Validation of 13C NMR measurement of human skeletal muscle glycogen by direct biochemical assay of needle biopsy samples

Effects of carbohydrate availability on cycling endurance at the maximal lactate steady state

The impacts of carbohydrate (CHO) availability on time to task failure (TTF) and physiological responses to exercise at the maximal lactate steady state (MLSS) have not been studied. Ten participants (3 females, 7 males) completed this double-blinded, placebo-controlled study that involved a ramp incremental test, MLSS determination, and four TTF trials at MLSS, all performed on a cycle ergometer. With the use of a combination of nutritional (CHO, 7 g/kg, and placebo, PLA, 0 g/kg drinks) and exercise interventions [no exercise (REST) and glycogen-reducing exercise (EX)], the four conditions were expected to differ in preexercise CHO availability (RESTCHO > RESTPLA > EXCHO > EXPLA). TTF at MLSS was not improved by CHO loading, as RESTCHO (57.1 [16.6] min) and RESTPLA (57.1 [15.6] min) were not different (P = 1.00); however, TTF was ∼50% shorter in EX conditions compared with REST conditions on average (P < 0.05), with EXCHO (39.1 [9.2] min) ∼90% longer than EXPLA (20.6 [6.9] min; P < 0.001). There were effects of condition for all perceptual and cardiometabolic variables when compared at isotime (P < 0.05) and task failure (TF; P < 0.05), except for ventilation, perceptual responses, and neuromuscular function measures, which were not different at TF (P > 0.05). Blood lactate concentration was stable in all conditions for participants who completed 30 min of exercise. These findings indicate that TTF at MLSS is not enhanced by preexercise CHO supplementation, but recent intense exercise decreases TTF at MLSS even with CHO supplementation. Extreme fluctuations in diet and strenuous exercise that reduce CHO availability should be avoided before MLSS determination.NEW & NOTEWORTHY Carbohydrate (CHO) loading did not increase participants' ability to cycle at their maximal lactate steady state (MLSS); however, performing a glycogen depletion task the evening before cycling at MLSS reduced the time to task failure, even when paired with a high dose of CHO. These diet and exercise interventions influenced blood lactate concentration ([BLa]) but not the stability of [BLa]. Activities that reduce CHO availability should be avoided before MLSS determination.

The early days of ex vivo 1 H, 13 C, and 31 P nuclear magnetic resonance in the laboratory of Dr. Robert G. Shulman from 1975 to 1995

This paper provides a brief description of the early use of ex vivo nuclear magnetic resonance (NMR) studies of tissue and tissue extracts performed in the laboratory of Dr. Robert G. Shulman from 1975 through 1995 at Bell Laboratories, then later at Yale University. During that period, ex vivo NMR provided critical information in support of resonance assignments and the quantitation of concentrations for magnetic resonance spectroscopy studies. The period covered saw rapid advances in magnet technology, starting with studies of microorganisms in vertical bore high-resolution NMR studies, then by 1981 studies of small mammals in a horizontal bore magnet, and then studies of humans in 1984. Ex vivo NMR played a critical role in all these studies. A general strategy developed in the lab for using ex vivo NMR to support in vivo studies is presented, as well as illustrative examples.

Liver glycogen stores via 13C magnetic resonance spectroscopy in healthy children: randomized, controlled study

BACKGROUND

Owing to its role in glucose homeostasis, liver glycogen concentration ([LGly]) can be a marker of altered metabolism seen in disorders that impact the health of children. However, there is a paucity of normative data for this measure in children to allow comparison with patients, and time-course assessment of [LGly] in response to feeding has not been reported. In addition, carbon-13 magnetic resonance spectroscopy (¹³C-MRS) is used extensively in research to assess liver metabolites in adult health and disease noninvasively, but similar measurements in children are lacking.

OBJECTIVES

The main objectives were to quantify the depletion of [LGly] after overnight fasting and the subsequent response to feeding.

METHODS

In a randomly assigned, open-label, incomplete block design study, healthy, normal-weight children (8-12 y) attended 2 evening visits, each separated by ≥5 d and directly followed by a morning visit. An individually tailored, standardized meal was consumed 3-h prior to evening assessments. Participants then remained fasted until the morning visit. [LGly] was assessed once in the fed (20:00) and fasted state (08:00) using ¹³C-MRS. After the 8:00 assessment, 200 ml of a mixed-macronutrient drink containing 15.5 g (402 kJ) or 31 g carbohydrates (804 kJ), or water only, was consumed, with ¹³C-MRS measurements then performed hourly for 4 h. Each child was randomly assigned to 2 of 3 drink options across the 2 mornings. Data are expressed as mean (SD).

