Trioctanoin infusion increases in vivo leucine oxidation: a lesson in isotope modeling

Anabolic Resistance in the Pathogenesis of Sarcopenia in the Elderly: Role of Nutrition and Exercise in Young and Old People

The development of sarcopenia in the elderly is associated with many potential factors and/or processes that impair the renovation and maintenance of skeletal muscle mass and strength as ageing progresses. Among them, a defect by skeletal muscle to respond to anabolic stimuli is to be considered. Common anabolic stimuli/signals in skeletal muscle are hormones (insulin, growth hormones, IGF-1, androgens, and β-agonists such epinephrine), substrates (amino acids such as protein precursors on top, but also glucose and fat, as source of energy), metabolites (such as β-agonists and HMB), various biochemical/intracellular mediators), physical exercise, neurogenic and immune-modulating factors, etc. Each of them may exhibit a reduced effect upon skeletal muscle in ageing. In this article, we overview the role of anabolic signals on muscle metabolism, as well as currently available evidence of resistance, at the skeletal muscle level, to anabolic factors, from both in vitro and in vivo studies. Some indications on how to augment the effects of anabolic signals on skeletal muscle are provided.

Growth hormone and testosterone interact positively to enhance protein and energy metabolism in hypopituitary men

We investigated the impact of growth hormone (GH) alone, testosterone (T) alone, and combined GH and T on whole body protein metabolism. Twelve hypopituitary men participated in two studies. Study 1 compared the effects of GH alone with GH plus T, and study 2 compared the effects of T alone with GH plus T. IGF-I, resting energy expenditure (REE), and fat oxidation (F(ox)) and rates of whole body leucine appearance (R(a)), oxidation (L(ox)), and nonoxidative leucine disposal (NOLD) were measured. In study 1, GH treatment increased mean plasma IGF-I (P < 0.001). GH did not change leucine R(a) but reduced L(ox) (P < 0.02) and increased NOLD (P < 0.02). Addition of T resulted in an additional increase in IGF-I (P < 0.05), reduction in Lox (P < 0.002), and increase in NOLD (P < 0.002). In study 2, T alone did not alter IGF-I levels. T alone did not change leucine R(a) but reduced L(ox) (P < 0.01) and increased NOLD (P < 0.01). Addition of GH further reduced L(ox) (P < 0.05) and increased NOLD (P < 0.05). In both studies, combined treatments on REE and F(ox) were greater than either alone. In summary, GH-induced increase of circulating IGF-I is augmented by T, which does not increase IGF-I in the absence of GH. T and GH exerted independent and additive effects on protein metabolism, F(ox) and REE. The anabolic effects of T are independent of circulating IGF-I.

Combined growth hormone/insulin-like growth factor I in addition to glutamine-supplemented TPN results in net protein anabolism in critical illness

Protein loss leading to reduced lean body mass is recognized to contribute to the high levels of morbidity and mortality seen in critical illness. This prospective, randomized, controlled study compared the effects of conventional parenteral nutrition (TPN), glutamine-supplemented (0.4 g.kg-1.day-1) TPN (TPNGLN), and TPNGLN with combined growth hormone (GH, 0.2 IU.kg-1.day-1) and IGF-I (160 microg.kg-1.day-1) on protein metabolism in critical illness. Nineteen mechanically ventilated subjects [64 +/- 3 yr, body mass index (BMI) 23.8 +/- 1.3, kg/m2] were initially studied in the fasting state (study 1) and subsequently after 3 days of nutritional with/without hormonal support (study 2). All had recently been admitted to the ICU and the majority were postemergency abdominal surgery (APACHE II 17.5 +/- 1.0). Protein metabolism was assessed using a primed constant infusion of [1-13C]leucine. Conventional TPN contained mixed amino acids, Intralipid, and 50% dextrose. TPNGLN, unlike TPN alone, resulted in an increase in plasma glutamine concentration ( approximately 50%, P < 0.05). Both TPN and TPNGLN decreased the rate of protein breakdown (TPN 15%, P < 0.002; TPNGLN 16%, P < 0.05), but during these treatments the patients remained in a net negative protein balance. Combined treatment with TPNGLN + GH/IGF-I increased plasma IGF-I levels (10.3 +/- 0.8 vs. 48.1 +/- 9.1 nmol/l, study 1 vs. study 2, P < 0.05), and in contrast to therapy with nutrition alone, resulted in net protein gain (-0.75 +/- 0.14 vs. 0.33 +/- 0.12 g protein.kg-1.day-1, study 1 vs. study 2, P < 0.05). Therapy with GH/IGF-I + TPNGLN, unlike nutrition alone, resulted in net positive protein balance in a group of critically ill patients.

