Effects of parenteral lipid on leucine metabolism: dependence of fatty acid chain length

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.

Muscle protein synthesis and balance responsiveness to essential amino acids ingestion in the presence of elevated plasma free fatty acid concentrations

CONTEXT

Elevated plasma free fatty acid (FFA) concentrations are observed under various clinical circumstances and are associated with impaired glucose disposal in skeletal muscle.

OBJECTIVE

The aim of the study was to determine the effects of elevated plasma FFA concentrations on the response of protein synthesis and balance in muscle after essential amino acids (EAAs) ingestion.

DESIGN

Leg protein kinetics were determined in young healthy individuals before and after the ingestion of EAAs at 10 h after the initiation of either lipid (Liposyn/heparin+EAA) or saline (saline+EAA) infusions.

RESULTS

Plasma insulin responses where higher (P <0.05) in the Liposyn/heparin+EAA group than the saline+EAA group both before (14 +/- 4 vs. 6 +/- 1 microIU . ml(-1)) and after (1038 +/- 257 vs. 280 +/- 87 microIU . ml(-1) . 210 min(-1)) the EAA ingestion. After the EAA ingestion, the rates of both leg phenylalanine disappearance (Rd; nmol . min(-1) . kg lean leg mass(-1)) and muscle proteins fractional synthesis (FSR; % . h(-1)) increased (P <0.05) in both the Liposyn/heparin+EAA and saline+EAA groups, but these changes were not different between the two groups (Rd, 102 +/- 32 vs. 118 +/- 34; FSR, 0.014 +/- 0.005 vs. 0.018 +/- 0.007; P > 0.05). Although the leg phenylalanine rate of appearance (Ra; nmol . min(-1) . kg lean leg mass(-1)) was lower (381 +/- 47 vs. 518 +/- 40) and the balance was greater (-109 +/- 20 vs. -172 +/- 17) in the Liposyn/heparin+EAA group compared to the saline+EAA group before the EAA ingestion (P <0.05), the changes in both of these parameters were not different between groups after the EAA ingestion (P > 0.05).

CONCLUSIONS

Elevated plasma FFA concentrations do not interfere with the response of muscle protein synthesis and balance to a bolus ingestion of EAAs.

Effect of L-glutamine supplementation on impaired glucose regulation during intravenous lipid administration

In contrast to L-glutamine, lipid emulsions are routinely administered to patients receiving nutritional support. The provision of fat during intravenous feeding is essential, but the potentially toxic byproducts of fatty acid oxidation may have adverse metabolic consequences. In the present study, we have examined the effect of L-glutamine, an inhibitor of fatty acid oxidation, on the development of defective blood glucose regulation caused by a 48-hour infusion of 10% intralipid in rats. Male Sprague-Dawley rats (200-290 g) were anesthetized with sodium pentobarbital, the right femoral vein cannulated, and baseline blood samples were taken. Each rat was placed in a metabolic cage with access to water, in the presence or absence of rodent chow. Two hours after waking, the rats were infused with 10% intralipid with either saline (control), 2% L-glutamine, or 2% L-alanine. After 48 hours, all animals were sacrificed and blood samples were again obtained. The mean +/- SEM plasma glucose levels before and after lipid infusion at the rate of 1 mL/hr in control rats fed ad libitum, were 125 +/- 13 and 170 +/- 5 mg/dL (p < 0.01, n = 7). Similarly, plasma free fatty acids (FFA) in these animals rose from 0.74 +/- 0.11 to 1.34 +/- 0.32 mmol/L (p < 0.05). Plasma insulin levels also increased from 337 +/- 44 to 1278 +/- 88 pg/mL (p < 0.01). Reduction of intralipid dose infusion did not prevent insulin resistance characterized by hyperglycemia and hyperinsulinemia. However, addition of L-glutamine to the high-dose lipid infusion with chow feeding prevented changes in plasma glucose, insulin levels, and FFA but not triglyceride levels. Also, glutamine but not alanine supplementation in intralipid infused rats without chow feeding prevented changes in plasma glucose, insulin, and malondialdehyde levels. In conclusion, these data show that glutamine supplementation during intravenous lipid administration in rats prevents the development of impaired glucose regulation associated with hyperlipidemia.

Leucine as a regulator of whole body and skeletal muscle protein metabolism in humans

Leucine has been proposed as an in vivo regulator of protein metabolism, although the evidence for this in humans remains inconclusive. To test this hypothesis, we infused either L-leucine (154 +/- 1 mumol.kg-1 x h-1) or saline intravenously in six healthy men in two separate studies. L-Leucine infusion increased plasma concentrations of leucine and alpha-ketoisocaproate from 112 +/- 6 and 38 +/- 3 mumol/l to 480 +/- 27 (P < 0.001) and 94 +/- 13 mumol/l (P < 0.001), respectively, without any significant change in circulating insulin or C peptide levels. Leucine infusion decreased plasma concentrations of several amino acids and decreased whole body valine flux and valine oxidation (using L-[1-13C]valine as a tracer) and phenylalanine flux (using [2H5]-phenylalanine as a tracer). According to arteriovenous differences across the leg, the net balance of phenylalanine, valine, and lysine shifted toward greater retention during leucine infusion, whereas alanine balance did not change. Valine release and phenylalanine release from the leg (estimated from the dilution of respective tracers) decreased, indicating inhibition of protein degradation by leucine infusion. We conclude that leucine decreases protein degradation in humans and that this decreased protein degradation during leucine infusion contributes to the decrease in plasma essential amino acids. This study suggests a potential role for leucine as a regulator of protein metabolism in humans.