Dietary antioxidant intake and its association with cognitive function in an ethnically diverse sample of US adults

Abstract

INTRODUCTION

Impaired cognitive function, a major cause for functional disability in old age, leads to loss of independence ascribed mostly to age-related dementing illnesses, most commonly, Alzheimer’s Disease (AD). With the rise in the proportion of people over 65 years of age in the United States and elsewhere, it is expected that AD will quadruple in prevalence by 2050 to more than 100 million people worldwide, when 1 in 85 will be living with AD (1–5). However, efforts are currently under way to uncover modifiable risk factors, including dietary patterns, nutrient and antioxidant intakes, that might reduce the prevalence of cognitive impairment, dementing illnesses and AD.

Several findings suggest oxidative stress plays an important role in neurodegenerative processes accompanying cognitive impairment and dementia, particularly AD. The brain is particularly vulnerable to reactive oxygen species (ROS) as its metabolism accounts for approximately 20% of all oxygen consumption within the body. (6). The exposure of ROS has been shown to result in oxidative modification of DNA in brain tissue that in some cases has been shown to accumulate due to reductions of DNA repair. (7–9) Oxidative stress among AD patients is marked by increased antioxidant brain levels, acting as free radical scavengers. (10) In vitro studies suggest that exogenous antioxidants may reduce β-amyloids toxicity in AD patients’ brains (10–12). Dietary antioxidants, mainly β-carotene (as well as other carotenoids), vitamin C and vitamin E, were shown to inhibit lipid peroxidation (6), the production of ROS (13), apoptosis (13), and oxidative damage to protein (14) and DNA (15). It is hypothesized that dietary antioxidants can potentially improve middle-aged adults’ cognitive performance and ultimately delay onset of AD in older age.

However, many previous studies used global cognition tests (16–20) or assessed dementia/AD diagnosis in older adults (21–24), and did not examine the association of antioxidants with various cognitive function domains in middle-age adults. It is important to test whether dietary intakes of carotenoids, vitamins A, C and E are differentially associated with areas of cognition among middle-aged US adults and to examine whether associations differ by sex and race. In addition, although many recent studies have found a protective effect of antioxidants against depressive symptoms (25, 26), and depressive symptoms have been directly related to poor cognitive functioning in middle-age (27–30), no study has assessed the mediating or moderating roles of depressive symptoms in association with antioxidants and middle-aged adults’ cognitive functioning. Indeed, dietary antioxidants may affect cognition directly through reducing oxidative stress at the neuronal level in areas of the brain that relate to memory and other cognitive domains or indirectly by affecting oxidative stress in areas that were linked to depressive symptoms which in turn can affect cognitive performance. Associations between antioxidants and cognition were shown to be significant only for specific cognitive domains in some studies (31–33) while other studies have shown that those associations were restricted to specific socio-demographic groups or genotypes in others (34–36). Moreover, the bioavailability of many nutrients, including antioxidants, is highly dependent socio-demographic variables and thus the variable recommendations for different age and sex groups. (37–40). Therefore, it is hypothesized that sex, race and age may moderate the association between antioxidants and cognition while depressive symptoms may act as a mediating factor.

In this report we describe the adjusted associations between dietary antioxidants and cognitive function in various cognitive domains in a large biracial population, while examining socio-demographic differences in those associations and the mediating role of depressive symptoms.

METHODS

RESULTS

Study characteristics by sex, race and age group (<median=48y vs. ≥median) are shown in Table 1. Women (vs. men), African-Americans (vs. Whites) and younger participants (vs. older) were more likely to be below poverty. African-Americans and younger participants had on average lower educational attainment and literacy compared to Whites and older participants, respectively. Men and younger participants were more likely to be current smokers than women and older participants, respectively. Similarly, African-Americans, men and younger participatns had a higher prevalence of illicit drug ever use compared to Whites, women and older participants, respectively. Average BMI was higher among women than men. Men, Whites and younger individuals were more likely to be married compared to women, African-Americans and older inidividuals, respectively.

Women had higher dietary intakes than men in vitamins C and E per 1,000 kcal, whereas Whites had higher intakes of total carotenoids, vitamin E, folate than African-Americans but lower intakes of n-3 HUFA (%energy) and vitamin C. Older participants had higher intakes of vitamin B-6 and folate compared to their younger counterparts. Mean CES-D score was higher among women compared to men (11.9 vs. 10.3, p<0.001), though no significant differences were noted by race or age. Women outperformed men on the MMSE, CVLT-List A and CVLT-DFR, the BTA and IP. Men performed better on BVRT, AF, CR and CDT. No sex differences were detected for DS-F, DS-B, Trails A or B. Whites outperformed African-Americans on all cognitive tests, and older participants had a poorer performance than younger participants, on all tests except Digits span backwards and forward as well as the clock drawing test.

