Body Position Modulates Gastric Emptying and Affects the Post-Prandial Rise in Plasma Amino Acid Concentrations Following Protein Ingestion in Humans

Dietary protein digestion and amino acid absorption kinetics determine the post-prandial muscle protein synthetic response. Body position may affect gastrointestinal function and modulate the post-prandial rise in plasma amino acid availability. We aimed to assess the impact of body position on gastric emptying rate and the post-prandial rise in plasma amino acid concentrations following ingestion of a single, meal-like amount of protein. In a randomized, cross-over design, eight healthy males (25 +/- 2 years, 23.9 +/- 0.8 kg.m(-2)) ingested 22 g protein and 1.5 g paracetamol (acetaminophen) in an upright seated position (control) and in a -20 degrees head-down tilted position (inversion). Blood samples were collected during a 240-min post-prandial period and analyzed for paracetamol and plasma amino acid concentrations to assess gastric emptying rate and post-prandial amino acid availability, respectively. Peak plasma leucine concentrations were lower in the inversion compared with the control treatment (177 +/- 15 vs. 236 +/- 15 mmol.L-1, p <0.05), which was accompanied by a lower plasma essential amino acid (EAA) response over 240 min (31,956 +/- 6441 vs. 50,351 +/- 4015 AU; p <0.05). Peak plasma paracetamol concentrations were lower in the inversion vs. control treatment (5.8 +/- 1.1 vs. 10.0 +/- 0.6 mg.L-1, p <0.05). Gastric emptying rate and post-prandial plasma amino acid availability are significantly decreased after protein ingestion in a head-down tilted position. Therefore, upright body positioning should be considered when aiming to augment post-prandial muscle protein accretion in both health and disease.