Intelligence and Carotid Atherosclerosis in Older People: Cross‐sectional Study

To the Editor: Children who score higher on tests of intelligence have a lower subsequent risk of coronary heart disease.1,2 One explanation may lie in observations linking higher intelligence in childhood with healthier behaviors and lower levels of some cardiovascular risk factors in adulthood.3–5 Such individuals may be less susceptible to atherosclerosis. Intelligence differences measured in childhood tend to remain stable into old age.6 Crystallized intelligence is often assessed using vocabulary-based tests and shows little deterioration with aging. Fluid intelligence, often assessed using time-limited reasoning tests, ages like many physical abilities. At older ages, cognitive performance reflects not just peak ability earlier in life, but also the cumulative effects of disease processes. However, crystallized intelligence, being relatively resistant to the influences of age-related pathologies, provides an indicator of peak prior intelligence that is strongly correlated with intelligence in childhood.7 The relationship between crystallized and fluid intelligence and carotid atherosclerosis in older people was investigated. The participants were 466 men and women aged 66 to 75 from Sheffield, United Kingdom. The North Sheffield Research Ethics Committee approved the study. Fluid intelligence was assessed using the Alice Heim 4 test, Part 1 (AH4), which consists of 65 reasoning items of increasing difficulty.8 Crystallized intelligence was assessed using the Mill Hill vocabulary scale's synonyms subtest, which assesses knowledge of word meaning.9 It consists of 33 words of increasing difficulty. Participants underwent a color duplex ultrasonographic examination of the carotid arteries with an HDI3000 high-resolution, real-time scanner (Advanced Technology Laboratories, Bothell, WA). The ultrasonographer examined the carotid arteries and the bifurcation and estimated the maximum degree of stenosis. Intima-media thickness (IMT) of the far wall was measured three times in the common carotid artery and twice in the internal carotid artery on both sides and a mean value calculated. Linear regression was used to examine the relationship between cognitive test scores and IMT, and logistic regression was used to examine the relationship between cognitive test scores and having carotid stenosis greater than 30%. Men who scored higher on the Mill Hill test of crystallized intelligence had less carotid atherosclerosis as measured according to IMT and degree of stenosis (Table 1). For a standard deviation increase in Mill Hill score, IMT decreased 2.26 μm (95% confidence interval (CI)=0.36–4.17) and the odds ratio for having carotid stenosis greater than 30% was 0.72 (95% CI=0.56–0.93). Separate adjustment for education, vitamin C concentration, and cardiovascular risk factors attenuated the relationship between Mill Hill score and IMT. Adjustment only slightly affected the relationship between Mill Hill score and carotid stenosis. Relationships between scores on the AH4 test of fluid intelligence and markers of carotid atherosclerosis in men were in a similar direction but considerably weaker and nonsignificant (data not shown). In women, there were no significant associations with either test. No previous study has examined the relationship between crystallized intelligence and carotid atherosclerosis. Because the effects of aging and disease affect this area of cognitive function little, if at all,7 scores on tests such as the Mill Hill provide an estimate of peak prior intelligence in older people. The findings of the current study suggest that higher premorbid intelligence may protect men against atherogenesis. Associations were not found between fluid intelligence test scores and carotid atherosclerosis, but age-related pathologies can impair fluid intelligence, which may thus provide a less accurate indicator of peak mental ability in elderly populations. Education, plasma vitamin C, and other cardiovascular risk factors attenuated the relationship between Mill Hill scores and IMT in men, but these and other factors explained little of the significant association between Mill Hill scores and risk of carotid stenosis. Although intima-media thickening and plaque formation are features of atherogenesis, they are pathologically distinct processes.10 The mechanisms underlying the associations between Mill Hill score and IMT and carotid stenosis may not be identical. No relationship was found in women between Mill Hill scores and carotid atherosclerosis. One explanation might be that they had less evidence of plaque or intimal thickening than men. Studies that have found associations between childhood intelligence and risk of heart disease have been carried out in men1 or have not reported sex-specific analyses.2 Whether intelligence is a stronger predictor of atherosclerotic disease in men than women remains unclear. These findings suggest that higher peak prior intelligence may reduce susceptibility to atherosclerosis in men. Future research will be needed to investigate other mechanisms that might underlie these findings and examine whether lower intelligence is a more powerful risk factor for atherosclerotic disease in men than in women. Conflict of Interest: The editor in chief has reviewed the conflict of interest checklist provided by the author and has determined that none of the authors have any financial or any other kind of personal conflicts with this letter. David Batty is a Wellcome Fellow. Ian Deary is the recipient of a Royal Society-Wolfson Research Merit Award. This research was funded by the Medical Research Council and the Stroke Association. Author Contributions: David Batty and Catharine Gale had the idea for the present study. Catharine Gale planned and coordinated the data collection, conducted the statistical analyses, and wrote the first draft of the manuscript. Ian Deary and David Batty made substantial contributions to the final version. Sponsor's Role: The funders had no role in the design, methods, subject recruitment, data collections, analysis, or preparation of the paper.