The metabolism and metabolic effects of ketoacids

Branched-chain amino acid and branched-chain ketoacid ingestion increases muscle protein synthesis rates in vivo in older adults: a double-blind, randomized trial

BACKGROUND

Protein ingestion increases muscle protein synthesis rates. However, limited data are currently available on the effects of branched-chain amino acid (BCAA) and branched-chain ketoacid (BCKA) ingestion on postprandial muscle protein synthesis rates.

OBJECTIVE

The aim of this study was to compare the impact of ingesting 6 g BCAA, 6 g BCKA, and 30 g milk protein (MILK) on the postprandial rise in circulating amino acid concentrations and subsequent myofibrillar protein synthesis rates in older males.

METHODS

In a parallel design, 45 older males (age: 71 ± 1 y; BMI: 25.4 ± 0.8 kg/m2) were randomly assigned to ingest a drink containing 6 g BCAA, 6 g BCKA, or 30 g MILK. Basal and postprandial myofibrillar protein synthesis rates were assessed by primed continuous l-[ring-13C6]phenylalanine infusions with the collection of blood samples and muscle biopsies.

RESULTS

Plasma BCAA concentrations increased following test drink ingestion in all groups, with greater increases in the BCAA and MILK groups compared with the BCKA group (P < 0.05). Plasma BCKA concentrations increased following test drink ingestion in all groups, with greater increases in the BCKA group compared with the BCAA and MILK groups (P < 0.05). Ingestion of MILK, BCAA, and BCKA significantly increased early myofibrillar protein synthesis rates (0-2 h) above basal rates (from 0.020 ± 0.002%/h to 0.042 ± 0.004%/h, 0.022 ± 0.002%/h to 0.044 ± 0.004%/h, and 0.023 ± 0.003%/h to 0.044 ± 0.004%/h, respectively; P < 0.001), with no differences between groups (P > 0.05). Myofibrillar protein synthesis rates during the late postprandial phase (2-5 h) remained elevated in the MILK group (0.039 ± 0.004%/h; P < 0.001), but returned to baseline values following BCAA and BCKA ingestion (0.024 ± 0.005%/h and 0.024 ± 0.005%/h, respectively; P > 0.05).

CONCLUSIONS

Ingestion of 6 g BCAA, 6 g BCKA, and 30 g MILK increases myofibrillar protein synthesis rates during the early postprandial phase (0-2 h) in vivo in healthy older males. The postprandial increase following the ingestion of 6 g BCAA and BCKA is short-lived, with higher myofibrillar protein synthesis rates only being maintained following the ingestion of an equivalent amount of intact milk protein. This trial was registered at Nederlands Trial Register (www.trialregister.nl) as NTR6047.

Oral amino acid administration in patients with diabetes mellitus: supplementation or metabolic therapy?

Amino acids are essential for body protein synthesis. Moreover, they can be used to produce energy within the cells. For protein turnover, normal plasma amino acid concentration enhances proteolytic suppression by insulin; furthermore, hyperaminoacidemia can stimulate protein synthesis both in the presence of baseline insulin and in hyperinsulinemic subjects with type 1 diabetes. In humans, the availability of amino acids represents a factor more important than insulin in maintaining protein synthesis in skeletal muscle. Among amino acids, branched-chain amino acids exert an anabolic effect on heart protein metabolism, and their uptake by the myocardium is increased by increasing their circulating concentrations. An important aspect of branched-chain amino acid metabolism in the heart (mainly in the ischemic heart) is that branch-chain amino acid infusion can diminish myocardial lactate; in this way, the inhibition of anaerobic energy phosphate caused by accumulation of lactate can be overridden. Plasma amino acid availability plays an important role in promoting protein synthesis and in energy production, both in peripheral skeletal muscle and in the myocardium.

Low protein diet in uremia: effects on glucose metabolism and energy production rate

Low-protein diets (LPD) increase insulin-mediated glucose disposal in chronic renal failure (CRF), but the fate of the better utilized glucose and the effect on energy production rate are unknown. Using a two-step (1 and 5 mU x kg(-1) x min(-1)) euglycemic hyperinsulinemic clamp combined with indirect calorimetry, we studied the effects of a LPD (0.3 g x kg(-1) x day(-1), supplemented with essential amino acids and ketoanalogs) in six patients suffering from chronic renal failure. After three months of diet, no significant change was observed concerning glomerular filtration rate, body wt, or arterial pH. In the postabsorptive state, plasma glucose and insulin levels were significantly lower, and energy production rose from 15.72 +/- 0.48 to 17.16 +/- 0.67 Cal x kg(-1) x min(-1) (P < 0.05). Insulin-stimulated glucose oxidation (2.36 +/- 0.29 vs. 3.37 +/- 0.35 mg x kg(-1) x min(-1); P < 0.05 at first clamp step) and nonoxidative disposal (P < 0.05 at both clamp steps) increased after LPD. This confirms that LPD ameliorates insulin sensitivity in CRF, even for low plasma insulin concentrations. Since energy production rate is increased by LPD, the caloric intake should be increased when protein intake is restricted.

