Effects of D-beta-hydroxybutyrate and long- and medium-chain triglycerides on leucine metabolism in humans

A Systematic Review of Intravenous β-Hydroxybutyrate Use in Humans - A Promising Future Therapy?

Therapeutic ketosis is traditionally induced with dietary modification. However, owing to the time delay involved, this is not a practical approach for treatment of acute conditions such as traumatic brain injury. Intravenous administration of ketones would obviate this problem by rapidly inducing ketosis. This has been confirmed in a number of small animal and human studies. Currently no such commercially available product exists. The aim of this systematic review is to review the safety and efficacy of intravenous beta-hydroxybutyrate. The Web of Science, PubMed and EMBASE databases were searched, and a systematic review undertaken. Thirty-five studies were included. The total beta-hydroxybutyrate dose ranged from 30 to 101 g administered over multiple doses as a short infusion, with most studies using the racemic form. Such dosing achieves a beta-hydroxybutyrate concentration >1 mmol/L within 15 min. Infusions were well tolerated with few adverse events. Blood glucose concentrations occasionally were reduced but remained within the normal reference range for all study participants. Few studies have examined the effect of intravenous beta-hydroxybutyrate in disease states. In patients with heart failure, intravenous beta-hydroxybutyrate increased cardiac output by up to 40%. No studies were conducted in patients with neurological disease. Intravenous beta-hydroxybutyrate has been shown to increase cerebral blood flow and reduce cerebral glucose oxidation. Moreover, beta-hydroxybutyrate reduces protein catabolism and attenuates the production of counter-regulatory hormones during induced hypoglycemia. An intravenous beta-hydroxybutyrate formulation is well tolerated and may provide an alternative treatment option worthy of further research in disease states.

Effects of 3-hydroxybutyrate and free fatty acids on muscle protein kinetics and signaling during LPS-induced inflammation in humans: anticatabolic impact of ketone bodies

Background

Acute inflammation, and subsequent release of bacterial products (e.g. LPS), inflammatory cytokines, and stress hormones, is catabolic, and the loss of lean body mass predicts morbidity and mortality. Lipid intermediates may reduce protein loss, but the roles of free fatty acids (FFAs) and ketone bodies during acute inflammation are unclear.

Objective

We aimed to test whether infusions of 3-hydroxybutyrate (3OHB), FFAs, and saline reduce protein catabolism during exposure to LPS and Acipimox (to restrict and control endogenous lipolysis).

Design

A total of 10 healthy male subjects were randomly tested 3 times, with: 1) LPS, Acipimox (Olbetam) and saline, 2) LPS, Acipimox, and nonesterified fatty acids (Intralipid), and 3) LPS, Acipimox, and 3OHB, during a 5-h basal period and a 2-h hyperinsulinemic, euglycemic clamp. Labeled phenylalanine, tyrosine, and urea tracers were used to estimate protein kinetics, and muscle biopsies were taken for Western blot analysis of protein metabolic signaling.

Results

3OHB infusion increased 3OHB concentrations (P < 0.0005) to 3.5 mM and decreased whole-body phenylalanine-to-tyrosine degradation. Basal and insulin-stimulated net forearm phenylalanine release decreased by >70% (P < 0.005), with both appearance and phenylalanine disappearance being profoundly decreased. Phosphorylation of eukaryotic initiation factor 2α at Ser51 was increased in skeletal muscle, and S6 kinase phosphorylation at Ser235/236 tended (P = 0.074) to be decreased with 3OHB infusion (suggesting inhibition of protein synthesis), whereas no detectable effects were seen on markers of protein breakdown. Lipid infusion did not affect phenylalanine kinetics, and insulin sensitivity was unaffected by interventions.

Conclusion

During acute inflammation, 3OHB has potent anticatabolic actions in muscle and at the whole-body level; in muscle, reduction of protein breakdown overrides inhibition of synthesis. This trial was registered at clinicaltrials.gov as NCT01752348.

