Leucine metabolism in type II diabetes mellitus

Long-term rates of mitochondrial protein synthesis are increased in mouse skeletal muscle with high-fat feeding regardless of insulin-sensitizing treatment

Skeletal muscle mitochondrial protein synthesis is regulated in part by insulin. The development of insulin resistance with diet-induced obesity may therefore contribute to impairments to protein synthesis and decreased mitochondrial respiration. Yet the impact of diet-induced obesity and insulin resistance on mitochondrial energetics is controversial, with reports varying from decreases to increases in mitochondrial respiration. We investigated the impact of changes in insulin sensitivity on long-term rates of mitochondrial protein synthesis as a mechanism for changes to mitochondrial respiration in skeletal muscle. Insulin resistance was induced in C57BL/6J mice using 4 wk of a high-fat compared with a low-fat diet. For 8 additional weeks, diets were enriched with pioglitazone to restore insulin sensitivity compared with nonenriched control low-fat or high-fat diets. Skeletal muscle mitochondrial protein synthesis was measured using deuterium oxide labeling during weeks 10-12 High-resolution respirometry was performed using palmitoyl-l-carnitine, glutamate+malate, and glutamate+malate+succinate as substrates for mitochondria isolated from quadriceps. Mitochondrial protein synthesis and palmitoyl- l-carnitine oxidation were increased in mice consuming a high-fat diet, regardless of differences in insulin sensitivity with pioglitazone treatment. There was no effect of diet or pioglitazone treatment on ADP-stimulated respiration or H₂O₂ emission using glutamate+malate or glutamate+malate+succinate. The results demonstrate no impairments to mitochondrial protein synthesis or respiration following induction of insulin resistance. Instead, mitochondrial protein synthesis was increased with a high-fat diet and may contribute to remodeling of the mitochondria to increase lipid oxidation capacity. Mitochondrial adaptations with a high-fat diet appear driven by nutrient availability, not intrinsic defects that contribute to insulin resistance.

Nutrition from the Inside Out

Nearly 50 years ago, I set out to investigate the clinical problem of hypoglycemia in children with illnesses that limited their food intake. My goal was to gather accurate and precise measurable data. At the time, I wasn't interested in nutrition as a discipline defined in its more general or popular sense. To address the specific problem that interested me required development of entirely new methods based on stable, nonradioactive tracers that satisfied the conditions of accuracy and precision. At the time, I had no inclination of the various theoretical and practical problems that would have to be solved to achieve this goal. Some are briefly described here. Nor did I have the slightest idea that developing the field would result in a fundamental change in how human clinical investigation was conducted, with the eventual replacement of radiotracers with stable isotopically labeled ones, even for adult clinical investigation. Additionally, I had no inclination that the original questions would open avenues to much broader questions of practical nutritional relevance. Moreover, only much later as the editor of The American Journal of Clinical Nutrition did I appreciate the policy implications of how nutritional data are presented in the scientific literature. At least in part, less accurate and precise measurements and less than full transparency in reporting nutritional data have resulted in widespread debate about the public policy recommendations and guidelines that are the intended result of collecting the data in the first place. This article provides a personal recollection (with all the known faults of self-reporting and retrospective memory) of the journey that starts with measurement certainty and ends with policy uncertainty.

The 2010 ESPEN Sir David Cuthbertson Lecture: new and old proteins: clinical implications

The past century had witnessed vast advances in biomedical research, particularly in the fields of genomics and proteomics, yet the translation of these discoveries into clinical practice has been hindered by gaps in mechanistic understanding of variability governing disease susceptibility and pathogenesis. Among the greatest challenges are the dynamic nature of the proteome and the imperfect methodologies currently available to study it. Here, we review key recently developed proteomic techniques that have allowed for dynamic characterization of protein quality, as well as quantity, and discuss their potential applications in understanding aging and metabolic disorders including diabetes. These methodologies revealed that senescence is characterized, in part, by decreased rates of de novo protein synthesis and potentially also degradation, in addition to concomitantly increased levels of oxidative stress, ultimately resulting in excessive accumulation of damaged and dysfunctional proteins. Insulin may be a key mediator in these pathologies, as hyperinsulinemia has been shown to hinder protein degradation while transient insulin deficiency may accelerate oxidative damage. We also discuss two interventions that have been proposed to delay, and possibly reverse, senescence by augmenting protein degradation: chronic caloric restriction and aerobic exercise.

