V. R. Young

Insulin-mediated reduction of whole body protein breakdown. Dose-response effects on leucine metabolism in postabsorptive men.

In vivo effects of insulin on plasma leucine and alanine kinetics were determined in healthy postabsorptive young men (n = 5) employing 360-min primed, constant infusions of L-[1-13C]leucine and L-[15N]alanine during separate single rate euglycemic insulin infusions. Serum insulin concentrations of 16.4 +/- 0.8, 29.1 +/- 2.7, 75.3 +/- 5.0, and 2,407 +/- 56 microU/ml were achieved. Changes in plasma 3-methyl-histidine (3-MeHis) were obtained as an independent qualitative indicator of insulin-mediated reduction in proteolysis. Hepatic glucose output was evaluated at the lowest insulin level using D-[6,6-2H2]glucose. The data demonstrate a dose-response effect of insulin to reduce leucine flux, from basal values of 77 +/- 1 to 70 +/- 2, 64 +/- 3, 57 +/- 3, and 52 +/- 4 mumol(kg X h)-1 at the 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.01). A parallel, progressive reduction in 3-MeHis from 5.8 +/- 0.3 to 4.3 +/- 0.3 microM was revealed. Leucine oxidation estimated from the 13C-enrichment of expired CO2 and plasma leucine (12 +/- 1 mumol[kg X h]-1) and from the 13C-enrichment of CO2 and plasma alpha-ketoisocaproate (19 +/- 2 mumol[kg X h]-1) increased at the 16 microU/ml insulin level to 16 +/- 1 and 24 +/- 2 mumol(kg X h)-1, respectively (P less than 0.05 for each), but did not increase at higher insulin levels. Alanine flux (206 +/- 13 mumol(kg X h)-1) did not increase during the clamp, but alanine de novo synthesis increased in all studies from basal rates of 150 +/- 13 to 168 +/- 23, 185 +/- 21, 213 +/- 29, and 187 +/- 15 mumol(kg X h)-1 at 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.05). These data indicate the presence of insulin-dependent suppression of leucine entry into the plasma compartment in man secondary to a reduction in proteolysis and the stimulation of alanine synthesis during euglycemic hyperinsulinemia.

Palmitate and glycerol kinetics during brief starvation in normal weight young adult and elderly subjects.

Data obtained in vitro suggest that the ability to mobilize fat decreases with age. We determined lipolytic rates in vivo in normal weight young adult (22-33 yr) and elderly (65-77 yr) subjects using a simultaneous infusion of [1,2-13C2]palmitate and [2H5]glycerol. The subjects were studied after a 12-h fast and again after 60-82 h of fasting. When lipolysis was expressed per unit of adipose tissue the values for the young adults were more than double those for the elderly (P less than 0.05). However, the amount of body fat in the elderly was twice that of the young adults, so that lipolysis per unit of body weight was similar in both groups. These results demonstrate that lipolysis per unit of adipose tissue is lower in elderly subjects. This may be due to their increase in body fat, however, since the total amount of potential energy mobilized from adipose tissue was similar to that of the young adults.

Metabolic effects of very low calorie weight reduction diets.

A randomized comparison trial of two very low calorie weight reduction diets was carried out for 5 or 8 wk in 17 healthy obese women. One diet provided 1.5 g protein/kg ideal body weight; the other provided 0.8 g protein/kg ideal body weight plus 0.7 g carbohydrate/kg ideal body weight. The diets were isocaloric (500 kcal). Amino acid metabolism was studied by means of tracer infusions of L-[1-13C]leucine and L-[15N]alanine. After 3 wk of adaptation to the diets, nitrogen balance was zero for the 1.5 g protein diet but -2 g N/d for the 0.8 g protein diet. Postabsorptive plasma leucine and alanine flux decreased from base line by an equal extent with both diets by approximately 20 and 40%, respectively. It was concluded that protein intakes at the level of the recommended dietary allowance (0.8 g/kg) are not compatible with nitrogen equilibrium when the energy intake is severely restricted, and that nitrogen balance is improved by increasing the protein intake above that level. Basal rates of whole body nitrogen turnover are relatively well maintained, compared with total fasting, at both protein intakes. However, turnover in the peripheral compartment, as evidenced by alanine flux, may be markedly diminished with either diet.

