Childhood IQ and cardiovascular disease in adulthood: prospective observational study linking the Scottish Mental Survey 1932 and the Midspan studies

Abstract

This study investigated the influence of childhood IQ on the relationships between risk factors and cardiovascular disease (CVD), coronary heart disease (CHD) and stroke in adulthood. Participants were from the Midspan prospective cohort studies which were conducted on adults in Scotland in the 1970s. Data on risk factors were collected from a questionnaire and at a screening examination, and participants were followed up for 25 years for hospital admissions and mortality. 938 Midspan participants were successfully matched with their age 11 IQ from the Scottish Mental Survey 1932, in which 1921-born children attending schools in Scotland took a cognitive ability test. Childhood IQ was negatively correlated with diastolic and systolic blood pressure, and positively correlated with height and respiratory function in adulthood. For each of CVD, CHD and stroke, defined as either a hospital admission or death, there was an increased relative rate per standard deviation decrease (15 points) in childhood IQ of 1.11 (95% confidence interval 1.01–1.23), 1.16 (1.03–1.32) and 1.10 (0.88–1.36), respectively. With events divided into those first occurring before and those first occurring after the age of 65, the relationships between childhood IQ and CVD, CHD and stroke were only seen before age 65 and not after age 65. Blood pressure, height, respiratory function and smoking were associated with CVD events. Relationships were stronger in the early compared to the later period for smoking and FEV1, and stronger in the later compared to the earlier period for blood pressure. Adjustment for childhood IQ had small attenuating effects on the risk factor–CVD relationship before age 65 and no effects after age 65. Adjustment for risk factors attenuated the childhood IQ–CVD relationship by a small amount before age 65. Childhood IQ was associated with CVD risk factors and events and can be considered an important new risk factor.

Introduction

Although rates of coronary heart disease (CHD) and stroke are declining in the west (Office for National Statistics, 1997; Warlow, 1998), they were the cause of 21% and 12% of deaths, respectively, in Scotland in 2000 (Registrar General for Scotland, 2001). Cardiovascular disease (CVD), which includes CHD and stroke, was the underlying cause in 41% of deaths (Registrar General for Scotland, 2001). In addition, CVD is a major contributor to the burden of illness, hospital admissions and disability.

There are well-established risk factors for CVD, such as tobacco smoking and raised blood pressure. People with poorer socioeconomic profiles, determined by education, social class and deprivation are known to be at higher risk of CVD (Davey Smith et al., 1998; Davey Smith, Bartley, & Blane, 1990; Carstairs & Morris, 1991). Personal traits, such as hostility, submissiveness and the type A personality more generally, have a link to some CVD outcomes (Whiteman, Deary, & Fowkes, 2000; Hemingway & Marmot, 1999). Childhood IQ, an intriguing new risk factor, has been found to be inversely related to all cause mortality, CVD and CHD (Whalley, & Deary, 2001; Hart et al., 2003b; Osler et al., 2003). In this paper, we expand our previous findings on mortality to investigate how childhood IQ contributes to CVD events (hospital admissions and deaths). We next examine the associations between childhood IQ and specific CVDs, viz., CHD and stroke. Given the suggestion that childhood IQ influences health behaviours throughout life (Deary, Whalley, & Starr, 2003), and the fact that childhood IQ is associated with birthweight (Shenkin et al., 2001), which is also linked to CVD in adulthood (Barker, 1994), it is possible that childhood IQ is associated with CVD outcomes relatively early in adult life. We therefore also explore whether the effects of childhood IQ are different in CVD, CHD and stroke occurring before or after the age of 65. Lastly, we hypothesise that the effect of childhood IQ on CVD may be mediated by traditional risk factors for CVD.

The Scottish Mental Survey 1932 (SMS1932), conducted under the auspices of the Scottish Council for Research in Education (SCRE), obtained data about the whole distribution of the intelligence of Scottish pupils (Scottish Council for Research in Education, 1933). On June 1, 1932, children born in the calendar year 1921 and attending schools in Scotland were given the same, well-validated mental ability test. The total number of children who sat the Moray House Test was 87,498 (44,210 boys and 43,288 girls). As far as we are aware, no other country collected information about the childhood mental ability differences of an entire year-of-birth cohort. The Moray House test is described in detail elsewhere (Deary, Whalley, Lemmon, Crawford, & Starr, 2000). Briefly, it has 71 items with a maximum possible score of 76. The time limit was 45 min for completion. It contains a preponderance of verbal reasoning items but also some numerical, spatial and abstract reasoning items. The score is expressed as a total score; there are no subscores.

