Am J Physiol Endocrinol Metab. 2008 Apr 22 Protein co-ingestion stimulates muscle protein synthesis during resistance type exercise. Beelen M, Koopman R, Gijsen AP, Vandereyt H, Kies AK, Kuipers H, Saris WH, van Loon LJ. In contrast to the impact of nutritional intervention on post-exercise muscle protein synthesis, little is known about the potential to modulate protein synthesis during exercise. This study investigates the impact of protein co-ingestion with carbohydrate on muscle protein synthesis during resistance type exercise. Ten healthy males were studied in the evening after consuming a standardized diet throughout the day. Subjects participated in 2 experiments, in which they ingested either carbohydrate or carbohydrate with protein during a 2h resistance exercise session. Subjects received a bolus of test drink prior to and every 15 min during exercise, providing 0.15 g.kg(-1).h(-1) carbohydrate with (CHO+PRO) or without (CHO) 0.15 g.kg(-1).h(-1) protein hydrolysate. Continuous intravenous infusions with L-[ring-(13)C6]phenylalanine and L-[ring-(2)H2] tyrosine were applied, and blood and muscle biopsies were collected to assess whole-body and muscle protein synthesis rates during exercise. Protein co-ingestion lowered whole-body protein breakdown rates by 8.4+/-3.6% (P=0.066), compared to the ingestion of carbohydrate only, and augmented protein oxidation and synthesis rates by 77+/-17 and 33+/-3%, respectively (P<0.01). As a consequence, whole-body net protein balance was negative in CHO, whereas a positive net balance was achieved following the CHO+PRO treatment (-4.4+/-0.3 vs 16.3+/-0.4 micromol phe.kg(-1).h(-1), respectively; P<0.01). In accordance, mixed muscle protein fractional synthetic rate (FSR) was 49+/-22% higher following protein co-ingestion (0.088+/-0.012 and 0.060+/-0.004 %.h(-1) in CHO+PRO vs CHO treatment, respectively; P<0.05). We conclude that, even in a fed state, protein co-ingestion stimulates whole-body and muscle protein synthesis rates during resistance type exercise. Key words: muscle, protein synthesis, exercise, nutrition.