Original article Oral citrulline does not affect whole body protein metabolism in healthy human volunteers: Results of a prospective, randomized,double-blind,cross-over study Ronan Thibault a ,b,c , Laurent Flet d , Fabienne Vavasseur c , Marie Lemerle b , Véronique Ferchaud-Roucher e , Denis Picot f , Dominique Darmaun a ,b,c , * a Service d’Hépato-gastro-entérologie et Assistance Nutritionnelle, CHU de Nantes, Nantes,France b UMR 1280 Physiologie des Adaptations Nutritionnelles, INRA,Université de Nantes Nantes, France c Centre d’Investigation Clinique, CRNH Nantes and Institut des Maladies de l’Appareil Digestif, CHU de Nantes, Nantes F-44093, France d Pharmacie, Hôtel Dieu,CHU de Nantes, Nantes F-44093, France e Plateforme de Spectrométrie de Masse, CRNH Nantes, CHU de Nantes, Nantes F-44093, France f Service de Réadaptation Digestive et Nutritionnelle, Clinique Saint Yves, Rennes F-35044, France a r t i c l e i n f o Article history: Received 11 January 2011 Accepted 12 June 2011 Keywords: Citrulline Protein metabolism Healthy humans Intestinal insufficiency Insulin s u m m a r y Background & aims: Citrulline increases protein synthesis during refeeding in rodents with short bowel syndrome, aging and malnutrition, and improves nitrogen balance in fed healthy humans. The aim of the current study therefore was to determine whether citrulline had affected protein metabolism in healthy volunteers. Methods: In a randomized, double-blind,cross-over study, 12 healthy adults received a 5-h intravenous infusion of L-[1- 13 C]-leucine in the post-absorptive state, after a 7-day oral supplementation with 0.18 g/ kg/day citrulline,or an iso-nitrogenous placebo. Treatment order was randomized, treatment periods were separated by 13-day wash out. Leucine appearance rate (Ra) was determined from plasma [1- 13 C]- keto-iso-caproate enrichmentand leucine oxidation from expired 13 CO 2 , and nitrogen balance was estimated from 6-h urinary urea excretion. Results:Compared with placebo,oral citrulline supplementation increased plasma citrulline, arginine and ornithine concentrations, but failed to affect albumin,transthyretin,free insulin and insulin-like growth factor (IGF)-1 plasma concentrations, urinary nitrate excretion,or nitrogen balance.Citrulline supplementation did not alter leucine Ra, leucine oxidation,nor whole-body protein synthesis. Conclusion: In healthy, well nourished volunteers, oral citrulline increases plasma citrulline and arginine availability but does not affect whole-body protein kinetics in the post-absorptive state. Ó 2011 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved. 1. Introduction Citrulline is a non essential amino acid that is not incorporated into protein. Endogenous citrulline is produced from glutamine by enterocytes in the small bowel. 1e5 In the absence of renal failure, plasma citrulline concentration is an index of intestinal mass and/ or absorptive function. 6e13 Citrulline is converted to arginine in kidney: in fasting humans 5e15% of overall arginine production is derived from citrulline e the remaining coming from proteolysis. 14 As citrulline escapessplanchnic uptake,oral citrulline supple- mentation might be an efficient way to enhance arginine avail- ability and thus impact whole-body protein synthesis as arginine may have a protein anabolic effect. 15,16 In addition,severalstudies suggest citrulline per se may have a protein anabolic effect: compared with an iso-nitrogenous mixture of non essentialamino acids,enteralcitrulline supplementation improved nitrogen balance in enterectomized rats 17 ; and refeeding with enteral citrulline increased muscle protein synthesis, together with a rise in serum insulin, in old malnourished rats. 18 However, little is known about the effect of citrulline on protein metabolism in humans. Recently we showed that oral citrulline acutely increased plasma citrulline and arginine concentrations,and improved nitrogen balance in healthy volunteers in the fed state. 19 The aims of this study were: i) to determine whether a 7-day dietary citrulline administration would enhance whole-body protein synthesis in the * Corresponding author. INRA, UMR 1280 Physiologie des Adaptations Nutri- tionnelles,CHU Hotel-Dieu,HNB1, 1 place Alexis Ricordeau, 44093 Nantes cedex 1, France.Tel.: þ33 2 53 48 20 02; fax: þ33 2 53 48 20 03. E-mail address: dominique.darmaun@chu-nantes.fr (D. Darmaun). Contents lists available at ScienceDirect Clinical Nutrition j o u r n a l homepage: h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / c l n u 0261-5614/$ e see front matter Ó 2011 Elsevier Ltd and European Society for Clinical Nutrition and Metabolism. All rights reserved. doi:10.1016/j.clnu.2011.06.005 Clinical Nutrition 30 (2011) 807e811