On average, women completed five dietary assessments during the analysis period and three assessments after diabetes diagnosis. We observed few meaningful differences in health and lifestyle across tertiles of several major fat types, with intake averaged since midlife (). However, women with higher intakes of saturated and trans fat were slightly less likely to take vitamin E supplements and had slightly higher prevalence of obesity and current smoking. Women with higher saturated fat intake were also somewhat less likely to exercise and have a history of hypercholesterolemia (women with high cholesterol may have initiated dietary changes). In addition, women in increasing tertiles of polyunsaturated fat intake were somewhat more likely to use vitamin E supplements.
Characteristics of women at initial cognitive assessment, by tertile of dietary fat intake since midlife
For diet since midlife, we found significantly worse cognitive decline on the global score with increasing intakes of saturated (P trend = 0.02) and trans fat (P trend = 0.002) (). For example, women in the highest tertile of trans fat intake had a mean decline in the global score that was 0.15 standard units (95% CI −0.24 to −0.06) worse than those in the lowest tertile, after multivariable adjustment for age, education, BMI, physical activity, and measures of diabetes severity. Results were virtually identical with further adjustment for depression, vitamin E supplement use, alcohol intake, smoking status, and history of high blood pressure, high cholesterol, or myocardial infarction (mean difference in decline = 0.16 standard units; 95% CI −0.25 to −0.07, comparing extreme tertiles of trans fat) (data not shown in ). For saturated fat, the mean difference in global decline was −0.12 standard units (95% CI −0.22 to −0.01) comparing top and bottom tertiles. To help interpret these results, we compared these effect estimates to those we found for the relation of age to cognitive decline in our population. We found that a 1-year age increase was associated with a mean global score decline of 0.02 standard units; thus, the association we observed for high consumption of trans fat was equivalent to ~7 years of cognitive aging, and the observed relation for high saturated fat intake was equivalent to 6 years of cognitive aging.
Mean differences in change in cognitive function scores, by tertile of dietary fat intake since midlife
In contrast, women with higher intake of monounsaturated fat maintained better cognitive function than those with lower intake, although this finding was only borderline significant (P trend = 0.06). Polyunsaturated fat intake, considered alone, was not significantly associated with cognitive decline (P trend = 0.5); however, women with a higher ratio of polyunsaturated to saturated fat intake had significantly lower rates of cognitive decline for the global score (P trend = 0.03). Specifically, compared with women in the lowest tertile of polyunsaturated fat relative to saturated fat intake, those in the highest tertile declined an average of 0.08 standard units less (95% CI 0.008–0.16).
In secondary analyses, results were not substantially different when we adjusted for A1C levels, although P values were higher in the small subset of women with this information. For example, the mean difference in global score was −0.12 standard units before adjustment for A1C versus −0.11 after adjustment, when extreme tertiles of saturated fat were compared.
Substitution models yielded results that were consistent with those given above (data not shown in ); specifically, replacement of 1% of total energy from “bad” fat (saturated and trans unsaturated) with the same percentage of energy from “good” fat (mono- and polyunsaturated) was associated with significantly less cognitive decline. For example, for a 5% substitution, the mean difference in global score decline was 0.15 standard units (95% CI 0.005–0.30). That is, replacing 5% of energy from bad fat with good fat could be considered cognitively equivalent to delaying aging by ~7 years.
In analyses of diet after diabetes diagnosis, women had an average of 9 years between diagnosis and initial cognitive interview. Relations of postdiabetes fat intake and cognitive decline were similar to those observed when we considered diet since midlife (). Increasing intake of saturated fat was related to worse cognitive decline across global (P = 0.003) and verbal scores (P trend = 0.02). Specifically, women in the highest tertile of saturated fat declined an average of 0.18 standard units more in the global score than those in the lowest tertile (95% CI −0.29 to −0.06). We also found that women with higher intake of trans fat had worse decline on the global score (P trend = 0.07), a finding that was borderline significant. As anticipated, average trans fat intake in these analyses of more recent diet was lower (by 19%) than in analyses of diet since midlife. In addition, a higher polyunsaturated fat–to–saturated fat ratio was related to better maintenance of cognition; this relation was also borderline significant (P trend = 0.08). Finally, results were similar in models that simultaneously included separate terms for fat intake before and after diabetes diagnosis, indicating that diet could be equally important during the period before and after clinical diagnosis of diabetes.
Mean differences in change in cognitive function scores, by tertile of dietary fat intake after diabetes diagnosis
In preliminary analyses of decline over three cognitive assessments, we found results consistent with those reported above. Greater intakes of saturated and trans fat were associated with worse cognitive decline for both global and verbal scores, although not all trends were statistically significant (e.g., for trans fat, P trend = 0.2 for the global score and P trend = 0.05 for the verbal score).