Phenylalanine kinetics in human adipose tissue

Very little is known about the regulation of protein metabolism in adipose tissue. In this study systemic, adipose tissue, and forearm phenylalanine kinetics were determined in healthy postabsorptive volunteers before and during a 2-h glucose infusion (7 mg.kg-1.min-1). [3H]Phenylalanine was infused and blood was sampled from a radial artery, a subcutaneous abdominal vein, and a deep forearm vein. Adipose tissue and forearm blood flow were measured with 133Xe and plethysmography, respectively, and body fat mass was determined by dual energy x-ray absorptiometry. During glucose infusion, glucose concentration increased from 86 +/- 2 to 228 +/- 13 mg/dl and insulin concentration increased from 6.6 +/- 0.6 to 35.0 +/- 3.9 mU/liter, both P < 0.001. Systemic phenylalanine appearance decreased from 40.3 +/- 1.9 to 37.0 +/- 1.6 mumol/min during glucose infusion (P < 0.05). Baseline whole body adipose tissue phenylalanine release (5.2 +/- 1.4 mumol/min) was approximately 12% of systemic phenylalanine appearance and decreased (P < 0.05) to 2.3 +/- 0.9 mumol/min during glucose infusion. In contrast, phenylalanine release from the forearm did not change during glucose infusion. These results indicate that adipose tissue is a small but significant contributor to systemic phenylalanine appearance. Phenylalanine release from adipose tissue like lipolysis, is relatively sensitive to hyperinsulinemia.

Administration of structured lipid composed of MCT and fish oil reduces net protein catabolism in enterally fed burned rats

The effects of enteral feeding with safflower oil or a structured lipid (SL) derived from 60% medium-chain triglyceride (MCT) and 40% fish oil (MCT/fish oil) on protein and energy metabolism were compared in gastrostomy-fed burned rats (30% body surface area) by measuring oxygen consumption, carbon dioxide production, nitrogen balance, total liver protein, whole-body leucine kinetics, and rectus muscle and liver protein fractional synthetic rates (FSR, %/day). Male Sprague-Dawley rats (195 +/- 5g) received 50 ml/day of an enteral regimen containing 50 kcal, 2 g amino acids, and 40% nonprotein calories as lipid for three days. Protein kinetics were estimated by using a continuous L-[1-14C] leucine infusion technique on day 2. Thermally injured rats enterally fed MCT/fish oil yielded significantly higher daily and cumulative nitrogen balances (p less than or equal to 0.025) and rectus muscle (39%) FSR (p less than or equal to 0.05) when compared with safflower oil. MCT/fish oil showed a 22% decrease (p less than or equal to 0.005) in per cent flux oxidized and a 7% (p less than or equal to 0.05) decrease in total energy expenditure (TEE) versus safflower oil. A 15% increase in liver FSR was accompanied by a significant elevation (p less than or equal to 0.025) in total liver protein with MCT/fish oil. This novel SL shares the properties of other structured lipids in that it reduces the net protein catabolic effects of burn injury, in part, by influencing tissue protein synthetic rates. The reduction in TEE is unique to MCT/fish oil and may relate to the ability of fish oil to diminish the injury response.