Effects of type 2 diabetes and insulin on whole-body, splanchnic, and leg protein metabolism

CONTEXT

Type 2 diabetes (T2D) is characterized by insulin resistance to glucose metabolism. Most studies suggest that protein metabolism is unaffected by T2D, but regional protein metabolism and response to multiple doses of insulin have not been examined.

OBJECTIVE

Our objective was to determine whether insulin regulation of splanchnic and leg protein metabolism are affected by T2D during hyperglycemia and graded insulin levels.

DESIGN AND SETTING

We conducted a cross-sectional study at an academic medical center.

PARTICIPANTS

T2D and non-T2D adults were matched for age (62 yr) and body mass index (30 kg/m(2)).

INTERVENTIONS

Glucose was maintained at approximately 9 mmol/liter while insulin was infused at three progressively higher rates, achieving circulating concentrations of approximately 150, 350, and 700 pmol/liter, respectively.

MAIN OUTCOME MEASURES

Protein kinetics were measured using labeled phenylalanine (Phe) and tyrosine (Tyr).

RESULTS

Whole-body protein breakdown and synthesis rates were higher in T2D but declined with increasing insulin in both groups. Leg Phe and Tyr appearance and disappearance and estimates of protein breakdown and synthesis, respectively, were higher in T2D but did not decline significantly with insulin, resulting in similar net balance between groups. Splanchnic response to insulin was blunted in T2D, shown by a smaller reduction in rates of disappearance and net balance of Phe and Tyr as insulin increased. Splanchnic conversion of Phe to Tyr was lower in T2D and less sensitive to insulin, whereas nonsplanchnic Phe to Tyr tended to be higher in T2D.

CONCLUSIONS

T2D results in higher whole-body, splanchnic, and leg protein turnover and blunts the insulin-mediated suppression of splanchnic protein anabolism under hyperglycemic, hyperinsulinemic conditions.

Age effect on myocellular remodeling: response to exercise and nutrition in humans

Aging is associated with decline in muscle mass and muscle functions. Muscle strength declines disproportionate to the decline in muscle mass indicating that muscle quality or protein quality also declines with age. Human studies have shown a progressive decline in muscle protein synthesis including proteins in the contractile apparatus and mitochondria with age. However, the decline in muscle protein synthesis is disproportionate to the decline in muscle mass that occurs with age prompting to hypothesize that muscle protein degradation also declines with age. A decline in mitochondrial capacity to synthesize ATP is likely a limiting factor of both synthesis and degradation, which are ATP dependent processes. In support of the above hypothesis, several studies have shown a decline in whole body protein turnover (synthesis and degradation). The timely and efficient degradation of irreversibly damaged or modified proteins is critical to maintain the quality of protein. It is proposed that a failure to degrade the damaged proteins and replacing them with newly synthesized proteins contribute to age related decline in muscle mass and quality of muscle proteins. The underlying molecular mechanism of these age related changes in human muscle needs further investigation.

Emerging perspectives on essential amino acid metabolism in obesity and the insulin-resistant state

Dysregulation of insulin action is most often considered in the context of impaired glucose homeostasis, with the defining feature of diabetes mellitus being elevated blood glucose concentration. Complications arising from the hyperglycemia accompanying frank diabetes are well known and epidemiological studies point to higher risk toward development of metabolic disease in persons with impaired glucose tolerance. Although the central role of proper blood sugar control in maintaining metabolic health is well established, recent developments have begun to shed light on associations between compromised insulin action [obesity, prediabetes, and type 2 diabetes mellitus (T2DM)] and altered intermediary metabolism of fats and amino acids. For amino acids, changes in blood concentrations of select essential amino acids and their derivatives, in particular BCAA, sulfur amino acids, tyrosine, and phenylalanine, are apparent with obesity and insulin resistance, often before the onset of clinically diagnosed T2DM. This review provides an overview of these changes and places recent observations from metabolomics research into the context of historical reports in the areas of biochemistry and nutritional biology. Based on this synthesis, a model is proposed that links the FFA-rich environment of obesity/insulin resistance and T2DM with diminution of BCAA catabolic enzyme activity, changes in methionine oxidation and cysteine/cystine generation, and tissue redox balance (NADH/NAD+).