RESULTS

Twenty-four children including females and males (13F:11M) completed the study [9.9 (1.1) y, BMI percentile 45.7 (25.9)]. [LGly] decreased from 377.9 (141.3) to 277.3 (107.4) mmol/L overnight; depletion rate 0.14 (0.15) mmol/L min. Incremental responses of [LGly] to test drinks differed (P < 0.001), with incremental net area under the curve of [LGly] over 4 h being higher for 15.5 g [-67.1 (205.8) mmol/L·240 min; P < 0.01] and 31 g carbohydrates [101.6 (180.9) mmol/L·240 min; P < 0.005] compared with water [-253.1 (231.2) mmol/L·240 min].

CONCLUSIONS

After overnight fasting, [LGly] decreased by 22.9 (25.1)%, and [LGly] incremental net area under the curve over 4 h was higher after subsequent consumption of 15.5 g and 31 g carbohydrates, compared to water. Am J Clin Nutr 20XX;xx:xx-xx.

Muscle Glycogen Assessment and Relationship with Body Hydration Status: A Narrative Review

Muscle glycogen is a crucial energy source for exercise, and assessment of muscle glycogen storage contributes to the adequate manipulation of muscle glycogen levels in athletes before and after training and competition. Muscle biopsy is the traditional and gold standard method for measuring muscle glycogen; alternatively, ¹³C magnetic resonance spectroscopy (MRS) has been developed as a reliable and non-invasive method. Furthermore, outcomes of ultrasound and bioimpedance methods have been reported to change in association with muscle glycogen conditions. The physiological mechanisms underlying this activity are assumed to involve a change in water content bound to glycogen; however, the relationship between body water and stored muscle glycogen is inconclusive. In this review, we discuss currently available muscle glycogen assessment methods, focusing on ¹³C MRS. In addition, we consider the involvement of muscle glycogen in changes in body water content and discuss the feasibility of ultrasound and bioimpedance outcomes as indicators of muscle glycogen levels. In relation to changes in body water content associated with muscle glycogen, this review broadens the discussion on changes in body weight and body components other than body water, including fat, during carbohydrate loading. From these discussions, we highlight practical issues regarding muscle glycogen assessment and manipulation in the sports field.

Leptin receptor defect with diabetes causes skeletal muscle atrophy in female obese Zucker rats where peculiar depots networked with mitochondrial damages

Tibialis anterior muscles of 45-week-old female obese Zucker rats with defective leptin receptor and non-insulin dependent diabetes mellitus (NIDDM) showed a significative atrophy compared to lean muscles, based on histochemical-stained section's measurements in the sequence: oxidative slow twitch (SO, type I) < oxidative fast twitch (FOG, type IIa) < fast glycolytic (FG, type IIb). Both oxidative fiber's outskirts resembled 'ragged' fibers and, in these zones, ultrastructure revealed small clusters of endoplasm-like reticulum filled with unidentified electron contrasted compounds, contiguous and continuous with adjacent mitochondria envelope. The linings appeared crenated stabbed by circular patterns resembling those found of ceramides. The same fibers contained scattered degraded mitochondria that tethered electron contrasted droplets favoring larger depots while mitoptosis were widespread in FG fibers. Based on other interdisciplinary investigations on the lipid depots of diabetes 2 muscles made us to propose these accumulated contrasted contents to be made of peculiar lipids, including acyl-ceramides, as those were only found while diabetes 2 progresses in aging obese rats. These could interfere in NIDDM with mitochondrial oxidative energetic demands and muscle functions.

The Source of Glycolytic Intermediates in Mammalian Tissues

Glycolysis plays a central role in organismal metabolism, but its quantitative inputs across mammalian tissues remain unclear. Here we use ¹³C-tracing in mice to quantify glycolytic intermediate sources: circulating glucose, intra-tissue glycogen, and circulating gluconeogenic precursors. Circulating glucose is the main source of circulating lactate, the primary end product of tissue glycolysis. Yet circulating glucose highly labels glycolytic intermediates in only a few tissues: blood, spleen, diaphragm, and soleus muscle. Most glycolytic intermediates in the bulk of body tissue, including liver and quadriceps muscle, come instead from glycogen. Gluconeogenesis contributes less but also broadly to glycolytic intermediates, and its flux persists with physiologic feeding (but not hyperinsulinemic clamp). Instead of suppressing gluconeogenesis, feeding activates oxidation of circulating glucose and lactate to maintain glucose homeostasis. Thus, the bulk of the body slowly breaks down internally stored glycogen while select tissues rapidly catabolize circulating glucose to lactate for oxidation throughout the body.