When examining the relationship between CES-D score and all other variables (Table S1, Supplemental Digital Content 2), certain patterns emerged. Importantly, a higher level of depressive symptoms was associated with poorer cognitive performance on all tests, with the exception of Trailmaking Test, A. Moreover, depressive symptoms were inversely but weakly related to age, they were significantly higher among women, and among individuals with lower educational attainment, income and literacy. Current smokers and unmarried individuals also had higher levels of depressive symptoms compared to non-smokers and married participants, respectively. Among dietary intake covariates, only total carotenoids were found to be significantly and inversely related to CES-D.

Table 2 presents findings from a series of OLS regression models independent associations between socio-demographic, lifestyle and dietary correlates with the main dietary antioxidant exposures. Among key findings, age and education were positively linked to vitamin E intake per 1,000 kcal/d, African-Americans had higher intakes of vitamin C compared to Whites, whereas men had lower intakes of both vitamins A and E compared to women. Current smokers had lower intake of vitamin C which was also inversely related to BMI and ever use of drugs but positively associated with total energy intake.

Among dietary correlates of four antioxidant exposures, total carotenoids were positively associated with intakes of vitamin A, C and E, n-3 HUFA (% energy) but inversely related to vitamin B-12 (per 1,000 kcal/d). Vitamin A was positively related to vitamin B-12 and folate intakes (and total carotenoids), but inversely related to vitamin B-6 and n-3 HUFA (%energy) intakes. We additionally detected a positive relationship between vitamin C intake and vitamins E, B-6 and folate intakes, and an inverse relationship with vitamin B-12 intake. Finally, vitamins B-6, folate and n-3 HUFA (in addition to vitamin C) were positively and independently related to vitamin E intake.

Table 3 displays associations between the four dietary antioxidant exposures and cognitive performance in separate models, based on multiple regression analyses. Among those exposures, vitamin C was not associated with cognitive performance in the total population. After Bonferroni correction, 1 SD (~2.02 mg) higher intake of vitamin E per 1,000 kcal was associated with a 0.64-point higher score on the CVLT-List A (reflecting the verbal memory domain, p=0.001), independently of other antioxidants, dietary and socio-demographic, lifestyle and health-related factors included in the model. Moreover, higher intake of vitamin E in the diet was linked to better performance on AF (reflecting language/verbal test performance, β=+0.53, p=0.001). Table 3 also presents sub-group analyses by sex. Although the associations between vitamin E and verbal memory and fluency were restricted to women, there was no statistically significant effect modification by sex. Furthermore, vitamin E was positively associated with performance on IP (reflecting psychomotor speed) among women after familywise Bonferroni correction (β=+0.68, p=0.001). Moreover, Table S2 (Supplemental Digital Content 3) shows findings from covariates entered into the model for the total population, presenting only those that were statistically significant at a type I error 0.05. Aside from the findings for age, sex and race which were consistent with the bivariate results in Table 1, other notable findings included the following: Cognitive performance was better with higher literacy (all tests), higher educational attainment (MMSE, CVLT-List A, BVRT, AF, Trails B and IP), above poverty income (MMSE, BVRT, AF, BTA, Trails A and B, IP), lower CES-D score (all tests), higher BMI (CVLT-List A and CVLT-DFR), lower vitamin B-12 intake (BVRT), drug users vs. not (AF, BTA, Trails B) and higher energy intake (CR and IP).

Table 4 shows findings from sub-group analyses by race and age group (<48y vs. ≥48y), while separately evaluating two-way interactions between dietary antioxidant exposures and these potential effect modifiers in relation to cognitive performance. Among stratum-specific associations that survived multiple testing, higher intake of vitamin E in the younger group (<48y) was linked to better performance on CVLT-List A (β=+1.06, p<0.001), CVLT-DFR (β=+0.46, p=0.001; p for interaction by age group<0.05) and AF (β=+0.77, p=0.003). Tests of effect modification only showed a few other instances of heterogeneity between strata (e.g. vitamin E vs. MMSE by race and age group; total carotenoids vs. BVRT by race). A consistent, though mariginally significant result was found whereby vitamin E was positively related to IP among the younger group (β=+0.67, p=0.016). Similarly, higher vitamin E intake was marginally related to smaller number of errors among among Whites in both MMSE and BVRT (0.004<p<0.05, with a signficant racial difference in the case of MMSE (p=0.033).

