We examined the effectiveness of dietary counseling performed within a trimodal prehabilitation study for patients with cancer awaiting hepato-pancreato-biliary (HPB) surgery. Additionally, we explored relationships between nutritional status and health-related quality of life (HRQoL). The dietary intervention aimed to achieve a protein intake of 1.5 g/kg/day and reduce nutrition-impact symptoms. Patients received dietary counseling 4 weeks prior to surgery (prehabilitation group); the rehabilitation group just before surgery. We used 3-day food journals to calculate protein intake and the abridged Patient-generated Subjective Global Assessment questionnaire (aPG-SGA) to determine nutritional status. We utilized the Functional Assessment of Cancer Therapy-General questionnaire to measure HRQoL. Sixty-one patients participated in the study (30 = prehabilitation). Dietary counseling achieved a significant increase in preoperative protein intake (+0.3 ± 0.1 g/kg/day, P = 0.007), with no change in the rehabilitation group. Dietary counseling did not mitigate a significant increase in aPG-SGA postoperatively (prehabilitation: +5.8 ± 1.0; rehabilitation: +3.3 ± 1.0; P < 0.05). aPG-SGA was predictive of HRQoL (β = -1.77, P < 0.0001). HRQoL remained unchanged in both groups over the study period. Dietary counseling within a HPB prehabilitation program improves preoperative protein intake, but not aPG-SGA, which is predictive of HRQoL. Future studies should examine whether specialized medical management of nutrition-impact symptoms would improve HRQoL outcomes within a prehabilitation model.