Multimodal prehabilitation programs to improve physical fitness before surgery often include nutritional interventions. This study evaluates the efficacy of and adherence to a nutritional intervention among colorectal and esophageal cancer patients undergoing the multimodal Fit4Surgery prehabilitation program. The intervention aims to achieve an intake of ≥1.5 g of protein/kg body weight (BW) per day through dietary advice and daily nutritional supplementation (30 g whey protein). This study shows 56.3% of patients met this goal after prehabilitation. Mean daily protein intake significantly increased from 1.20 ± 0.39 g/kg BW at baseline to 1.61 ± 0.41 g/kg BW after prehabilitation (p < 0.001), with the main increase during the evening snack. BW, BMI, 5-CST, and protein intake at baseline were associated with adherence to the nutritional intervention. These outcomes suggest that dietary counseling and protein supplementation can significantly improve protein intake in different patient groups undergoing a multimodal prehabilitation program.