Background: We studied the association between participation in the longitudinal follow-up study and cardiometabolic disorders in two longitudinal studies which started prospectively in the antenatal period: the Northern Finland Cohort 1966 (NFBC1966) and the Northern Finland Birth Cohort 1986 (NFBC1986). Both birth cohorts have been followed up since birth with multiple follow-ups including questionnaires, and clinical examinations.

Methods: The NFBC studies were compared to comparison cohorts of individuals who were born in the same area as the study cohorts, but in different years. The data for the comparison cohort were obtained from registers. The cumulative incidence rates of hospital-treated cardiometabolic disorders were calculated for study and comparison cohorts covering the age of 7-50 years in NFBC1966 and the age of 0-29 years in NFBC1986. Cardiometabolic-related causes of death were analysed in NFBC1966 and the comparison cohort from the age of 0-50 years. The analysed cardiometabolic disorders were diabetes mellitus, coronary artery disease, hyperlipidaemia, obesity, hypertension, and cerebrovascular disorders. The risk ratio (RR) with 95% confidence intervals (CI) was calculated by sex.

Results: In NFBC1966, no differences in cumulative incidences of cardiometabolic disorders or cardiometabolic-related deaths compared to the comparison cohort were found. Male members of NFBC1986 had decreased risk of obesity (RR: 0.45, 95% CI: 0.27-0.75) and any cardiometabolic disorders (RR: 0.75, 95% CI: 0.59-0.95) compared to the comparison cohort.

Conclusions: The results suggest that participation in the NFBC1986 may have a weak positive health effect among men. Agreement to follow-up studies focusing on diet, substance use, and physical activity, may slightly decrease the incident risk of cardiometabolic disorders in the study population.KEY MESSAGESEven mild interventions, such as follow-up studies in the prospective follow-up studies, might affect participants' behaviour and consequently the incidence of cardiometabolic disordersThe fact that follow-up itself might affect the study population in terms of risk factors, has to be taken into account when estimating the representativeness of the followed population.