Multiple linear regression models were conducted to test mediation of the antioxidant-cognition association through CES-D scores (Table S3, Supplemental Digital Content 4). In the overall population, the total effects of vitamin E on CVLT-List A and CVLT-DFR indicated a putative protective effect, whereas there were positive association between vitamin E and CES-D scores and an inverse relationship between those two cognitive test scores and CES-D. When CES-D was entered into the model with antioxidants vs. CVLT-List A and CVLT-DFR, the net effect of vitamin E was markedly altered compared to the total effect, with a significant S-G test (p=0.032 and 0.035, respectively). In addition, the respective MPs were −13% and −16%. In all other total effects under study, CES-D did not show an appreciable mediating effect, especially when examining the S-G test.

Figure 1 displays the findings from a structural equations model where CVLT-List A is shown as an example for cognitive test score outcomes and predicted by the four antioxidants whose total effect is allowed to be partially mediated by CES-D score, by including a direct effect from each antioxidant into the cognitive test score. Our findings are in line with the S-G test and the MP estimate (~−13%). In addition, when stratifying the SEM by race, we found that associations in which vitamin E was positively associated with CES-D, CES-D inversely related to CVLT-List A while vitamin E had a positive and significant direct effect on CVLT-List A mainly among African-Americans. Among whites, the path coefficient (α) from vitamin E into CES-D was non-significant.

Open in a separate window

FIGURE 1

Structural Equation Model (SEM) for associations between antioxidants and a test of verbal memory (CVLT-List A): mediating effects of depressive symptoms (CES-D): HANDLS study

Key: AA=African-American; HANDLS=Healthy Aging in Neighborhoods of Diversity across the Life Span; HS=High School; HUFA=Highly Unsaturated Fatty Acids; OLS=Ordinary Least Square; PIR=Poverty Income Ratio; RE=Retinol Equivalent; SEE=Standard error of the estimate; WRAT=Wide Range Achievement Test.

Note: Path coefficients between antioxidant exoosure and CES-D or cognitive scores are denoted by α and labelled by to the predictor and otucome variables of each path.

When relaxing the assumption of additivity between the CES-D score (mediator) and each of the antioxidant exposures by allowing for interaction, we computed four distinctive estimates to assess mediation of antioxidant-CS relationship through CES-D (Table 5). Our findings were in line with the previous mediation analysis (Table S1), whereby vitamin E’s total effect on CVLT-List A and CVLT-DFR appeared to be partially mediated through CES-D with a significant NIE and a MP estimated at 13–16%.

DISCUSSION

This is one of very few studies that examined the association between antioxidants and cognitive functioning in various domains among young and middle-aged US adults, and is the first to examine potential moderation by sex, race and age and mediation by depressive symptoms. Among key findings, dietary vitamin E intake was positively associated with performance in domains of verbal memory (total population and the younger group (<48y)), verbal fluency (total population and the younger group (Age<48y)) and psychomotor speed (women). Vitamin E was positively linked to CES-D among African-Americans, yet its positive association with verbal memory was only partially mediated by depressive symptoms (total population).

Vitamin E has not only antioxidant activity but also functions in other independent roles such as inhibiting brain protein kinase C activity. This ability is most likely attributed to the multiple isoforms of Vitamin E. (37, 81) It is recognized that the diet contains several Vitamin E isoforms while the results of this study reflect intake of only one form of Vitamin E, alpha tocopherol. Due to the increased use of oils in the US diet, there has been an increase in the gamma tocopherol, the form that has similar antioxidant capacity but greater anti-inflammatory properties compared alpha tocopherol, Morris reported that only α and γ-tocopherols found in foods were linked to slower rate of cognitive decline over a 6y period (82). This observation was corroborated by Commenges and colleagues (20).

It is worth noting, however, that in the case of vitamin E, our study population had on average a level of alpha tocopherol consumption that meets roughly 50% of Estimated Average Requirement (EAR) (Mean±SD: 6.8±5.0 mg/d vs. EAR=12 mg./d), with only 10.1% of the distribution being adequate. This is in contrast with vitamin A (38.8% meeting the EAR) and vitamin C (41.4% meeting the EAR). (83) Thus, our findings should be interpreted in light of this difference in distributions of intakes compared to other studies whose selected population consumed higher amounts of vitamin E or included supplemental intake in addition to dietary sources. (e.g. (32–35)) It is unclear from other publications when the Vitamin E intake included all isoforms due to a lack of description in the diet methodology. Morris and colleagues (34) reported that dietary intake of vitamin E, but not other antioxidants, was associated with a reduced risk of incident AD, although this association was restricted to individuals without the Apolipoprotein E ε4 genotype. Similar findings were reported with cognitive decline as an outcome. (35)