Plasma metabolomic profiles reflective of glucose homeostasis in non-diabetic and type 2 diabetic obese African-American women

Insulin resistance progressing to type 2 diabetes mellitus (T2DM) is marked by a broad perturbation of macronutrient intermediary metabolism. Understanding the biochemical networks that underlie metabolic homeostasis and how they associate with insulin action will help unravel diabetes etiology and should foster discovery of new biomarkers of disease risk and severity. We examined differences in plasma concentrations of >350 metabolites in fasted obese T2DM vs. obese non-diabetic African-American women, and utilized principal components analysis to identify 158 metabolite components that strongly correlated with fasting HbA1c over a broad range of the latter (r = −0.631; p<0.0001). In addition to many unidentified small molecules, specific metabolites that were increased significantly in T2DM subjects included certain amino acids and their derivatives (i.e., leucine, 2-ketoisocaproate, valine, cystine, histidine), 2-hydroxybutanoate, long-chain fatty acids, and carbohydrate derivatives. Leucine and valine concentrations rose with increasing HbA1c, and significantly correlated with plasma acetylcarnitine concentrations. It is hypothesized that this reflects a close link between abnormalities in glucose homeostasis, amino acid catabolism, and efficiency of fuel combustion in the tricarboxylic acid (TCA) cycle. It is speculated that a mechanism for potential TCA cycle inefficiency concurrent with insulin resistance is “anaplerotic stress” emanating from reduced amino acid-derived carbon flux to TCA cycle intermediates, which if coupled to perturbation in cataplerosis would lead to net reduction in TCA cycle capacity relative to fuel delivery.

Protein balance in nondiabetic versus diabetic patients undergoing colon surgery: effect of epidural analgesia and amino acids

BACKGROUND

Surgical injury provokes a stress response that is thought to be pronounced in patients with diabetes mellitus type 2 (DM2) leading to intensified catabolism. The aim of this study was to compare the effects of perioperative epidural analgesia (EDA) versus patient controlled analgesia (PCA) and amino acid infusion on postoperative metabolism in patients with and without DM2.

METHODS

For this study, 12 nondiabetic patients and 12 diabetic patients undergoing colorectal surgery were randomly assigned to 4 groups (n = 6 per group) receiving either EDA (nondiabetic EDA and diabetic EDA [DEDA group]) or PCA with morphine (nondiabetic PCA and diabetic PCA) for perioperative pain control. Protein and glucose kinetics were measured on the second postoperative day using L-[1-13C]leucine and [6,6-2H2]glucose infusion during a fasted state and a 3-hr fed state with amino acid infusion.

RESULTS

The transition from the fasted to fed state suppressed endogenous rate of appearance (Ra) of glucose (P G 0.001) with a distinct effect for the DEDA group (P G 0.001). The Ra of leucine and the endogenous rate of appearance of leucine tended to be lower in the DEDA group(P = 0.056 and P = 0.07). Leucine oxidation was more suppressed in the DEDA group (P = 0.02) and when receiving amino acids(P = 0.001). Diabetic patients achieved a higher protein balance than nondiabetic patients (P = 0.032) and when receiving EDA instead of PCA (P = 0.012) or infusion of amino acids (P = 0.014).

CONCLUSIONS

A short-term infusion of amino acids reduced protein breakdown, increased protein synthesis, and rendered protein balance positive. This anabolic effect was pronounced in diabetic patients with EDA compared with nondiabetic patients or PCA, respectively, and prevented an undesirable hyperglycemia.

The muscle protein synthetic response to carbohydrate and protein ingestion is not impaired in men with longstanding type 2 diabetes

Protein ingestion stimulates muscle protein synthesis and improves net muscle protein balance. Insulin resistance has been suggested to result in a reduced muscle protein synthetic response to food intake. As such, we hypothesized that type 2 diabetes patients have a impaired muscle protein synthetic response to food ingestion. To test this hypothesis, 10 male type 2 diabetes patients using their normal oral glucose-lowering medication (68 +/- 2 y) and 10 matched, normoglycemic men (65 +/- 2 y) were randomly assigned to 2 crossover treatments in which whole body and muscle protein synthesis were measured following the consumption of either carbohydrate (CHO) or carbohydrate with a protein hydrolysate (CHO+PRO). Primed, continuous infusions with L-[ring-13C6]phenylalanine and L-[ring-2H2]tyrosine were applied and blood and muscle samples were collected to assess whole-body protein balance and mixed muscle protein fractional synthetic rate over a 6-h period. Whole-body phenylalanine and tyrosine flux were higher after the CHO+PRO treatment compared with the CHO treatment in the diabetes and control group (P < 0.01). Protein balance was negative following CHO but positive following CHO+PRO treatment in both groups. Muscle protein synthesis rates were higher in both groups following the CHO+PRO (0.086 +/- 0.014%/h) treatment than in the CHO treatment (0.040 +/- 0.003%/h; P < 0.01) with no difference between the diabetes patients and normoglycemic controls. We conclude that the muscle protein synthetic response to CHO or CHO+PRO ingestion is not substantially impaired in longstanding, type 2 diabetes patients treated with oral blood glucose-lowering medication.