Determination of protein replacement rates by deuterated water: validation of underlying assumptions

H(2)O administration has recently been proposed as a simple and convenient method to measure protein synthesis rates. (2)H(2)O administration results in deuterium labeling of free amino acids such as alanine, and incorporation into proteins of labeled alanine can then be used to measure protein synthesis rates. We examined first whether during (2)H(2)O administration plasma free alanine enrichment is a correct estimate of the enrichment in the tissue amino acid pools used for protein synthesis. We found that, after (2)H(2)O administration, deuterium labeling in plasma free alanine equilibrated rapidly with body water, and stable enrichment values were obtained within 20 min. Importantly, oral administration of (2)H(2)O induced no difference of labeling between portal and peripheral circulation except for the initial 10 min after a loading dose. The kinetics of free alanine labeling were comparable in various tissues (liver, skeletal muscle, heart) and in plasma with identical plateau values. We show next that increased glycolytic rate or absorption of unlabeled amino acids from ingested meals do not modify alanine labeling. Calculated synthesis rates of mixed proteins were much higher (20- to 70-fold) in plasma and liver than in muscle and heart. Last, comparable replacement rates of apoB100-VLDL were obtained in humans by using the kinetics of incorporation into apoB100 of infused labeled leucine or of alanine labeled by (2)H(2)O administration. All of these results support (2)H(2)O as a safe, reliable, useful, and convenient tracer for studies of protein synthesis, including proteins with slow turnover rate.

Skeletal muscle protein anabolic response to increased energy and insulin is preserved in poorly controlled type 2 diabetes

Type 2 diabetes (T2DM) subjects failing diet treatment are characterized by hyperinsulinemia and insulin resistance leading to fasting and postprandial hyperglycemia and hyperlipidemia. Energy is essential for allowing the process of protein synthesis to proceed. Additionally, insulin can stimulate protein synthesis in human muscle. The aims of this study were to determine if poorly controlled T2DM affects postabsorptive muscle protein anabolism, and if the muscle anabolic response to hyperinsulinemia with high energy availability is maintained. Control (n = 6) and T2DM subjects (n = 6) were studied in the postabsorptive state and during an isoenergetic high nutritional energy clamp (relative to postabsorptive state). Muscle protein synthesis and breakdown (nmol . min(-1) . 100 g leg muscle(-1)) were assessed using stable isotope methodology, femoral arterio-venous sampling, muscle biopsies, and a three-pool model to calculate protein turnover. Postabsorptive phenylalanine net balance and whole body rate of appearance (Ra) were not different between groups; however, basal muscle protein breakdown was higher in T2DM (94 +/- 9) than in controls (58 +/- 12) (P < 0.05) and muscle protein synthesis tended (P = 0.07) to be elevated in T2DM (66 +/- 14) compared with controls (39 +/- 6). During the clamp, net balance increased, whole body Ra and muscle protein breakdown decreased (P < 0.05), and muscle protein synthesis tended to decrease (P = 0.08) to a similar extent in both groups. We conclude that postabsorptive muscle protein turnover is elevated in poorly controlled T2DM, however, there is no excessive loss of muscle protein because net balance is not different from controls. Moreover, the anabolic response to increased insulin and energy availability is maintained in T2DM.

Amino acids do not suppress proteolysis in premature neonates

To determine whether increased amino acid availability can reduce proteolysis in premature neonates and to assess the capacity of infants born prematurely to acutely increase the irreversible catabolism of the essential amino acids leucine (via oxidation) and phenylalanine (via hydroxylation to form tyrosine), leucine and phenylalanine kinetics were measured under basal conditions and in response to a graded infusion of intravenous amino acids (1.2 and 2.4 g. kg(-1). day(-1)) in clinically stable premature (approximately 32 wk gestation) infants in the 1st wk of life. In contrast to the dose-dependent suppression of proteolysis seen in healthy full-term neonates, the endogenous rates of appearance of leucine and phenylalanine (reflecting proteolysis) were unchanged in response to amino acids (297 +/- 21, 283 +/- 19, and 284 +/- 31 micromol. kg(-1). h(-1) for leucine and 92 +/- 6, 92 +/- 4, and 84 +/- 7 micromol. kg(-1). h(-1) for phenylalanine). Similar to full-term neonates, leucine oxidation (40 +/- 5, 65 +/- 6, and 99 +/- 7 micromol. kg(-1). h(-1)) and phenylalanine hydroxylation (12 +/- 1, 16 +/- 1, and 20 +/- 2 micromol. kg(-1). h(-1)) increased in a stepwise fashion in response to graded amino acids. This capacity to increase phenylalanine hydroxylation may be crucial to meet tyrosine needs when exogenous supply is limited. Finally, to determine whether amino acids stimulate glucose production in premature neonates, glucose rate of appearance was measured during each study period. In response to amino acid infusion, rates of endogenous glucose production were unchanged (and near zero).