The Midspan studies were large cardiorespiratory studies of adults carried out in Scotland in the 1960s and 1970s. Two studies are included in this analysis, the Collaborative study and the Renfrew/Paisley study. The Collaborative study was conducted between 1970 and 1973 in 27 workplaces in the west and central belt of Scotland (Davey Smith et al., 1998). The full sample consisted of 6022 men and 1006 women of working age. The Renfrew/Paisley general population study was carried out between 1972 and 1976 and involved 7052 men and 8354 women aged 45–64 years who were resident in Renfrew and Paisley (Hawthorne et al., 1995). The age ranges of the studies were such that some participants were born in 1921. Ethical permission was obtained from the Multi-Centre Research Ethics Committee for Scotland to link the SMS1932 data set with the 1921-born participants of the Midspan data sets.

There were 1251 Midspan participants who were born in 1921 and of these, 938 (75%) were matched to a mental ability score from the SMS1932. Full details of the matching procedures are reported elsewhere (Hart et al., 2003a). Since the children's ages varied between 10$\text{1}\text{2}$ and 11$\text{1}\text{2}$ years at the time of testing, the test scores were corrected for age in days and converted to usual IQ-type scores with mean 100 and standard deviation 15.

In both Midspan studies, participants completed a questionnaire and attended a physical examination (Davey Smith et al., 1998; Hawthorne et al., 1995). The questionnaire included questions about smoking habit, home address and occupation. Social class was coded according to the Registrar General's Classification (General Register Office, 1966) for occupation at the time of screening. The social class of women was allocated according to their own occupation, except for those women in the Renfrew/Paisley study who gave their occupation as housewives. In these cases, the social class was that of their husband. The home address at the time of screening was retrospectively postcoded, enabling deprivation category as defined by Carstairs and Morris to be ascertained (Carstairs & Morris, 1991). This measure is an area-based measure of deprivation, obtained from four census variables—male unemployment, overcrowding, car ownership and the proportion of heads of households in social classes IV and V. A deprivation score for each postcode sector is obtained which is converted to seven categories ranging from 1 (least deprived) to 7 (most deprived). The few missing values for social class (n=13) and deprivation category (n=3) were imputed as social class III manual and deprivation category 5 as these were the most commonly occurring. The physical examination included measurement of blood pressure, height, weight, respiratory function, plasma cholesterol and blood glucose (in the Renfrew/Paisley study only). Blood pressure was measured with the subject seated, using the London School of Hygiene sphygmomanometer, with a cuff of 12cm×22 cm. Diastolic pressure was recorded at the disappearance of the fifth Korotkoff sound. Two blood pressure readings were taken and the average diastolic and systolic pressure were recorded. The adjusted FEV1 was defined as the actual FEV1 as a percentage of the expected FEV1, derived from sex-specific linear regressions of age and height from healthy subsets of the study populations (Davey Smith et al., 1998; Hole et al., 1996). Body mass index in kg/m² was calculated from the weight and height.

Midspan study participants were flagged at the National Health Service Central Register in Edinburgh. Causes and dates of death in a 25 year follow-up period were provided. Underlying causes of death were identified and grouped as all cardiovascular disease (CVD) (ICD9 codes 390-459), coronary heart disease (CHD) (ICD9 410-414) and stroke (ICD9 430-438). The few more recent deaths were coded in ICD10 and equivalent codes were used for each underlying cause. In addition, a computerised linkage with acute hospital discharges in Scotland provided records of acute hospital admissions in up to 25 years of follow-up, although hospital admissions were only provided to the end of 1995 for Renfrew/Paisley study participants (Hanlon et al., 1998). First occurrence of each cause of hospital admission from the discharge diagnosis was coded in the same way as the deaths.

Cox's models (Cox, 1972) were used to calculate proportional hazards regression coefficients for one standard deviation change in risk factor and for current and ex smokers compared to never smokers. The exponentiated proportional hazards regression coefficients are referred to as relative rates. Survival time in 25 years was taken from the date of screening until either the date of hospital admission, or the date of death if no hospital admission was found. One participant had embarked from the UK during the follow-up period and survival time was taken until the date of embarkation. Hospital admissions and deaths for the whole follow-up period were divided into those first occurring up to and including age 65 (the early period) and those first occurring after age 65 (the later period).