Whole-body proteolysis rate is elevated in HIV-associated insulin resistance

Type 2 diabetes is characterized by impaired glucose tolerance (IGT) and insulin resistance with respect to glucose metabolism but not amino acid metabolism. We examined whether whole-body leucine and protein metabolism are dysregulated in HIV-infected individuals with IGT. Glucose and leucine kinetics were measured under fasting insulin conditions and during euglycemic hyperinsulinemia using primed-constant infusions of 2H2-glucose and 13C-leucine in 10 HIV-seronegative control subjects, 16 HIV+ subjects with normal glucose tolerance, and 21 HIV+IGT subjects. Glucose disposal rate during hyperinsulinemia was lower in HIV+IGT than the other two groups. Absolute plasma leucine levels and rate of appearance (whole-body proteolysis) were higher in HIV+IGT at all insulin levels but declined in response to hyperinsulinemia in parallel to those in the other two groups. HIV+IGT had greater visceral adiposity, fasting serum interleukin (IL)-8 and free fatty acid levels, and higher lipid oxidation rates during the clamp than the other two groups. These findings implicate several factors in the insulin signaling pathway, which may be further dysregulated in HIV+IGT, and support the notion that insulin signaling pathways for glucose and leucine metabolism may be disrupted by increased proinflammatory adipocytokines (IL-8) and increased lipid oxidation. Increased proteolysis may provide amino acids for gluconeogenesis, exacerbating hyperglycemia in HIV.

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.

Evidence for impaired assimilation of protein in chronic renal failure

BACKGROUND

Protein malnutrition is a common finding in chronic renal failure (CRF) and is associated with poor outcome. We hypothesized that besides inadequate dietary protein intake and alterations in metabolism, deficient protein assimilation (digestion and absorption) might contribute to the pathogenesis of protein malnutrition in uremia.

METHODS

Protein assimilation was evaluated in 64 healthy volunteers and 119 CRF patients by means of a 13C protein breath test and/or quantification of p-cresol in a 24-hour urine collection. Both approaches provide reliable information on the efficiency of protein assimilation. Breath test results were expressed as maximum percentage of administered dose of 13C (%max) and cumulative percentage at the end of the test (%cum(end)). Data were stratified according to renal function.

RESULTS

As compared to subjects with glomerular filtration rate (GFR) > or = 60 mL/min/1.73 m2, subjects with GFR < 30 mL/min/1.73 m2 demonstrated significantly lower breath test-derived parameters of protein assimilation (%max 3.97 +/- 0.23 vs. 5.20 +/- 0.23, P=0.0017; %cum(end) 13.91 +/- 0.86 vs. 17.40 +/- 0.80, P= 0.013) and significantly higher urinary output of p-cresol (54.88 mg/24 hours vs. 28.65 mg/24 hours, P= 0.0005). %max (r=0.399, P < 0.0001), %cum(end) (r=0.347, P=0.0007), and urinary p-cresol (r=-0.229, P=0.007) correlated significantly with GFR. Serum albumin correlated significantly with %max (r=0.399, P=0.0002), %cum(end) (r=0.408, P=0.0001), and urinary p-cresol output (r=-0.186, P=0.035).

CONCLUSION

Our data provide evidence that protein assimilation is impaired in CRF. This impairment might contribute to protein malnutrition in CRF.

