Scientific Study

Background: Improving brain insulin sensitivity, which can be assessed by measuring regional cerebral blood flow (CBF) responses to intranasal insulin, may prevent age-related metabolic and cognitive diseases. Objective: This study aimed to investigate longer-term effects of mixed nuts on brain insulin sensitivity in older individuals with overweight/obesity. Methods: In a randomized, single-blinded, controlled, crossover trial, twenty-eight healthy adults (mean±SD; 65±3 years; BMI: 27.9±2.3 kg/m2) received either daily 60 g mixed nuts (15 g of walnuts, pistachio, cashew, and hazelnuts) or no nuts (control) for 16 weeks, separated by an 8-week washout period. Throughout the study, participants were instructed to adhere to the Dutch food-based dietary guidelines. During follow-up, brain insulin action was assessed by quantifying acute effects of intranasal insulin on regional CBF using arterial spin labeling magnetic resonance imaging. Furthermore, effects on peripheral insulin sensitivity (oral glucose tolerance test), intrahepatic lipids, and cardiometabolic risk markers were assessed. Results: Body weight and composition did not change. Compared with control, mixed nut consumption improved regional brain insulin action in five clusters located in the left (difference in CBF responses to intranasal insulin: -4.5±4.7 mL/100g/min; P<0.001; -4.6±4.8 mL/100g/min; P<0.001; and -4.3±3.6 mL/100g/min; P=0.007) and right occipital lobe (-4.3±5.6 mL/100g/min; and -3.9±4.9 mL/100g/min; P=0.028). A fifth cluster was part of the left frontal lobe (-5.0±4.6 mL/100g/min; P<0.001). Peripheral insulin sensitivity was not affected. Intrahepatic lipid content (-0.7 %-point; 95%CI: -1.3 to -0.1; P=0.027), serum LDL cholesterol (-0.24 mmol/L; 95%CI: -0.44 to -0.04; P=0.019), and systolic blood pressure (-5 mmHg; 95%CI: -8 to -1; P=0.006) were lower after the mixed nut intervention. Conclusions: Longer-term mixed nut consumption affected insulin action in brain regions involved in the modulation of metabolic and cognitive processes in older adults with overweight/obesity. Intrahepatic lipid content and different cardiometabolic risk markers also improved, but peripheral insulin sensitivity was not affected.
https://doi.org/10.1016/j.ajcnut.2023.12.010