Threshold Effects of Total Copper Intake on Cognitive Function in US Older Adults and the Moderating Effect of Fat and Saturated Fatty Acid Intake

Abstract

BackgroundEvidence for a relationship between total copper intake and cognition is lacking, and few studies have assessed the moderating effect of dietary fat and saturated fatty acid (SFA) intake on this relationship. ObjectiveOur aim was to explore the curvilinear association between total copper intake and cognitive function in older adults, and to clarify the moderating effect of dietary fat and SFA intake on the association. DesignThis was a cross-sectional analysis of data from National Health and Nutrition Examination Surveys 2011-2014. ParticipantsThe analysis included 2,483 participants aged 60 years and older. Main outcome measuresCognitive function was evaluated by the Consortium to Establish a Registry for Alzheimer’s Disease (CERAD) Word Learning subtest, the Animal Fluency Test, and the Digit Symbol Substitution Test (DSST). Statistical analyses performedSmooth curve fitting and two-piecewise linear regression models were performed to address the nonlinear association between total copper intake and cognitive function. Multivariable quadratic regression models and analyses stratified by total fat or SFA intake were used to assess the effects of the interaction between copper and fat intake and between copper and SFA intake on cognitive function. ResultsThere was a nonlinear association between total copper intake and cognitive test scores. The inflection point of copper was 0.8 mg/d for the Consortium to Establish a Registry for Alzheimer’s Disease Word Learning subtest and 1.4 mg/d for both the Animal Fluency test and the DSST. When copper intake was below the inflection point, positive associations were apparent for copper intake and Consortium to Establish a Registry for Alzheimer’s Disease Word Learning subtest scores (β = 3.9; 95% CI 1.2 to 6.5), Animal Fluency test scores (β = 1.7, 95% CI .9 to 2.6), and DSST scores (β = 6.0, 95% CI 3.8 to 8.3). When copper intake was above the inflection point, a nonsignificant downward trend was found. Interactive effects between total copper and total fat intake (P interaction = .000) and between total copper and SFA intake (P interaction = .011) on the DSST scores were observed. In the low fat intake and low SFA intake groups, DSST scores first increased and then decreased with increasing copper. However, in the high fat intake and high SFA intake groups, DSST scores first increased and then flattened with increasing copper. ConclusionsThe present study suggests a nonlinear association between copper intake and cognitive function and identifies threshold effects of copper intake on cognitive function. Copper intake below the inflection point was positively and independently associated with cognitive function. High fat and high SFA intake may protect older adults against a decline in DSST scores related to high copper intake.