Critique 169: Effects of IQ on risk of morbidity and mortality related to alcohol consumption — 25 August 2015
Sjölund S, Hemmingsson T, Gustafsson JE, Allebeck P. IQ and alcohol-related morbidity and mortality among Swedish men and women: the importance of socioeconomic position. Journal of Epidemiology and Community Health 2015;69:858-864.
Aims: To investigate the association between intelligence in childhood and later risk of alcohol-related disease and death by examining (1) the mediating effect of social position as an adult and (2) gender as a possible moderator.
Design: Cohort study.
Setting and Participants: 21,809 Swedish men and women, born in 1948 and 1953, from the Swedish “Evaluation Through Follow-up” database were followed until 2006/2007.
Measurements: IQ was measured in school at the age of 13 and alcohol-related disease and death (International Classification of Disease codes) were followed from 1971 and onwards.
Findings: We found an increased crude HR of 1.23 (95% CI 1.18 to 1.29) for every decrease in group of IQ test results for alcohol-related admissions and 1.14 (95% CI 1.04 to 1.24) for alcohol-related death. Social position as an adult was found to mediate both outcomes. Gender was not found to moderate the association. However, adjusting for socioeconomic position lowered the risk more among men than among women.
Conclusions: There was an inverse, graded association between IQ and alcohol-related disease and death, which at least partially was mediated by social position as an adult. For alcohol-related death, complete mediation by socioeconomic position as an adult was found. Gender does not moderate this association. The role of socioeconomic position may differ between the genders.
Epidemiologists have noted for many years that a factor that modifies the relation between alcohol intake and many diseases is the education, income, or other index of socio-economic status (SES) of subjects. Higher SES subjects are known to be at lower risk of alcohol-related diseases than subjects with lower SES. Suggested reasons why lower-SES subjects may have more adverse effects from alcohol include (1) different drinking patterns, with more binge drinking, (2) clustering of poor lifestyle factors (e.g., greater smoking rates), and (3) less access to health care.
Our Forum recently reviewed a systematic review paper that dealt with this subject by Jones et al. The key finding of that study was a tendency for greater risk for lower-SES subjects than for higher-SES subjects to develop a number of diseases that are associated with alcohol consumption, especially head and neck cancers and stroke. In addition to the above mentioned factors, Forum members suggested that lower SES may also affect susceptibility to many diseases, most likely due to effect modification of hitherto unrecognized factors. The Forum concluded that “While the inverse pattern between income and SES and alcohol-related problems is clear, there are too many confounders to make a conclusion about the determinants of this relation. Much more research will be needed to understand how socio-economic factors affect the risk of disease and may modify the relation between alcohol consumption and disease (www.bu.edu/alcohol-forum/critique-164).”
“Intelligence” is positively associated with education, income, and other measures of SES. It is commonly assessed by tests that give an intelligence quotient, IQ, defined as a number representing a person’s reasoning ability, measured using problem-solving tests, as compared to the statistical norm or average for their age, taken as 100.
The present paper is based on a large cohort of subjects in Sweden who had IQ tests as children (when they were 13 years old) and were then followed for more than 30 years. The study reports an inverse association between childhood IQ and hospitalization for alcohol-related diseases or death. The association is strongly dose-dependent, with a marked increase in disease/mortality risk for each decrease in the childhood IQ score. The authors also report that the attained socio-economic status (SES) of the subjects at age 32 explained the association to a large degree.
For the underlying causes for such an association, in their background the authors state: “Proposed hypotheses imply mainly two ways: either intelligence has effects on health, through, for example, choice in lifestyle behaviour or socioeconomic environment achieved as an adult, or, the relationship between intelligence and health is confounded by other factors such as biological or socioeconomic conditions early in life.”
Specific comments on paper by Forum members: Forum members considered this to be a very well-done analysis with results consistent with much previous research. As stated by member Barrett-Connor, “I agree that this is a worthwhile study,” and Svilaas thought it “A well performed study with not surprising results.” Forum member de Gaetano commented: “The paper is well written and the study was well performed. One should have included income and education, as separated entities, although they are usually correlated with each other.”
Several Forum members considered especially noteworthy the strong effect, especially among men, of the attained SES of subjects at age 32 on the association between childhood IQ and alcohol-related outcomes. The association of IQ with the risk of alcohol-related hospital admissions (e.g., for habitual excessive drinking, alcohol abuse, or alcohol dependence) was attenuated by taking the attained SES of the subject into account; for alcohol-related death (e.g, from alcoholism, toxic effects of alcohol), the association with childhood IQ was completely abolished. As stated by the authors, “Social position as an adult was found to be a partial mediator of alcohol-related disease and a complete mediator for alcohol-related death in our causal steps analysis.” Further, the adult SES level of the subject was noted to have a much stronger effect than the SES level of the parents when the subject was a child. Hence, being from a poor, less-educated family may not be as important as your status as a young adult in terms of the risk of adverse alcohol-related outcomes.
Reviewer Ellison commented that there is considerable interaction between intelligence and job position and income. “This makes it difficult to tease apart the mechanisms by which intelligence in itself affects the relation of alcohol consumption to adverse health outcomes: is it the underlying intelligence or the achieved social position of the adult? (The latter may have many genetic and environmental determinants).
“Given the association between attained SES and disease outcomes, it raises the question as to what degree native intelligence is an independent factor in SES. Further, as stated by Galbraith, ‘Nothing gives the illusion of intelligence so much as personal association with large sums of money.’ (Some have cited this phenomenon in the current race for the Republican nomination for president of the USA!) It is obvious that adult SES has many determinants, and it is questionable as to the degree to which childhood IQ is an unbiased estimate of intelligence.”