Diurnal variation in the glycogen content of the human liver using 13 C MRS

Glycogen in tissues functions not only as carbohydrate reserves, but also as molecular sensors capable of activating signaling pathways in response to physical activity. While glycogen in the skeletal muscles is mainly a local energy substrate, glycogen in the liver serves as a glucose reserve to maintain normal blood glucose levels in the body, even during the sleep state. The aim of this study is to compare the diurnal variation of glycogen in the muscle and liver of human subjects under normal conditions. The glycogen content was measured in the muscle and liver of 10 young, healthy, male volunteers using ¹³ C MRS, a non-invasive technique. The subjects remained sedentary, and glycogen concentration was measured six times daily. Experimental meals were provided to achieve individual energy balance, estimated according to the energy requirement guideline for patients from Japan. The largest variation in muscle glycogen compared with 1 h after supper (20:00 on Day 1) was 3.1 ± 8.2 mmol/L (16:00 on Day 2). In the liver, however, the glycogen content decreased by 80.6 ± 40.4 mmol/L through the overnight fasting period (07:00 on Day 2). This study demonstrated that the glycogen content in the liver was significantly lower in the morning, while the glycogen content in the calf muscles underwent minimal diurnal variation. The overnight fast is a characteristic daily condition, in which liver glycogen content is low, whereas muscle glycogen content is relatively unaffected.

Restoration of Muscle Glycogen and Functional Capacity: Role of Post-Exercise Carbohydrate and Protein Co-Ingestion

The importance of post-exercise recovery nutrition has been well described in recent years, leading to its incorporation as an integral part of training regimes in both athletes and active individuals. Muscle glycogen depletion during an initial prolonged exercise bout is a main factor in the onset of fatigue and so the replenishment of glycogen stores may be important for recovery of functional capacity. Nevertheless, nutritional considerations for optimal short-term (3–6 h) recovery remain incompletely elucidated, particularly surrounding the precise amount of specific types of nutrients required. Current nutritional guidelines to maximise muscle glycogen availability within limited recovery are provided under the assumption that similar fatigue mechanisms (i.e., muscle glycogen depletion) are involved during a repeated exercise bout. Indeed, recent data support the notion that muscle glycogen availability is a determinant of subsequent endurance capacity following limited recovery. Thus, carbohydrate ingestion can be utilised to influence the restoration of endurance capacity following exhaustive exercise. One strategy with the potential to accelerate muscle glycogen resynthesis and/or functional capacity beyond merely ingesting adequate carbohydrate is the co-ingestion of added protein. While numerous studies have been instigated, a consensus that is related to the influence of carbohydrate-protein ingestion in maximising muscle glycogen during short-term recovery and repeated exercise capacity has not been established. When considered collectively, carbohydrate intake during limited recovery appears to primarily determine muscle glycogen resynthesis and repeated exercise capacity. Thus, when the goal is to optimise repeated exercise capacity following short-term recovery, ingesting carbohydrate at an amount of ≥1.2 g kg body mass⁻¹·h⁻¹ can maximise muscle glycogen repletion. The addition of protein to carbohydrate during post-exercise recovery may be beneficial under circumstances when carbohydrate ingestion is sub-optimal (≤0.8 g kg body mass⁻¹·h⁻¹) for effective restoration of muscle glycogen and repeated exercise capacity.

State-of-the-Art Methods for Skeletal Muscle Glycogen Analysis in Athletes-The Need for Novel Non-Invasive Techniques

Muscle glycogen levels have a profound impact on an athlete’s sporting performance, thus measurement is vital. Carbohydrate manipulation is a fundamental component in an athlete’s lifestyle and is a critical part of elite performance, since it can provide necessary training adaptations. This paper provides a critical review of the current invasive and non-invasive methods for measuring skeletal muscle glycogen levels. These include the gold standard muscle biopsy, histochemical analysis, magnetic resonance spectroscopy, and musculoskeletal high frequency ultrasound, as well as pursuing future application of electromagnetic sensors in the pursuit of portable non-invasive quantification of muscle glycogen. This paper will be of interest to researchers who wish to understand the current and most appropriate techniques in measuring skeletal muscle glycogen. This will have applications both in the lab and in the field by improving the accuracy of research protocols and following the physiological adaptations to exercise.

Increased liver fat and glycogen stores after consumption of high versus low glycaemic index food: A randomized crossover study

AIM

To investigate the acute and longer-term effects of low (LGI) versus high glycaemic index (HGI) diets on hepatic fat and glycogen accumulation and related blood measures in healthy volunteers.

METHODS

Eight healthy men (age 20.1 ± 0.4 years, body mass index 23.0 ± 0.9 kg/m² ) attended a test day before and after a 7-day macronutrient- and energy-matched HGI or LGI diet, followed by a minimum 4-week wash-out period, and then returned to repeat the intervention with the alternative diet. During test days, participants consumed either an HGI or an LGI test meal corresponding to their diet week, and liver fat [ ¹ H magnetic resonance spectroscopy (MRS)], glycogen ( ¹³ C MRS) and gastric content volume (MRI) were measured. Blood samples were obtained regularly throughout the test day to assess plasma glucose and insulin levels.