Effect of protein restriction on sulfur amino acid catabolism in insulin-dependent diabetes mellitus

Persons with conventionally treated insulin-dependent diabetes mellitus (IDDM) appear to be impaired in their ability to reduce fed-state urea production appropriately in response to dietary protein restriction (Hoffer LJ, Taveroff A, and Schiffrin A. Am J Physiol 272: E59-E67, 1997). To determine whether these conclusions apply to whole body sulfur amino acid (SAA) catabolism, we used samples from this protocol to measure daily urinary sulfate excretion and fed-state sulfate production after a high-protein test meal before and after dietary protein restriction. Eight normal subjects and six IDDM subjects treated with twice-daily intermediate- and short-acting insulin consumed a mixed test meal containing 0.50 g protein/kg after adaptation to 4 days of high protein intake (1.28 g protein/kg body wt) and again after 5 days of dietary protein restriction (0.044 g/kg). Adaptation to protein restriction decreased daily urinary sulfate and urea-N excretion by approximately 80%. Over the first 24 h of protein restriction, urinary sulfate excretion decreased more than urea-N excretion for both the normal and IDDM subjects. Under conditions of a high prior protein intake, fed-state sulfate production was normal for the IDDM subjects; protein restriction reduced fed-state sulfate production by 51% (normal subjects) and 59% (IDDM subjects; not significant). We conclude that whole body SAA metabolism is normal in conventionally treated IDDM before and after dietary protein restriction. SAA catabolism, as measured by fed-state sulfate production, may be a convenient and useful method to determine the extent of whole body protein dysregulation in IDDM.

Type 2 diabetic patients may have a mild form of an injury response: a clinical research center study

Patients with type 2 diabetes (DM) demonstrate inadequate insulin release, elevated gluconeogenesis, and diminished nonoxidative glucose disposal. Similar metabolic changes occur during systemic injury caused by infection, trauma, or cancer. Described here are metabolic changes occurring in 16 DM and 11 lung cancer patients (CA) and 13 normal volunteers (NV). After a 10-h overnight fast, all subjects had fasting hormone and substrate concentrations determined, along with rates of glucose production, leucine appearance (LA), and leucine oxidation (LO). Fasting insulin (data not shown) and C-peptide concentrations were elevated in DM and CA compared with weight-matched NV (0.72 +/- 0.09 and 0.64 +/- 0.08 vs. 0.51 +/- 0.03 mg/l, P < 0.05). C-reactive protein concentration was elevated in CA compared with DM and NV (23.3 +/- 6.0 vs. 4.2 +/- 1.4 and 2.1 +/- 0.5 mg/l, P < 0.01). All counterregulatory hormones were normal except for serum cortisol (11.4 +/- 1.0 and 12.1 +/- 1.0 vs. 8.9 +/- 0.7 microg/dl, DM and CA vs. NL, respectively, P < 0.05). Glucose production was increased in DM and CA compared with NV (4.22 +/- 0.6 and 3.53 +/- 0.3 vs. 2.76 +/- 0.2 mg x kg lean body wt(-1) x min(-1), P < 0.01). LO and LA were increased in DM and CA compared with NV (LO: 27.3 +/- 1.5 and 19.7 +/- 1.5 vs. 12.5 +/- 1.1 mmol x kg lean body wt(-1) x min(-1), P < 0.05; LA: 91.9 +/- 6.6 and 90.7 +/- 7.0 vs. 79.1 +/- 6.0 mmol. kg lean body wt(-1) x min(-1), P < 0.01). DM share similar metabolic derangements with CA. The increase in LA may be secondary to an increased glucose production where amino acids are mobilized to provide the liver with adequate substrate to make glucose. The increase in glucose production may also be part of the injury response, or it may represent a form of insulin resistance that exists in both the DM and (non-DM) CA patients.

Increased synthesis rate of fibrinogen as a basis for its elevated plasma levels in obese female adolescents

Increased concentrations of plasma fibrinogen, an independent risk factor for cardiovascular disease (CVD), in obese children have been reported. The underlying mechanism for this, however, remains to be defined. In the current study, we measured the fractional synthesis rates (FSR) of plasma fibrinogen in six healthy postpubertal obese girls [body mass index (BMI) 36.6 +/- 1.8 kg/m(2); age 16.6 +/- 0.5 yr] and six age-matched lean normal control girls (BMI 20.8 +/- 0.7 kg/m(2); age 16.4 +/- 0.4 yr) during a primed, continuous infusion of L-[1-(13)C]leucine in the postabsorptive state. The method involved purification of plasma fibrinogen by use of immunoaffinity chromatography followed by measurement of [(13)C]leucine enrichment using gas chromatography-combustion-isotope ratio mass spectrometry. The FSR of fibrinogen in obese girls (35.06 +/- 2.61%/day) was almost double that in lean girls (17.02 +/- 1.43%/day), and this increase was associated with a relative increase in plasma concentration of fibrinogen as well as BMI in the subjects studied. Obese subjects had high fasting insulin levels (138 +/- 47 pmol/l) compared with lean subjects (54 +/- 11 pmol/l), whereas their glucose concentrations were similar (4.5 +/- 0.3 mmol/l in obese and 4.4 +/- 0.4 mmol/l in lean subjects), suggesting insulin resistance. The doubling of the FSR of fibrinogen provides novel insight into the mechanism of elevated levels of plasma fibrinogen and suggests a primary role for increased synthesis in producing the hyperfibrinogenemia associated with obesity. This finding may have important implications in the design of therapies for modulating plasma fibrinogen levels in obesity and/or CVD in childhood.

