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Nutrient Physiology, Metabolism, and Nutrient-Nutrient Interactions

The Leucine Content of a Complete Meal Directs Peak Activation but Not Duration of Skeletal Muscle Protein Synthesis and Mammalian Target of Rapamycin Signaling in Rats^1,2

³ Division of Nutritional Sciences, Department of Food Science and Human Nutrition, and ⁴ Department of Animal Sciences University of Illinois at Urbana-Champaign, Urbana, IL 61801; and ⁵ Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Evansville, IN 47712

This study examined the impact of leucine (Leu) derived from complete meals on stimulation of skeletal muscle protein synthesis (MPS). Expt. 1 examined time course changes in translation initiation and MPS after a meal. Male rats (300 g) were trained for 5 d to eat 3 meals/d providing 20, 50, and 30% of energy from whey protein, carbohydrates, and fats, respectively. Plasma and skeletal muscle were collected at time 0 (baseline) after 12 h of food deprivation and then at 45, 90, 135, 180, and 300 min after a 4-g meal. Plasma Leu increased at 45 min and remained elevated through 180 min. MPS peaked at 45–90 min and returned to baseline by 180 min. Plasma Leu correlated with phosphorylation of ribosomal protein p70 S6 kinase (r = 0.723; P < 0.05), eukaryotic initiation factor 4E binding protein-1 (r = 0.773; P < 0.05), and MPS (r = 0.608; P < 0.05) over time. Expt. 2 examined 3 levels of protein intake (10, 20, and 30% of energy) from 2 sources (wheat and whey) with different Leu contents (6.8 and 10.9%, respectively) on stimulation of initiation and MPS. Rats were trained to eat 3 meals/d providing 14, 56, and 30% of energy from protein, carbohydrates, and fats. On d 6, MPS was evaluated at 90 min after rats consumed 1 of the 6 test meals. Whey protein stimulated initiation and MPS more than wheat and the differential response related to greater plasma Leu responses in the whey groups. These studies demonstrate that peak activation but not duration of MPS is proportional to the Leu content of a meal.

^* To whom correspondence should be addressed. E-mail: lnorton2{at}illinois.edu.

Manuscript received 22 December 2008. Initial review completed 17 January 2009. Revision accepted 24 March 2009.

Published online 29 April 2009.