Rationale: Protein intake recommendations after metabolic and bariatric surgery range from 60 g/day to 2.1 g/kg ideal body weight but is based on weak evidence and does not consider time since surgery1.
Aim: This study determined long-term protein requirements in adult men after Roux-en-Y gastric bypass (RYGB) using the Indicator Amino Acid Oxidation (IAAO) method.
Methodology: In a randomized, within-subject feeding study, 7 protein intake levels (0.2–2.0 g/kg body weight) were compared in adult men (18–65 years), 1-10 years after RYGB surgery and without diabetes. Each study day followed a 2-day standardized run-in diet, whereafter participants received eight hourly isocaloric meals, with different test protein intakes as free amino acids based on egg protein composition with excess phenylalanine (Phe) and tyrosine. Energy intake was individualized based on measured energy expenditure. From the fifth meal, 1-¹³C-phe was added, and breath ¹³CO₂ measured during isotopic steady state to assess oxidation. Protein requirements were estimated using mixed effects changepoint regression on expired ¹³CO₂. Sample size was estimated based on the expected difference in amino acid oxidation between protein intakes below and above the predicted breakpoint, using an effect size derived from prior IAAO data; power analysis (80% power, α = 0.05) indicated that 7 participants at each intake level would be sufficient. The study protocol was approved by the Research Ethics Committee of UZ Leuven Hospital and KU Leuven University, registered at ClinicalTrials.gov (NCT06853652), and conducted in accordance with the Declaration of Helsinki, Good Clinical Practice (GCP) guidelines, and applicable regulatory requirements.
Results: Eight participants completed 49 study days [median age 57 years (IQR 38–63)], median 34 months (IQR 18–39) post-surgery, achieving 76% (IQR 66–80) excess weight loss. Estimated average protein requirements were 1.42 g/kg/day (95% Confidence Interval [CI] 1.31–1.53; R² 0.85).
Conclusion: This study provides the first direct estimate of protein requirements long-term after RYGB. Requirements appear substantially elevated relative to IAAO derived requirements for healthy younger or older men (average 0.93 g/kg, upper 95% CI 1.24 g/kg), potentially reflecting reduced postprandial efficiency in protein utilization2,3. Translating these findings into recommendations will need careful consideration of aspects such as protein type, meal timing, sex, obesity and diabetes.