Effects of Recommended Supplementation and Mediterranean Diet Adherence on Post-Metabolic Bariatric Surgery Outcomes

<b>Background/Objectives:</b> Metabolic bariatric surgery is an effective treatment for severe obesity but is associated with an increased risk of postoperative nutritional deficiencies. This study aimed to assess the impact of adherence to the Mediterranean diet on selected laboratory parameters and excess weight loss after bariatric surgery. <b>Methods:</b> Eighty adults with obesity were evaluated before and 6 months after surgery. Based on dietary questionnaires, patients were classified as adhering to the Mediterranean diet (<i>n</i> = 32) or not adhering (<i>n</i> = 48). Laboratory parameters, including vitamin D, vitamin B12, folic acid, iron, ferritin, calcium, hemoglobin, and total protein, were assessed pre- and postoperatively. Excess weight loss percentage (EWL%) was calculated using standard methodology. <b>Results:</b> Postoperatively, vitamin D and total protein levels increased in both groups, with median increases of 9.45 ng/mL and 1.1 g/dL, respectively. A significant increase in iron concentration was observed only in Mediterranean diet adherents (median +24 µg/dL), while a decrease was noted in non-adherent patients (-4 µg/dL). Iron deficiency and iron-deficiency anemia occurred exclusively in the non-Mediterranean diet group (10.4% vs. 0%). Median EWL% was higher in Mediterranean diet adherents (44% vs. 31%), although the difference was not statistically significant. <b>Conclusions:</b> Adherence to the Mediterranean diet may reduce the risk of iron deficiency after bariatric surgery, whereas total protein concentration alone appears insufficient for assessing nutritional status or weight loss effectiveness.