Vitamin C, which is essential for the reduction of vitamin E, was not associated with cognitive performance in the HANDLS study population. This finding differs from the results of a few recent prospective cohort studies which examined the association of other dietary antioxidants with various cognitive outcomes. One cross-sectional study found that participants in the lowest 10^th percentile of vitamin C intakes had poorer performance on abstract thinking and problem-solving task. (33) Another study reported that high dietary intake of vitamins C and E may reduce the risk of AD (22). This relationship most pronounced among smokers. The inverse association between vitamin C intake and cognitive impairment as assessed by the MMSE was corroborated by Paleologos and colleagues, (84) while Sato and colleagues only found this association in men (36).

When evaluating the association between antioxidants and cognitive domains, one study found that past intakes of vitamins A and E were associated with better performance on visuospatial recall and/or abstraction performance. (32) These results were similar to ours, though some of our related findings did not survive multiple testing correction. Another study, suggested that dietary antioxidants were not able to reduce AD risk. (24) Similarly, Laurin and colleagues (23) found no association between midlife dietary intake of vitamins E and C and dementia incidence. At least four other cohort studies came to a similar conclusion, especially after adjustment for potentially confounding factors. (16, 85–87) In addition to examining associations of cognition with vitamins A, C and E, other studies found that carotenoids, particularly β-carotene intake, may have beneficial effects of various cognitive outcomes (16) though others were not able to detect such an association (35, 86, 87).

Epidemiologic studies examining relationships between supplemental antioxidants and various cognitive outcomes found mixed results. In fact, vitamin C supplement use was related to lower AD risk in one cohort study, (88) whereas vitamin E and vitamin C supplements in combination were associated with reduced prevalence and incidence of AD and cognitive decline in three other cohort studies (89–91). Yet Grodstein and colleagues, found this effect to be specific to Vitamin E supplements. (92) The putative protective effect of supplemental antioxidant use against adverse cognitive health outcomes was replicated in a large cohort study (93). In addition, a post-hoc analysis of a large trial found that a combination of supplements including but not limited to β-carotene, vitamin C and E can improve verbal memory in the long-term (six years after the trial). (31) However, there was little evidence of a cognitive benefit from use of antioxidant supplements, particularly vitamins C and E, according to at least five independent cohort studies. (24, 94–97) Finally, a recent randomized controlled trial examining transition from mild cognitive impairment to AD, found no significant association between treatment with supplemental vitamin E and the outcome of interest. (98)

Our study has notable strengths. First, it is one of the largest studies examining the primary question of interest, it made use of extensive cognitive function tests, a valid measure of depressive symptoms (the CES-D) and advanced multivariate techniques such as structural equations modeling, mediation analyses, and Heckman selection models among others. Second, it is one of few to use the average of two 24-hr recalls while estimating usual dietary intakes of antioxidants.

Our study also has limitations. First, its cross-sectional design precluded temporality ascertainment, highlighting the importance of conducting further longitudinal studies in a U.S. community. Moreover, due to lack of factorial invariance between race and sex groups as well as poverty income ratio groups, the use of cognitive domains --obtained from confirmatory factor analysis-- that were comparable between those groups was not feasible in this study. Data on supplemental intakes of antioxidants were not available for the baseline wave and thus were not accounted for in estimating total intakes. Generally speaking, it was estimated that between 2003–2006, more than half of adults used dietary supplements. (99) In addition, supplement users also had higher intakes of vitamins A, C and E from foods and in total than non-users. However, they also tended to exceed the tolerable upper intake level for vitamins A and C compared to non-users. (100) Whether antioxidants are obtained from diet or supplements, plasma concentration would be a more sensitive indicator of antioxidant and oxidative stress status, while reducing reporting bias. (45, 88–92) Although, the baseline HANDLS study did not incorporate these measures, future waves may support this analysis, thereby enhancing our understanding of these associations in an urban, low-income population.

In conclusion, our study indicated that dietary intakes of selected antioxidant nutrients and cognition are closely related, although these relationships may vary according to sex, race and depressive status and are specific to certain domains of cognition as well as the nutrient. In particular, and among key findings, we found that vitamin E was positively associated with performance in domains of verbal memory and fluency in the total population and psychomotor speed among women. The association between vitamin E and verbal memory was only partially mediated by depressive symptoms. Future cohort studies and dietary antioxidant interventions should focus on association of dietary vitamin E with age-related cognitive decline, particularly in the domains of verbal memory, verbal fluency, and psychomotor speed.

Supplementary Material

Acknowledgements

Abbreviations

Footnotes

REFERENCES