HDL cholesterol between the ages of 35 and 80
based on the Whitehall II study, a longitudinal dataset of 10,308 individuals with 36,692 observations
obtained during over 25 years of follow up. Year of birth and its interaction term with age and age2 were
added to the models in order to evaluate cohort effects. Smooth kernel distributions were fitted for
subgroups of participants aged 57 to 61 at each of four study phases (5 years apart) to visualise the cohort
effect.
Results: BMI and waist circumference distributions for the age group 57-61 showed a higher degree of
right-skewing with each subsequent phase in both sexes, whereas a right-shift in the distribution was
observed for men but not for women. In both sexes distributions of diastolic blood pressure and total
cholesterol showed a left-shift with each subsequent phase, whereas HDL-cholesterol showed a slight
right-shift. There were marked differences in BMI, diastolic blood pressure and total cholesterol
trajectories depending on the year of birth (Figure 1). For any given age, younger birth cohorts had higher
BMI and a steeper increase in BMI with age. At any given age over 50 both total cholesterol and diastolic
blood pressure were lower in younger birth cohorts. A model that does not take birth year into account
(Figure 1, dashed line) underestimated the age-related decrease in total cholesterol in men and in diastolic
blood pressure in both sexes.
Conclusion: Lifetime exposure to cardiovascular risk factors differs markedly for people born as little as
5 years apart, with cohort effects of opposite directions and different magnitudes. These effects are likely
due to the different age and stage of life at which each subsequent generation experienced the changes in
society, diet, exercise, and developments in cardiovascular risk detection and management seen during
the past decades. Cohort effects should be taken into account when investigating the effects of ageing in
longitudinal datasets.
Supported by: MRC, BHF, NHLBI, NIA