Increased fibrinogen production in type 2 diabetic patients without detectable vascular complications: correlation with plasma glucagon concentrations

Fibrinogen is a strong cardiovascular risk factor in the general population, and increased fibrinogen plasma concentrations have been reported in type 2 diabetic patients. However, the mechanisms leading to hyperfibrinogenemia in type 2 diabetes are not known. It is also not known whether possible alterations of fibrinogen turnover may precede clinical diabetic micro- and macrovascular complications and therefore potentially contribute to their onset. To address these questions, fibrinogen production was determined in six male type 2 diabetic patients without detectable micro- and macrovascular complications (age, 45 +/- 4 yr; body mass index, 27 +/- 0.9 kg/m2) and in seven nondiabetic matched controls using leucine isotope precursor-product relationships. Plasma glucose (P < 0.001), insulin (P < 0.05), and glucagon concentrations (P < 0.01) were increased in the patients. Diabetic patients also had increased plasma fibrinogen concentration (+ approximately 50%; P < 0.01) and pool (+ approximately 40%; P < 0.01) as well as fractional (+ approximately 35%; P = 0.08) and absolute (+ approximately 100%; P < 0.01) synthetic rates. The plasma glucagon concentration was positively related (P < 0.005 or less) to the fibrinogen concentration as well as to fractional and absolute synthetic rates. Thus, fibrinogen production is markedly enhanced, and this alteration is likely to determine the observed hyperfibrinogenemia in type 2 diabetic patients. Hyperglucagonemia may contribute to the increased fibrinogen production. These findings in normoalbuminuric patients without clinical complications support the hypothesis that increased fibrinogen production and plasma concentrations may precede and possibly contribute to the onset of clinical cardiovascular complications in type 2 diabetes.

Protein nutritional status and function are associated with type 2 diabetes in Hispanic elders

BACKGROUND

Hispanic elders have a high prevalence of diabetes and poor glycemic control, leading to inadequate nutritional status, muscle wasting, and impaired function.

OBJECTIVE

We examined the association of type 2 diabetes with nutritional status measured by serum albumin concentrations and midupper arm muscle area (MAM) and with function measured by difficulty with at least one activity of daily living.

DESIGN

: Health history and disability were assessed by self report in 556 Hispanics with a mean (+/- SD) age of 69 +/- 7 y and 158 non-Hispanic whites (NHW; aged 71 +/- 7 y) from the Massachusetts Hispanic Elders Survey. Energy intake (in MJ/d) and protein intake (in g/d) were estimated with use of a food-frequency questionnaire. Body mass index (BMI; in kg/m(2)) and C-reactive protein concentrations (in mg/L) were also measured. Multiple logistic regression models by ethnic group were used.

RESULTS

There were no significant differences between Hispanics and NHWs in the proportion of those with low albumin concentrations or low MAM. Hispanic diabetic women had the lowest proportion of low MAM. The risk of low serum albumin concentration was twice as high in Hispanics taking insulin as in their NHW counterparts. Among Hispanics, low albumin concentration and low BMI were associated with low MAM; female sex, low albumin concentration, high BMI, and insulin use were significantly associated with risk of functional impairment.

CONCLUSION

Type 2 diabetes is associated with poor nutritional status, muscle loss, and functional impairment among Hispanic elders.

