The association between dietary dark green vegetable intake and cognitive function in US older adults

Dark green vegetables include dark green leafy vegetables and broccoli. They are sources of many essential nutrients, including vitamins A, B and C, folate, fibre, carotenoids and flavonoids. This study aimed to explore the association between dietary dark green vegetable intake and cognitive function in US older adults. We included 2344 older adults (≥60 years old) from the National Health and Nutrition Examination Survey (NHANES) 2011-2014 cycles. Dark green vegetable consumption was assessed using a continuous variable (cups/day) and two categorical variables. The first categorical variable classified participants into non-consumers and consumers based on whether they consumed dark green vegetables. The second categorical variable grouped participants into four levels of dark green vegetable consumption (non-consumers, consumers with low intakes, consumers with moderate intakes and consumers with high intakes). We used five continuous variables with non-normal distribution to assess cognitive function, including a composite z-score and the standardised scores of four individual cognitive tests. The four cognitive tests included the Immediate Recall Test (IRT), the Delayed Recall Test (DRT), the Animal Mobility Test (AFT) and the Digit Symbol Substitution Test (DSST). The standardised scores of the four cognitive tests were calculated using the mean and standard deviation of each cognitive test score. The composite z-score was calculated by averaging the standardised scores of four cognitive tests to evaluate global cognition. We used multiple linear regression models to examine the association between dietary dark green vegetable intake and cognitive function. Our findings indicated that dark green vegetable intake was positively associated with global cognition (β [95% CI]: 0.17 [0.04, 0.30]; p = 0.016) and IRT (β [95% CI]: 0.26 [0.08, 0.43]; p = 0.009) and DRT (β [95% CI]: 0.21 [0.05, 0.36]; p = 0.012) standardised scores. Individuals with high intake of dark green vegetables showed notably better global cognition (β [95% CI]: 0.16 [0.05, 0.28]; p = 0.010) and showed higher IRT (β [95% CI]: 0.22 [0.07, 0.38]; p = 0.010) and DRT standardised scores (β [95% CI]: 0.21 [0.07, 0.36]; p = 0.007) compared with the non-consumers. Blood neutrophil counts mediated the cognitive benefits of dark green vegetables (Proportion: 9.5%, p = 0.006). In conclusion, our findings suggest that dark green vegetable consumption may have favourable effects on cognitive function in US older adults, especially on immediate and delayed learning abilities. The underlying mechanisms include the ability of dark green vegetables to reduce blood neutrophil levels, an indicator of decreased systemic inflammation. Increasing dietary intake of dark green vegetables may be a beneficial intervention to improve cognitive health in the older US population.