Reduced Transamination of Valine and Isoleucine During Intake of a Leucine-Enriched Essential Amino Acid Mixture in Healthy Older Adults

Objectives: Leucine (LEU) rich meals are assumed to stimulate muscle protein synthesis in older adults, but long-term studies haven't shown increased muscle mass. Transamination of branched-chain amino acids (BCAAs) to ketoacids LEU >KIC, valine (VAL) >KIV, and isoleucine (ILE) >KMV is significant as KIV and KMV are anaplerotic substrates. Thus, we measured BCAA transamination during free essential amino acid (EAA) intake compared to intake of a LEU-enriched EAA mixture in healthy older adults. Methods: Eleven healthy participants (5f/6m), aged 60-80 y, consumed a liquid mixture every 20 mins for 6 hrs: a) EAA (20g, 4g LEU) b) EAA+LEU (4+3g LEU) c) Placebo (water) on 3 separate days. At 3h, a stable isotope i.v. pulse with L-LEU [13C6], L-ILE [1-13C], L-VAL [13C5], KIC [1,2-13C2], KMV [13C6], and KIV [dimethyl-13C2] was given followed by blood draws. Plasma amino acid concentrations and enrichments measured by LC/MS. Whole-body production (WBP) rates calculated from isotope enrichment decay curves. Stats: T-test (Graphpad Prism). Results: mean [95%CI], α< 0.05. Results: EAA+LEU had higher plasma concentrations (μM) of LEU (460 [431, 489] v 307 [276, 337] p< 0.0001) and KIC (76 [71, 82] v 61 [50, 73] p=0.0204) while VAL (409 [382, 436] v 502 [429, 575] p=0.019), KIV (14 [13, 15] v 18 [16, 19] p< 0.0001) ILE (149 [141, 157] v 185 [167, 204] p=0.0005), and KMV were reduced (29 [28, 30] v 37 [35, 40] p< 0.0001). Moreover, EAA+LEU had higher WBP (μmol/min) of LEU (238 [230, 247] v 167 [161, 173] p< 0.0001) and KIC (82 [78, 87] v 63 [59, 66] p< 0.0001), while WBPs were similar for VAL (109 [105, 112] v 110 [107, 114] p= 0.5043) and ILE (83 [79, 87] v 81 [77, 85] p=0.4741). WBP of KIV (20 [19, 22] v 27 [26, 29] p< 0.0001) and KMV (26 [24, 27] v 31 [29, 33] p< 0.0001) was lower in EAA+LEU. Conclusions: Adding 3g LEU to an 20g EAA mixture reduced the transamination of VAL to KIV by 26% and ILE to KMV by 17%. Our data suggests an imbalanced intake of LEU in free amino mixtures adversely affects BCAA metabolism in healthy older adults. This is crucial since KIV and KMV, as anaplerotic substrates, replenish TCA cycle intermediates to support mitochondrial metabolism. Funding Sources: N/A.