Diabetes mellitus is linked to an increased risk of cognitive impairment, particularly affecting verbal memory. The Mediterranean diet (MedD), characterized by high intakes of fruits, vegetables, cereals, nuts, and olive oil, moderate intakes of fish, wine, and dairy products, and low intakes of saturated fat and meat, has been suggested to improve cognitive performance. Studies have shown MedD's benefits on memory and fluency, primarily in healthy elderly individuals or those with cardiovascular risk factors, including diabetes. However, research specifically on type 2 diabetes patients shows inconsistent findings; some studies demonstrate a link between MedD adherence and reduced cognitive impairment risk, while others show no such association in individuals without diabetes. The effect of MedD on cognitive function in diabetic individuals, and whether this association differs between diabetes types and at varying disease durations, remains unclear. A previous study revealed an association between verbal memory and metabolic parameters in type 2 diabetes patients. This study aimed to (i) identify associations between MedD and cognitive performance, focusing on verbal memory, in patients with recently diagnosed or long-standing type 1 or type 2 diabetes and metabolically healthy individuals, and (ii) investigate potential differences between diabetes types.

Existing literature highlights a correlation between diabetes mellitus and cognitive decline, especially in verbal memory. Systematic reviews have indicated that the Mediterranean diet (MedD) may offer benefits to overall cognitive function and specific cognitive domains like memory. However, the majority of studies examining the relationship between MedD and cognition have focused on healthy elderly populations or those with elevated cardiovascular risk, with diabetes being one potential contributing factor. Studies focusing solely on type 2 diabetes patients yield mixed results, with some showing an association between higher MedD adherence and a reduced risk of cognitive impairment, while others report no such association in non-diabetic individuals. This inconsistency underscores the need for further investigation into the effect of MedD on cognitive function in individuals with diabetes, particularly exploring potential differences between diabetes types and the impact of disease duration.

This cross-sectional analysis included participants from the German Diabetes Study (GDS): 41 metabolically healthy individuals, 119 with type 1 diabetes, and 180 with type 2 diabetes (with disease duration <1 year or ≥5 years). Type 1 diabetes patients received insulin treatment, while type 2 diabetes patients were treated with oral glucose-lowering drugs (19%), insulin (6%), or lifestyle modifications (75%). Metabolically healthy individuals received no glucose-lowering medication. Participants were consecutively included if they completed cognition tests (the Brief Assessment of Cognition in Schizophrenia (BACS) and additional tests) and a food frequency questionnaire (FFQ). The FFQ, a validated 148-item semi-quantitative instrument from the EPIC Potsdam study, assessed habitual dietary intake over the past 12 months. The Modified Mediterranean Diet Scale (MMDS) was used to quantify MedD adherence, assigning scores based on median consumption of nine components. Data analysis involved SAS (version 9.4), with p<0.05 indicating statistical significance. Multiple linear regression models were used, with MMDS scores as independent predictors and cognition test results as dependent variables. Model 1 was unadjusted, Model 2 included age, sex, and total energy intake (TEI) as covariates, and Model 3 additionally adjusted for crystallized intelligence and fasting C-peptide. Adjusted means were calculated by MMDS tertiles, and interaction effects were investigated.

Metabolically healthy individuals and type 2 diabetes patients were older and had higher BMI than type 1 diabetes patients. Metabolic control (HbA1c) in all diabetic patients was within recommended guidelines. Mean cognition test values were within the normal range. Type 2 diabetes patients reported lower TEI (8.6 MJ/d) compared to type 1 diabetes patients and healthy individuals (≈10 MJ/d). Participants achieved a mean MMDS score of 4.5. In type 2 diabetes patients with ≥5 years of diagnosis, closer MMDS adherence correlated with higher verbal memory scores (p=0.043 after adjusting for confounders). This association was not seen in recently diagnosed type 2 diabetes patients, type 1 diabetes patients, or metabolically healthy individuals. The interaction analysis indicated that the association between MMDS and verbal memory only holds true for type 2 diabetes patients with at least 5 years of diabetes duration (P=0.024). No association was found between MMDS adherence and other cognitive performance parameters.

The findings suggest that higher MedD adherence is associated with better verbal memory performance in type 2 diabetes patients with a disease duration of at least five years. MedD's potential benefits on cardiovascular health and cognitive function have been documented previously, mainly in healthy elderly or high-cardiovascular risk individuals. The observed association between MedD and verbal memory in this study may be attributed to the diet's high antioxidant content, reducing reactive oxygen species and mitigating inflammatory processes, both linked to cognitive decline. The positive effects might also be mediated by n-3 fatty acids, known for their anti-inflammatory and antioxidative properties. The lack of association in type 1 diabetes patients might be due to differences in cognitive impairment patterns between type 1 and type 2 diabetes. This study's results align with other prospective studies showcasing a positive link between MedD and cognitive function after several years of follow-up, and mostly in well-controlled diabetic patients.

This study demonstrates a positive association between higher adherence to the Mediterranean diet and improved verbal memory in type 2 diabetes patients with a disease duration of at least five years. This finding suggests that a MedD may offer cognitive benefits in long-term type 2 diabetes management. Further research is needed to explore the underlying mechanisms and to investigate whether this association extends to other cognitive domains and different diabetes types. Longitudinal studies with larger samples are needed to confirm these findings and to establish causal relationships.

This study is limited by its cross-sectional design, preventing the establishment of causality. The use of a food frequency questionnaire (FFQ) for dietary assessment, while validated, might introduce recall bias. The relatively small sample size, particularly within subgroups, limited the ability to control for other potential confounding factors. Future research should employ larger, longitudinal studies with more comprehensive dietary assessments to confirm these findings and investigate the underlying mechanisms more thoroughly.