Smoking, childhood IQ, and cognitive function in old age
Introduction
Many people continue to smoke into old age despite public health messages warning of the long-term effects on health. Those smokers who survive to old age have an increased risk of significant morbidity [1]. Moreover, a growing body of evidence suggests that smokers perform more poorly in late life on tests of global cognitive function [2], [3] and across several cognitive domains, including memory [4], [5], [6], information processing speed [4], [6], executive function [7] psychomotor speed [8], [9] and cognitive flexibility [8], and especially on cognitively demanding tasks [10]. Numerous longitudinal studies have suggested a causal relationship between chronic smoking into late life and an increased risk of cognitive decline and dementia [5], [11], [12], [13], [14]. Some of these studies propose a dose–response effect of smoking, whereby the degree of cognitive decline experienced increases with the quantity of cigarettes smoked [12], [13]. Typically, ex-smokers cognitively outperform those who continue to smoke into old age [4], [15].
Several mechanisms have been proposed to explain this relationship. The most widely accepted suggest that cognitive impairment may be due to adverse effects of chronic smoking on cardiac, vascular [11], [16] and pulmonary functions [9], [17], [18]. However, recent findings point to alternative, non-physiological, underlying mechanisms. Firstly, population-based studies have identified an association between prior intelligence (IQ) and smoking behaviour. People with lower IQ-type scores are more likely to become smokers [19], [20], [21]. Those who quit are more likely to have had higher childhood intelligence than those who continue to smoke [22]. Secondly, there is a socioeconomic gradient in smoking prevalence [20], [23]. Socioeconomic factors may also explain some of the IQ-smoking relationship [19], [24]. A link between prior low IQ and/or low socioeconomic status (SES) and smoking habits may partly explain, by confounding, why long-term smokers perform more poorly on tests of cognition later in life than never-smokers, and why ex-smokers perform better than those who continue to smoke.
Many studies have been unable to rule out prior intelligence (before initiation of smoking) as a potential confounder as this is rarely available when examining cognitive function in older people. Those studies with this data have found that smoking was associated with a small relative decline in cognition after controlling for childhood IQ. However, in one study, the results were based on a single cognitive outcome (verbal reasoning at age 80) and did not control for SES [25]. In the other, of the five cognitive outcomes tested, only psychomotor speed was found to be adversely affected by smoking history (after adjusting for childhood IQ) in a sample of 64-year olds [9]. A 2-year follow-up of the same sample identified a small additional deficit in memory with a positive smoking history [6].
The present study addresses the gaps in the current knowledge of the smoking-cognition relationship in a large sample of older individuals (aged 70), for whom childhood intelligence, later life intelligence and extensive other cognitive data, adult SES, health measures (including lung function), and smoking history are all available. We examine: (1) the association between prior intelligence and smoking history; (2) the association between smoking and cognitive function in old age across a wide range of cognitive domains; and (3) the roles of prior intelligence and SES (independently and in combination) in order to evaluate more fully the underlying mechanisms in the smoking-cognition relationship which are as yet, unclear.
Section snippets
Participants
We examined individuals in the Lothian Birth Cohort 1936 Study (LBC1936) which comprises 1091 men and women. Almost all participants were residents in Edinburgh and the surrounding Lothian region. Early life (mean age 11 years) intelligence test data are available for this sample, because they are surviving participants of the Scottish Mental Survey of 1947 (SMS1947) [26]. Assessment in later life took place between 2004 and 2007 when participants were about 70 years old. Full recruitment and
Descriptive results — participants
Participants' characteristics are summarised in Table 1. At time of testing, 46% reported that they had never smoked, 42% were ex-smokers, and 12% were current smokers. Compared to never smokers, smokers were more likely to be male, unmarried (single, widowed, or divorced), belong to a ‘less professional’ social class, have fewer years of full-time education, a lower MMSE score, a higher weekly alcohol intake, and a lower BMI. Lung function (FEV1) was lowest in current smokers. Current smokers
Discussion
We examined a sample of individuals, aged about 70, for whom childhood intelligence scores are available. We replicated previous findings that lower IQ at age 11 was associated with a higher risk of becoming a smoker and continuing to smoke into old age, and with the amount of cigarettes smoked over the lifespan. We found an adverse effect of current smoking on general cognitive ability (g) and information processing speed — but not memory or verbal ability — even after controlling for
Conflict of interest
The authors declare no conflicts of interest.
Author contributions
JC performed the analyses and led the writing of the article. JC and AG were involved in data collection. AG, JS and ID contributed to the study design, interpretation of analysis and drafting of the manuscript.
Acknowledgments
The LBC1936 data were collected using a Research into Ageing programme grant; this research continues as part of the Age UK-funded Disconnected Mind project. We thank the Scottish Council for Research in Education for allowing access to the SMS1947. We thank the LBC1936 Study participants. AJG, JMS and IJD are members of the University of Edinburgh Centre for Cognitive Ageing and Cognitive Epidemiology, part of the cross council Lifelong Health and Wellbeing Initiative (G0700704/84698). Funding
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