Plasma arginine and leucine kinetics and urea production rates in burn patients

We measured plasma arginine and leucine kinetics and rates of urea production (appearance) in 12 severely burned patients (mean body surface burn area, 48%) during a basal state (low-dose intravenous glucose) and while receiving routine, total parenteral nutrition ([TPN] fed state) including an L-amino acid mixture, supplying a generous level of nitrogen (mean, 0.36 g N.kg-1.d-1). The two nutritional states were studied in random order using a primed 4-hour constant intravenous tracer infusion protocol. Stable-nuclide-labeled tracers were L-[guanidino-13C]arginine, L-[1-13C]leucine, [18O]urea, and NaH13CO3 (prime only), with blood and expired air samples drawn at intervals to determine isotopic abundance of arginine, citrulline, ornithine, alpha-ketoisocaproate ([KIC] for leucine), and urea in plasma and 13CO2 in breath. Results are compared with data obtained in these laboratories in healthy adults. Leucine kinetics (flux and disappearance into protein synthesis) indicated the expected higher turnover in burn patients than in healthy controls. Mean leucine oxidation rates are also higher and compared well with values predicted from urea production rates, provided that urea nitrogen recycling via intestinal hydrolysis is taken into account. The plasma urea flux was also higher than for normal subjects. Arginine fluxes as measured in the systemic whole body, via the plasma pool, were correspondingly higher in burned patients than in healthy controls and were in good agreement with values predicted from leucine-KIC kinetics. However, systemic whole-body arginine flux measured via the plasma pool was only 20% of the arginine flux estimated from the urea flux plus the rate of protein synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative aspects of glucose production and metabolism in healthy elderly subjects.

The metabolic basis for the reduced glucose tolerance that occurs during aging in humans has been explored with the aid of a primed constant intravenous infusion method of labeled glucose (6-3H; 6,6,2H- and U-13C-glucose). Healthy young adult men and women (24 ± 3 yr) and elderly men and women (75 ± 4 yr) participated in a series of studies designed to quantify rates of plasma glucose appearance, oxidation, and recycling while subjects were in the postabsorptive (basal) state and to determine rates of hepatic glucose production and glucose disappearance in response to intravenous glucose at approximately 1 and 2 mg · kg−1min−1 and also 4 mg · kg−1min−1 without or with a simultaneous infusion of insulin to maintain normoglycemia. Basal rates of glucose production were 2.41 ± 0.06 and 2.18 ± 0.05 mg · kg−1min−1 in the young adults and elderly, respectively (P < 0.05). Recycling of glucose carbon and glucose oxidation rates did not differ significantly between the two age groups. Infusion of unlabeled glucose reduced hepatic glucose production to the same extent in the two groups, indicating that the mechanisms responsible for altered hepatic glucose production with intravenous glucose administration remain intact during human aging. Plasma insulin changes were similar in young adult and elderly subjects receiving 4 mg · kg−1min−1 unlabeled glucose except that the higher plasma glucose levels in the elderly were associated with higher insulin levels. For elderly subjects, the amount of exogenous insulin required to maintain normoglycemia at the 4 mg · kg−1min−1 glucose infusion rate was about twice that necessary in young adults.

Abnormal phenylalanine hydroxylation and tyrosine oxidation in a patient with acute fulminant liver disease with correction by liver transplantation

Phenylalanine hydroxylation, tyrosine oxidation, and plasma appearance of phenylalanine and tyrosine were evaluated in a 49-yr-old woman with fulminant non-A, non-B hepatitis and encephalopathy using a continuous intravenous infusion of L-[ring-D5]phenylalanine and L-[U-14C]tyrosine. Despite marked elevations in plasma phenylalanine and tyrosine appearance and normal apparent albumin synthetic rates, phenylalanine clearance and hydroxylation to tyrosine were only 12% and 60%, respectively, of values observed in individuals with normal liver function. Three days after orthotopic liver transplantation, plasma phenylalanine and tyrosine appearances were not markedly changed. Phenylalanine clearance and conversion to tyrosine, however, were restored to normal. In addition, tyrosine oxidation and apparent albumin synthesis were increased. This case report represents the first in vivo demonstration of a selective diminution of enzyme function in an individual with fulminant liver disease. Liver replacement restored aromatic amino acid degradative capacity and increased albumin synthesis.

The impact of body composition on the regulation of lipolysis during short-term fasting.

