Intelligence and cardiovascular health : epidemiological studies of swedish men

Aim: A first aim of this thesis was to examine the relationship between intelligence in early adulthood and health outcomes, mainly cardiovascular morbidity and mortality, CVD, later in life using register-based data on Swedish men. A second aim was to increase understanding of the pathways between intelligence and CVD morbidity/mortality by studying the association between intelligence and smoking habits and nicotine dependence, who might act as mediators of the association between intelligence and mortality.

Material and methods: The study populations in this thesis were created through record linkage of several national registers where IQ was collected from the Military Conscription Register. The analyses were based on all Swedish men born 1951 to 1984, depending on the paper, that went through conscription examinations. Also, for paper II parents of the men were analysed. Paper IV and V were based on Swedish male twins. The association between IQ and mortality was analyzed with Cox proportional hazards regression and conditional logistic regression models. The association between IQ and smoking status was analysed with linear, logistic and polytomous regression models and the association between IQ and nicotine dependence with quantitative genetic analyses.

Results: In general, paper I-III support previous research about inverse associations between IQ and CVD morbidity/mortality. In addition to previous research our results revealed that IQ was associated with coronary heart disease, CHD, independently of socioeconomic position, SEP, (paper I) and that IQ was inversely and significantly associated with major subtypes of stroke (paper III). The strongest association found for hemorrhagic stroke. Further, the effect of IQ on mortality was found to be present also when offspring IQ was used as a proxy for parental IQ (paper II). In paper IV and V when the association of IQ and smoking and nicotine dependence was studied no evidence of a causal association was found. In paper IV smoking was associated with IQ but appeared to be the result of early environmental factors rather than resulting from a causal effect of intelligence. In paper V both IQ and nicotine dependence showed moderate heritability (0.58 and 0.39 respectively); however the phenotypic correlation was marginal (-0.08) and the overlap between genetic factors influencing IQ and nicotine dependence was small (-0.19).

Conclusion: In this thesis, inverse associations between IQ score at the age of 18 and mortality later in life, mainly CVD, is presented. These associations were found within all socioeconomic strata, were robust to adjustment for indicators of SEP in childhood and adulthood and were present also when using offspring IQ as a proxy for parental IQ. Further, our results gave no support for IQ to be causally associated with smoking or that smoking or nicotine dependence act as important mediators of the IQ-CVD association. Finally, no evidence was provided for a common genetic factor behind IQ and nicotine dependence. This thesis provides information about the pathways of intelligence and mortality. However, more research is needed before any conclusions can be drawn with regard to public health policy.

List of scientific papers

I. Silventoinen, K, Modig Wennerstad, K, Tynelius, P, Rasmussen, F. (2007). "Association between intelligence and coronary heart disease mortality: a population-based cohort study of 682,361 Swedish men" European Journal of Cardiovasc Prevention and Rehabilitation 14(4): 555-60.
https://doi.org/10.1097/HJR.0b013e328014672e

II. Modig Wennerstad, K, Silventoinen, K, Batty, G.D, Tynelius, P, Bergman, L, Rasmussen, F. (2008). "Association between offspring intelligence and parental mortality: a population-based cohort study of one million Swedish men and their parents." Journal of Epidemiol Community Health 62(8): 722-7.
https://doi.org/10.1136/jech.2007.065623

III. Modig Wennerstad, K, Silventoinen, K, Tynelius, P, Bergman, L, Rasmussen, F. (2009). "Association between intelligence and type specific stroke: a population-based cohort study of early fatal and non-fatal stroke in one million Swedish men". Journal of Epidemiol Community Health.
https://pubmed.ncbi.nlm.nih.gov/19833609

IV. Modig Wennerstad, K, Silventoinen, K, Tynelius, P, Bergman, L, Kaprio, J, Rasmussen, F. (2010). "Associations between IQ and cigarette smoking among Swedish male twins". Social Science & Medicine 70(4): 575-581.
https://doi.org/10.1016/j.socscimed.2009.10.050

V. Modig Wennerstad, K, Silventoinen, K, Tynelius, P, Kaprio, J, Rasmussen, F. (1970). "Genetics of the association between intelligence and nicotine dependence: a study of Swedish male twins". [Submitted]