Reduced synthesis of muscle proteins in chronic renal failure

Muscle wasting and weakness occur frequently in patients with chronic renal failure. The mechanism(s) by which these abnormalities occur is unclear. We hypothesized that such findings were due to defective muscle protein synthesis. We measured synthetic rates of mixed muscle proteins, myosin heavy chain, and mitochondrial proteins in serial muscle biopsy samples during a continuous infusion of L[1-(13)C]leucine from 12 patients with chronic renal failure and 10 healthy control subjects under identical study conditions. Patients with chronic renal failure have significantly lower synthetic rates of mixed muscle proteins and myosin heavy chain (27 and 37% reductions, respectively, P < 0.05 and P < 0.02). Significant declines in the synthetic rates of muscle mitochondrial protein (27%) (P < 0.05), muscle cytochrome c-oxidase activity (42%) (P < 0.007), and citrate synthase (27%) (P < 0.007) were also observed in patients with chronic renal failure. The synthetic rates of muscle proteins and activity of mitochondrial enzymes were negatively correlated to the severity of renal failure. These results indicate that in chronic renal failure there is a decrease in the synthesis of muscle contractile and mitochondrial proteins and a decrease in muscle mitochondrial oxidative enzymes. Reduced synthetic rate of several muscle proteins is the likely biochemical basis of muscle loss and muscle weakness in people with chronic renal failure.

Fed state protein metabolism in diabetes mellitus

Current knowledge concerning the physiologic regulation of protein anabolism during feeding is both limited and debated; little information is available on the effects of pathological conditions such as diabetes mellitus. This is due largely to methodological problems and the technical difficulties associated with the isotope dilution techniques used to estimate protein kinetics in the fed state. Data that are available suggest that meal intake induces protein anabolism in healthy subjects by reducing endogenous proteolysis and selectively stimulating protein synthesis, with amino acids and insulin playing the major regulatory role. Data obtained in subjects with noninsulin-dependent diabetes mellitus demonstrate that the impaired insulin action on glucose metabolism characteristic of that disease may not be associated with abnormal protein metabolism in the fed state. Severe insulin deficiency, as occurs in insulin-dependent diabetes, adversely affects nitrogen balance; at present, however, the effects of poor postprandial glycemic control on protein kinetics in subjects with insulin-dependent diabetes remain to be established.

Amino acids suppress proteolysis independent of insulin throughout the neonatal period

To determine how increased amino acid availability alters rates of whole body proteolysis and the irreversible catabolism of the essential amino acids leucine and phenylalanine throughout the neonatal period, leucine and phenylalanine kinetics were measured under basal conditions and in response to intravenous amino acids in two separate groups of healthy, full-term newborns (at 3 days and 3 wk of age). The endogenous rates of appearance of leucine and phenylalanine (reflecting proteolysis) were suppressed equally in both groups and in a dose-dependent fashion (by approximately 10% with 1.2 g x kg(-1) x day(-1) and by approximately 20% with 2.4 g x kg(-1) x day(-1)) in response to intravenous amino acid delivery. Insulin concentrations remained unchanged from basal values during amino acid administration. The irreversible catabolism of leucine and phenylalanine increased in a stepwise fashion in response to intravenous amino acids; again, no differences were observed between the two groups. This study clearly demonstrates that the capacity to acutely increase rates of leucine oxidation and phenylalanine hydroxylation is fully present early in the neonatal period in normal newborns. Furthermore, these data suggest that amino acid availability is a primary regulator of proteolysis in normal newborns throughout the neonatal period.

Fasting and postprandial phenylalanine and leucine kinetics in liver cirrhosis

To investigate body protein turnover and the pathogenesis of increased concentration of plasma phenylalanine in liver cirrhosis, we have studied phenylalanine and leucine kinetics in cirrhotic (diabetic and nondiabetic) patients, and in normal subjects, both in the postabsorptive state and during a mixed meal, using combined intravenous and oral isotope infusions. Postabsorptive phenylalanine concentration and whole body rate of appearance (Ra) were approximately 40% greater (P < 0.05) in patients than in controls. Leucine concentrations were comparable, but intracellular leucine Ra was also increased (P < 0.05), suggesting increased whole body protein breakdown. Postprandial phenylalanine Ra was also greater (P < 0.05) in the patients. This difference was due to a diminished fractional splanchnic uptake of the dietary phenylalanine (approximately 40% lower in the cirrhotics vs. controls, P < or = 0.05). Postprandial leucine Ra was also increased in the patients, but splanchnic uptake of dietary leucine was normal. Thus both increased body protein breakdown and decreased splanchnic extraction of dietary phenylalanine can account for the increased phenylalanine concentrations in liver cirrhosis.