Lipolytic rates were determined in normal-weight young adult (low body fat), normal-weight elderly (moderate body fat), and obese young adult (high body fat) subjects after an overnight (12-15 hr) and short-term (60-87 hr) fasting period. A simultaneous infusion of [1-13C] palmitate acid and [2H5]glycerol was used to measure the inflow of palmitate and glycerol into the bloodstream, and H2 18O dilution was used to measure body composition. The amount of body fat differed between the young adult (11 kg), elderly (23 kg) and obese (49 kg) subjects. Total lean body mass was similar in the normal-weight young adult (60 kg) and obese (63 kg) groups, but was lower in the elderly (46 kg). Lipolytic rates per unit of fat mass decreased in each group relative to the increase in the amount of body fat. Lipolytic rates, expressed as a function of lean body mass, however, were the same in all three groups. These results suggest that body composition contributes to the regulation of lipolysis during fasting. The decreased rate of lipolysis in subjects who have excess body fat may reflect the decreased need, per unit fat mass, for lipolysis to meet the energy requirements of the lean body mass.

Metabolic fate of extrahepatic arginine in liver after burn injury

Increased nitrogen loss in the form of urea is a hallmark of the metabolic aberrations that occur after burn injury. As the immediate precursor for urea production is arginine, we have conducted an investigation on the metabolic fate of arginine in the liver to shed light on the metabolic characteristics of this increased nitrogen loss. Livers from 25% total surface burn (n = 8) and sham burn rats (n = 8) were perfused in a recycling fashion with a medium containing amino acids and stable isotope labeled l-[(15) N(2)-guanidino, 5,5-(2)H(2)]arginine for 120 minutes. The rates of glucose and urea production and oxygen consumption were measured. The rate of unidirectional arginine transport and the intrahepatic metabolic fate of arginine in relation to urea cycle activity were quantified by tracing the disappearance rate of the arginine tracer from and the appearance rate of [(15)N(2)]urea in the perfusion medium. Perfused livers from burned rats showed higher rates of total urea production (mean +/- SE, 4.471 +/- 0.274 v 3.235 +/- 0.261 mumol. g dry liver(-1). min(-1); P <.01). This was accompanied by increased hepatic arginine transport (1.269 +/- 0.263 v 0.365 +/- 0.021 mumol. g dry liver(-1). min(-1)) and an increased portion of urea production from the transported extrahepatic arginine (12.9% +/- 2.9% v 3.5% +/- 0.4%, P <.05). The disposal of arginine via nonurea pathways was also increased (0.702 +/- 0.185 v 0.257 +/- 0.025 mumol/g dry weight(-1)/min(-1); P <.05). We propose that increased inward transport and utilization of extrahepatic arginine by the liver contributes to the accelerated urea production after burn injury and accounts, in part, for its conditional essentiality in the nutritional support of burn patients.

The role of protein in nutrition, with particular reference to the composition and use of enteral feeding formulas: a consensus report

In its proposed classification scheme for reimbursement for enteral feeding formulas, the United States Health Care Financing Administration (HCFA) has neglected to consider the importance of making available formulas of varying protein content. In responding to HCFAs proposal, this consensus report reviews information currently available regarding different protein requirements in healthy, ill, and undernourished adults and elderly individuals. HCFAs reimbursement guidelines should reflect sound medical and nutritional practice rather than influencing it adversely, which its original proposal could do. Clinicians should have at their disposal enteral feeding formulas of varying protein/calorie ratios, especially since the following groups often have higher than average protein requirements relative to calorie requirements: the elderly, individuals with physiologic stress such as infection and injury, and patients with protein-calorie undernutrition.

Synthesis of [1,2‐13C2] gly and [2,2‐2H2] gly glutathione

[1,2-13C2] Gly and [2,2-2H2] Gly isotopomers of the intracellular tripeptide glutathione were prepared by standard methods of solution phase peptide synthesis. The synthetic products were characterized by gas chromatography/mass spectroscopy (GC/MS) and 1H NMR spectroscopy. Optical purity was determined by hydrolysis, derivatization of the free amino acids with isopropanol-acetyl chloride and pentafluoropropionic anhydride and NCI/MS with a Chirasil-Val Heliflex column. These compounds should serve as useful tracers for the non-invasive study of glutathione synthesis and turnover rates in humans by GC/MS.