Reviewer Finkel did not believe that IQ is a simple and accurate measure of intelligence. “IQ is a stew, not a distinct variable. It no doubt repeats and reflects the influences of many other biological, social, economic, even political factors.” Forum member Van Velden noted: “Environmental influences such as diet and emotional wellness are important confounders, and difficult to evaluate.”
Forum members agreed with the authors that the strengths of this study are its prospective analysis of a large cohort chosen randomly from the population, good mechanisms for ascertaining the outcomes, a long follow-up period, and the inclusion of both men and women. Further, the study provides important data that may assist scientists who are seeking to determine the key factors relating alcohol consumption to disease outcomes and to mortality.
Reviewer Skovenborg commented: “Not mentioned in the Swedish study is a possible association between beverage choice and intelligence. In a Danish study by Mortensen et al, irrespective of socio-economic position, a high IQ was associated with preference for wine rather than other beverages containing alcohol, but IQ was not related similarly to alcohol consumption. The authors of the Mortensen et al study commented: “This study is set in the predominantly beer-drinking Danish population, in which wine drinking has traditionally been a sign of high social standing. Because factors such as income and vocational education are known to correlate with intelligence, an IQ gradient in wine drinking is to be expected. Thus, IQ in young adulthood might be considered a predictor of future life-style and social standing, which in turn affects beverage choices.” Thus, we have another possibility for the association shown in the Swedish paper, that subjects with higher SES in young adulthood may have preferentially consumed wine, which may have affected their risk of adverse alcohol-related outcomes. Type of beverage was not assessed in the present study.
References from Forum Review
Galbraith JK. “The 1929 Parallel,” The Atlantic Monthly, January 1987;259:62-66.
Jones L, Bates G, McCoy E, Bellis MA. Relationship between alcohol-attributable disease and socioeconomic status, and the role of alcohol consumption in this relationship: a systematic review and meta-analysis. BMC Public Health 2015;15:400. DOI 10.1186/s12889-015-1720-7. (Reviewed by Forum on 14 May 2015: www.bu.edu/alcohol-forum/critique-164.)
Mortensen LH, Sørensen TI, Grønbæk M. Intelligence in relation to later beverage preference and alcohol intake. Addiction 2005;100:1445-1452.
The present paper is based on a large cohort of subjects in Sweden who had IQ tests as children (when they were 13 years old) and were then followed for more than 30 years. The study reports an inverse association between childhood IQ and hospitalization for alcohol-related diseases or death. The association is strongly dose-dependent, with a marked increase in disease/mortality risk for each decrease in the childhood IQ score.
A key finding in this analysis is that the attained socio-economic status (SES) of the subjects at age 32 played a large role in explaining the association between IQ and alcohol-related disease and appeared to completely explain the association with death from alcohol-related causes. This suggests that much of the effect of IQ during childhood may be through its effect on later education, occupation, and income of the subjects, associations that have been noted in many previous epidemiologic studies to favor better health. It is reassuring that being born into a poor, uneducated family is not as important as a determinant of adverse effects of alcohol later in life as is the attained socio-economic status of the individual in young adulthood.
A possible explanation for the inverse relation of SES in young adulthood with adverse alcohol outcomes could relate to the type of beverage consumed, which was not evaluated in the present analysis. In a previous study from Denmark, it was shown that higher SES in young adults was associated with a strong preference for wine rather than for other types of alcoholic beverage. Wine consumption has been associated with better health in numerous studies, although it is unclear whether this is due primarily to the non-alcoholic constituents in wine or to confounding from associated lifestyle factors of wine drinkers.
Forum members agree with the authors that the underlying causes for an association between IQ (or SES) and adverse alcohol-related disease and death are not clearly defined. It could be that intelligence has effects on health through choices in lifestyle behavior or socioeconomic environment as an adult, or the relationship between intelligence and health is confounded by other factors such as biological or socioeconomic conditions early in life. As for now, we can only describe the association, but not explain it fully.
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Comments for this critique by the International Scientific Forum on Alcohol Research were provided by the following members:
Elizabeth Barrett-Connor, MD, Distinguished Professor, Division of Epidemiology, Department of Family Medicine and Public Health and Department of Medicine, University of California, San Diego, La Jolla, CA USA
R. Curtis Ellison, MD. Section of Preventive Medicine & Epidemiology, Department of Medicine, Boston University School of Medicine, Boston, MA, USA
Giovanni de Gaetano, MD, PhD, Department of Epidemiology and Prevention, IRCCS Istituto Neurologico Mediterraneo NEUROMED, Pozzilli, Italy
Harvey Finkel, MD, Hematology/Oncology, Boston University Medical Center, Boston, MA, USA
Tedd Goldfinger, DO, FACC, Desert Cardiology of Tucson Heart Center, University of Arizona School of Medicine, Tucson, AZ, USA
Fulvio Mattivi, PhD, Head of the Department of Food Quality and Nutrition, Research and Innovation Centre, Fondazione Edmund Mach, in San Michele all’Adige, Italy
Linda McEvoy, PhD, Department of Radiology, University of California at San Diego (UCSD), La Jolla, CA, USA
Erik Skovenborg, MD, specialized in family medicine, member of the Scandinavian Medical Alcohol Board, Aarhus, Denmark
Arne Svilaas, MD, PhD, general practice and lipidology, Oslo University Hospital, Oslo, Norway
David Van Velden, MD, Dept. of Pathology, Stellenbosch University, Stellenbosch, South Africa