Introduction: Leucine is both a substrate and signal for the synthesis of new muscle protein through its activation of the mechanistic target of rapamycin complex 1 (mTORC1) signalling pathway. Recent research has shown that modest protein doses enriched with leucine robustly stimulate muscle protein synthesis (MPS), akin to larger protein boluses. β-lactoglobulin (BLG) is a novelly processed milk protein containing a higher leucine content (~16%) than traditional whey protein (~12%). We determined the effects of two doses of BLG protein, both at rest and following a bout of resistance exercise in healthy older people.
Methods: 23 older men (70±4y) consumed either 5g or 10g of BLG protein during a primed constant [1,2-13C2] leucine stable isotope tracer infusion. Participants completed 6×8 repetitions of unilateral leg-extension at 75% of their one repetition maximum, such as to have an internally controlled rest and exercised-leg condition. Blood was collected for amino acid profiling and vastus lateralis muscle biopsies for quantifying MPS via mass spectrometry. All results presented are mean ± SEM.
Results: Both 5g and 10g BLG increased plasma essential amino acid (EAA) concentrations from baseline, 20 to 60 min and 20 to 80 min respectively (all P < 0.05). Based on the higher dosing, there was a significant difference in plasma EAA concentrations between feeds from 20 to 120 min (P < 0.05). On this basis, there was also a significant difference in plasma EAA area under the curve (AUC) (135.6 ± 14.2 vs 270.6 ± 16.3, P < 0.0001). Mean MPS rates were higher in the fed state compared to the fasted state for the 5g BLG No-Ex leg (0.042 ± 0.006%/h vs 0.060 ± 0.006%/h, P < 0.05), the 5g BLG Ex leg (0.046 ± 0.006%/h vs 0.072 ± 0.003%/h, P < 0.01), the 10g BLG No-Ex leg (0.046 ± 0.006%/h vs 0.088 ± 0.007%/h, P < 0.0001) and the 10g BLG Ex leg (0.049 ± 0.006%/h vs 0.081 ± 0.005%/h, P < 0.001). There was a greater fed-state MPS response in the non-exercised legs in the 10g BLG vs. 5g BLG (0.088 ± 0.007%/h vs. 0.060 ± 0.006%/h vs respectively, P < 0.01), with no other significant differences in fed-state MPS between any groups for all multiple comparisons (all P > 0.05).
Discussion: A dose of 10g BLG protein produced a greater plasma EAA and leucine response than 5g BLG, as indicated by the EAA AUC for 10g BLG being double that of 5g BLG. Despite this, 5g BLG protein was sufficient to produce a significant increase in MPS in older individuals in a non-exercised state, albeit not inducing a maximal MPS response, despite its high leucine content. There was no significant difference in postprandial MPS rates in the exercised leg in response to either 5g or 10g BLG protein consumption, practically indicating that as little as 5g of a leucine enriched protein source may be sufficient to maximally stimulate MPS following resistance exercise, even in anabolically resistant older individuals.