The digestion rate of protein is an independent regulating factor of postprandial protein retention

To evaluate the importance of protein digestion rate on protein deposition, we characterized leucine kinetics after ingestion of "protein" meals of identical amino acid composition and nitrogen contents but of different digestion rates. Four groups of five or six young men received an L-[1-13C]leucine infusion and one of the following 30-g protein meals: a single meal of slowly digested casein (CAS), a single meal of free amino acid mimicking casein composition (AA), a single meal of rapidly digested whey proteins (WP), or repeated meals of whey proteins (RPT-WP) mimicking slow digestion rate. Comparisons were made between "fast" (AA, WP) and "slow" (CAS, RPT-WP) meals of identical amino acid composition (AA vs. CAS, and WP vs. RPT-WP). The fast meals induced a strong, rapid, and transient increase of aminoacidemia, leucine flux, and oxidation. After slow meals, these parameters increased moderately but durably. Postprandial leucine balance over 7 h was higher after the slow than after the fast meals (CAS: 38 +/- 13 vs. AA: -12 +/- 11, P < 0.01; RPT-WP: 87 +/- 25 vs. WP: 6 +/- 19 micromol/kg, P < 0.05). Protein digestion rate is an independent factor modulating postprandial protein deposition.

Proteolysis and phenylalanine hydroxylation in response to parenteral nutrition in extremely premature and normal newborns

To determine to what extent intravenous nutrition can reduce proteolysis in very immature and normal newborns, and to assess the capacity of preterm and normal newborns to convert phenylalanine to tyrosine, phenylalanine and leucine kinetics were measured under basal conditions and during parenteral nutrition in clinically stable, extremely premature (approximately 26 wk of gestation) infants and in normal term newborns. In response to parenteral nutrition, there was significantly less suppression (P < 0.001) of endogenous leucine and phenylalanine rate of appearance in extremely premature infants compared with term infants. Phenylalanine utilization for protein synthesis during parenteral nutrition increased significantly (P < 0.01) and by the same magnitude (approximately 15%) in both extremely premature and term infants. Phenylalanine was converted to tyrosine at substantial rates in both extremely premature and term infants; however, this conversion rate was significantly higher (P < 0.05) in extremely premature infants during both the basal and parenteral nutrition periods. These data provide clear evidence that there is no immaturity in the phenylalanine hydroxylation pathway. Furthermore, although parenteral nutrition appears to produce similar increases in protein synthesis in extremely premature and term infants, proteolysis is suppressed much less in extremely premature newborns. The factors responsible for this apparent resistance to suppression of proteolysis in the very immature newborn remain to be elucidated.

Comparison of the effects of recombinant human insulin-like growth factor-I and insulin on glucose and leucine kinetics in humans

To compare the metabolic effects of elevated plasma concentrations of IGF-I and insulin, overnight-fasted normal subjects were studied twice, once receiving IGF-I and once insulin at doses that resulted in identical increases in glucose uptake during 8-h euglycemic clamping. Recombinant human IGF-I or insulin were infused in one group at high doses (30 micrograms/kg per h IGF-I or 0.23 nmol/kg per h insulin) and in another group at low doses (5 micrograms/kg per h IGF-I or 0.04 nmol/kg per h insulin). Glucose rate of disappearance (measured by [6,6-D2]-glucose infusions) increased from baseline by 239 +/- 16% during high dose IGF-I vs 197 +/- 18% during insulin (P = 0.021 vs IGF-I). Hepatic glucose production decreased by 37 +/- 6% during high dose IGF-I vs 89 +/- 13% during insulin (P = 0.0028 vs IGF-I). IGF-I suppressed whole body leucine flux ([1-13C]-leucine infusion technique) more than insulin (42 +/- 4 vs 32 +/- 3% during high doses, P = 0.0082). Leucine oxidation rate decreased during high dose IGF-I more than during insulin (55 +/- 4 vs 32 +/- 6%, P = 0.0001). The decreases of plasma concentrations of free fatty acids, acetoacetate, and beta-hydroxybutyrate after 8 h of IGF-I and insulin administration were similar. Plasma C-peptide levels decreased by 57 +/- 4% during high doses of IGF-I vs 36 +/- 6% during insulin (P = 0.005 vs IGF-I). The present data demonstrate that, compared to insulin, an acute increase in plasma IGF-I levels results in preferential enhancement of peripheral glucose utilization, diminished suppression of hepatic glucose production, augmented decrease of whole body protein breakdown (leucine flux), and of irreversible leucine catabolism but in similar antilipolytic effects. The data suggest that insulin-like effects of IGF-I in humans are mediated in part via IGF-I receptors and in part via insulin receptors.