Interrelationships of leucine and glutamate metabolism in cultured astrocytes

The aim was to study the extent to which leucine furnishes alpha-NH2 groups for glutamate synthesis via branched-chain amino acid aminotransferase. The transfer of N from leucine to glutamate was determined by incubating astrocytes in a medium containing [15N]leucine and 15 unlabeled amino acids; isotopic abundance was measured with gas chromatography-mass spectrometry. The ratio of labeling in both [15N]glutamate/[15N]leucine and [2-15N]glutamine/[15N]leucine suggested that at least one-fifth of all glutamate N had been derived from leucine nitrogen. At the same time, enrichment in [15N]leucine declined, reflecting dilution of the 15N label by the unlabeled amino acids that were in the medium. Isotopic abundance in [15N]isoleucine increased very quickly, suggesting the rapidity of transamination between these amino acids. The appearance of 15N in valine was more gradual. Measurement of branched-chain amino acid transaminase showed that the reaction from leucine to glutamate was approximately six times more active than from glutamate to leucine (8.72 vs. 1.46 nmol/min/mg of protein). However, when the medium was supplemented with alpha-ketoisocaproate (1 mM), the ketoacid of leucine, the reaction readily ran in the "reverse" direction and intraastrocytic [glutamate] was reduced by approximately 50% in only 5 min. Extracellular concentrations of alpha-ketoisocaproate as low as 0.05 mM significantly lowered intracellular [glutamate]. The relative efficiency of branched-chain amino acid transamination was studied by incubating astrocytes with 15 unlabeled amino acids (0.1 mM each) and [15N]glutamate. After 45 min, the most highly labeled amino acid was [15N]alanine, which was closely followed by [15N]leucine and [15N]isoleucine. Relatively little 15N was detected in any other amino acids, except for [15N]serine. The transamination of leucine was approximately 17 times greater than the rate of [1-14C]leucine oxidation. These data indicate that leucine is a major source of glutamate nitrogen. Conversely, reamination of alpha-ketoisocaproate, the ketoacid of leucine, affords a mechanism for the temporary "buffering" of intracellular glutamate.

Chronic metabolic acidosis accelerates whole body proteolysis and oxidation in awake rats

Previous work has documented an acceleration of proteolysis and branched-chain amino acid oxidation when muscles from rats with chronic metabolic acidosis were incubated in vitro. The present study examines the impact of chronic metabolic acidosis on whole body amino acid turnover and oxidation in chronically catheterized awake male Sprague-Dawley rats using stochastic modeling and a primed continuous infusion of L-[1-14C] leucine. Whole body protein turnover was accelerated by acidosis as reflected in a 70% increase in proteolysis and a 55% increase in protein synthesis. Amino acid oxidation was increased 145% in rats with chronic metabolic acidosis relative to control rats receiving diets identical in protein and calories based on a reciprocal pool model and plasma alpha-ketoisocaproate specific radioactivity. These changes were accompanied by a 104% increase in liver branched-chain ketoacid dehydrogenase (BCKAD) activity in rats with acidosis, similar to previously documented increases in skeletal muscle BCKAD activity caused by acidosis. In contrast, kidney BCKAD activity was decreased 38% by acidosis, illustrating the tissue-specificity of the changes that were present. We conclude that chronic metabolic acidosis accelerates whole body protein turnover and affects the reincorporation of amino acid into body proteins by accelerating amino acid oxidation.

Myocardial protein turnover in patients with coronary artery disease. Effect of branched chain amino acid infusion

The regulation of protein metabolism in the human heart has not previously been studied. In 10 postabsorptive patients with coronary artery disease, heart protein synthesis and degradation were estimated simultaneously from the extraction of intravenously infused L-[ring-2,6-3H]phenylalanine (PHE) and the dilution of its specific activity across the heart at isotopic steady state. We subsequently examined the effect of branched chain amino acid (BCAA) infusion on heart protein turnover and on the myocardial balance of amino acids and branched chain ketoacids (BCKA) in these patients. In the postabsorptive state, there was a net release of phenylalanine (arterial-cardiac venous [PHE] = -1.71 +/- 0.32 nmol/ml, P less than 0.001; balance = -116 +/- 21 nmol PHE/min, P less than 0.001), reflecting protein degradation (142 +/- 40 nmol PHE/min) in excess of synthesis (24 +/- 42 nmol PHE/min) and net myocardial protein catabolism. During BCAA infusion, protein synthesis increased to equal the degradation rate (106 +/- 24 and 106 +/- 28 nmol PHE/min, respectively) and the phenylalanine balance shifted (P = 0.01) from negative to neutral (arterial-cardiac venous [PHE] = 0.07 +/- 0.36 nmol/ml; balance = 2 +/- 25 nmol PHE/min). BCAA infusion stimulated the myocardial uptake of both BCAA (P less than 0.005) and their ketoacid conjugates (P less than 0.001) in proportion to their circulating concentrations. Net uptake of the BCAA greatly exceeded that of other essential amino acids suggesting a role for BCAA and BCKA as metabolic fuels. Plasma insulin levels, cardiac double product, coronary blood flow, and myocardial oxygen consumption were unchanged. These results demonstrate that the myocardium of postabsorptive humans is in negative protein balance and indicate a primary anabolic effect of BCAA on the human heart.