Intravenous glucose suppresses glucose production but not proteolysis in extremely premature newborns

To ascertain whether the inability to suppress glucose production and increase glucose utilization in response to glucose infusion is an inherent characteristic of immature individuals, we determined glucose rate of appearance (R(a)) in minimally stressed, clinically stable, extremely premature infants (approximately 26-wk gestation) at two glucose infusion rates (6.2 +/- 0.4 and 9.5 +/- 0.5 mg/kg per min). We also assessed whether an increase in glucose delivery suppresses proteolysis by measuring the R(a) of phenylalanine and leucine. Glucose R(a) (and utilization) increased significantly at the higher glucose infusion rate (7.9 +/- 0.5 vs. 9.8 +/- 0.6 mg/kg per min). Glucose production persisted at the lower glucose infusion rate but was suppressed to nearly zero at the higher rate (1.7 +/- 0.5 vs. 0.3 +/- 0.1 mg/kg per min). Proteolysis was unaffected by the higher glucose infusion rate as reflected by no change in the rates of appearance of either phenylalanine (96 +/- 5 vs. 95 +/- 3 mumol/kg per h) or leucine (285 +/- 20 vs. 283 +/- 14 mumol/kg per h). Thus, clinically stable, extremely premature infants suppress glucose production and increase glucose utilization in response to increased glucose infusion, demonstrating no inherent immaturity of these processes. In contrast, increasing the rate of glucose delivery results in no change in whole body proteolysis in these infants. The regulation of proteolysis in this population remains to be defined.

Fasting and postmeal phenylalanine metabolism in mild type 2 diabetes

We have investigated postabsorptive and postprandial phenylalanine kinetics in non-obese type 2 diabetic patients [non-insulin-dependent diabetes mellitus (NIDDM)], using a double-isotope technique and the constant oral administration of a synthetic mixed meal. Fasting and postmeal glucose levels were increased (P < 0.01) in NIDDM (165 +/- 16 to 226 +/- 24 mg/dl), with respect to normal controls (85 +/- 3 to 102 +/- 6 mg/dl). Fasting insulin concentrations were comparable in NIDDM (13 +/- 2 microU/ml) and in normals (12 +/- 2 microU/ml), but after the meal it increased less (P < 0.07) in NIDDM vs. normals (to 36 +/- 5 vs. 56 +/- 12 microU/ml, respectively; P < 0.01 vs. basal for both). Postabsorptive phenylalanine rate of appearance (R(a)) in NIDDM (0.63 +/- 0.08 mumol.kg-1 x min-1) was comparable to that of controls (0.73 +/- 0.05 mumol.kg-1 x min-1, not significant). During the meal, total and endogenous phenylalanine R(a), splanchnic uptake, oxidation, and nonoxidative disposal of the ingested phenylalanine were also comparable in the two groups. These data indicate that fasting and postprandial kinetics of the essential amino acid phenylalanine are normal in NIDDM.

Relationship of resting metabolic rate to body composition and protein turnover

This study examined whether variability among healthy young adults in resting metabolic rate, normalized for the amount of metabolically active tissue (assessed by total body potassium), is related to protein turnover. Resting metabolic rate was measured by indirect calorimetry for 2 h in 26 men and 21 women, 19-33 yr old, with simultaneous estimation of protein turnover during a 4-h infusion of L-[1-13C]leucine. After adjusting metabolic rate for total body potassium, the standard deviation was only 89 kcal/day, or 5.5% of the average value. There was a high correlation between leucine flux (an index of proteolysis) and metabolic rate (r = 0.84) and between the nonoxidized portion of leucine flux (an index of protein synthesis) and metabolic rate (r = 0.83). This relationship was weaker, but still significant, after adjusting leucine metabolism and metabolic rate for total body potassium (r = 0.36 for leucine flux vs. metabolic rate, r = 0.33 for nonoxidized portion of leucine flux vs. metabolic rate, P less than 0.05). The regression analysis suggested that the contribution of protein turnover to resting metabolic rate was approximately 20% in an average subject. Metabolic rate and protein turnover were highest in the subjects with the greatest amount of body fat, even after accounting for differences in whole body potassium. Neither resting metabolic rate nor protein turnover was related to total or free concentrations of thyroxine or triiodothyronine, within the euthyroid range.(ABSTRACT TRUNCATED AT 250 WORDS)