Serum Galectin-9 and Decorin in relation to brain aging and the green-Mediterranean diet: A secondary analysis of the DIRECT PLUS randomized trial

Background and aims: We explored whether changes in serum proteomic profiles differed between participants with distinct brain aging trajectories, and whether these changes were influenced by dietary intervention. Methods: In this secondary analysis of the 18-month DIRECT PLUS trial, 294 participants were randomized to one of three arms: 1) Healthy dietary guidelines (HDG); 2) Mediterranean (MED) diet (+440 mg/day polyphenols from walnuts); or 3) low red/processed meat green-MED diet (+1240 mg/day polyphenols from walnuts, Mankai plant, and green tea). We measured 87 serum proteins (Olink-CVDII). We used Magnetic-Resonance-Imaging (MRI)-assessed 3D-T1-weighted brain scans for brain age calculation (by convolutional neural network) to identify protein markers reflecting the brain age gap (BAG; deviation of MRI-assessed brain age from chronological age). Results: At baseline, lower weight, waist circumference, diastolic blood pressure, and HbA1c parameters were associated with a younger brain age than expected. Specifically, higher levels of two proteins, Galectin-9 (Gal-9) and Decorin (DCN), were associated with accelerated brain aging (larger BAG). A proteomics principal component analysis (PCA) revealed a difference in PC1 between the two time-points for participants with accelerated brain aging. Between baseline and 18 months, Gal-9 significantly decreased among individuals who completed the intervention with attenuated brain aging, while DCN significantly increased among those who completed the trial with accelerated brain aging. A significant interaction was observed between the green-MED diet and proteomics PCA, resulting in a beneficial change compared to the HDG. Participants in the green-MED diet significantly decreased Gal-9 compared to the HDG diet and from baseline. Conclusions: Higher serum levels of Gal-9 and DCN may indicate an acceleration of brain aging and could be reduced by a green-MED/high-polyphenol (green tea and Mankai) and low-red/processed meat diet. Trial registration number: NCT03020186.