RESULTS

Plasma glucose and insulin peak values and area under the curve were significantly greater after the HGI test meal compared with the LGI test meal, as expected. Hepatic glycogen concentrations increased more after the HGI test meal ( P < .05) and peak levels were significantly greater after 7 days of HGI dietary intervention compared with those at the beginning of the intervention ( P < .05). Liver fat fractions increased significantly after the HGI dietary intervention compared with the LGI dietary intervention (two-way repeated-measures analysis of variance P ≤ .05).

CONCLUSIONS

Compared with an LGI diet, a 1-week HGI diet increased hepatic fat and glycogen stores. This may have important clinical relevance for dietary interventions in the prevention and management of non-alcoholic fatty liver disease.

Liver glycogen metabolism during and after prolonged endurance-type exercise

Carbohydrate and fat are the main substrates utilized during prolonged endurance-type exercise. The relative contribution of each is determined primarily by the intensity and duration of exercise, along with individual training and nutritional status. During moderate- to high-intensity exercise, carbohydrate represents the main substrate source. Because endogenous carbohydrate stores (primarily in liver and muscle) are relatively small, endurance-type exercise performance/capacity is often limited by endogenous carbohydrate availability. Much exercise metabolism research to date has focused on muscle glycogen utilization, with little attention paid to the contribution of liver glycogen. (13)C magnetic resonance spectroscopy permits direct, noninvasive measurements of liver glycogen content and has increased understanding of the relevance of liver glycogen during exercise. In contrast to muscle, endurance-trained athletes do not exhibit elevated basal liver glycogen concentrations. However, there is evidence that liver glycogenolysis may be lower in endurance-trained athletes compared with untrained controls during moderate- to high-intensity exercise. Therefore, liver glycogen sparing in an endurance-trained state may account partly for training-induced performance/capacity adaptations during prolonged (>90 min) exercise. Ingestion of carbohydrate at a relatively high rate (>1.5 g/min) can prevent liver glycogen depletion during moderate-intensity exercise independent of the type of carbohydrate (e.g., glucose vs. sucrose) ingested. To minimize gastrointestinal discomfort, it is recommended to ingest specific combinations or types of carbohydrates (glucose plus fructose and/or sucrose). By coingesting glucose with either galactose or fructose, postexercise liver glycogen repletion rates can be doubled. There are currently no guidelines for carbohydrate ingestion to maximize liver glycogen repletion.

Mr Spectroscopy in Clinical Research

MR spectroscopy (MRS) offers unique possibilities for non-invasive evaluation of biochemistry in vivo. During recent years there has been a growing body of evidence from clinical research studies on human beings using 31P and 1H MRS. The results indicate that it is possible to evaluate phosphorous energy metabolism, loss of neurones, and lactate production in a large number of brain diseases. Furthermore, 31P and 1H MRS may be particularly clinically useful in evaluation of various disorders in skeletal muscle. In the heart 31P MRS seems at the moment the most suitable for evaluation of global affections of the myocardium. In the liver 31P MRS appears to be rather insensitive and non-specific, but absolute quantification of metabolite concentrations and using metabolic “stress models” may prove useful in the future. The clinical role of MRS in oncology is still unclear, but it may be useful for noninvasive follow-up of treatment.

Taken together, the evidence obtained so far certainly shows some trends for clinical applications of MRS. Methods are now available for the clinical research necessary for establishing routine clinical MRS examinations.

Experimental strategies for in vivo13C NMR spectroscopy

In vivo carbon-13 (¹³C) MRS opens unique insights into the metabolism of intact organisms, and has led to major advancements in the understanding of cellular metabolism under normal and pathological conditions in various organs such as skeletal muscles, the heart, the liver and the brain. However, the technique comes at the expense of significant experimental difficulties. In this review we focus on the experimental aspects of non-hyperpolarized ¹³C MRS in vivo. Some of the enrichment strategies which have been proposed so far are described; the various MRS acquisition paradigms to measure ¹³C labeling are then presented. Finally, practical aspects of ¹³C spectral quantification are discussed.

Methodological and physiological test-retest reliability of (13) C-MRS glycogen measurements in liver and in skeletal muscle of patients with type 1 diabetes and matched healthy controls

Glycogen is a major substrate in energy metabolism and particularly important to prevent hypoglycemia in pathologies of glucose homeostasis such as type 1 diabetes mellitus (T1DM). (13) C-MRS is increasingly used to determine glycogen in skeletal muscle and liver non-invasively; however, the low signal-to-noise ratio leads to long acquisition times, particularly when glycogen levels are determined before and after interventions. In order to ease the requirements for the subjects and to avoid systematic effects of the lengthy examination, we evaluated if a standardized preparation period would allow us to shift the baseline (pre-intervention) experiments to a preceding day. Based on natural abundance (13) C-MRS on a clinical 3 T MR system the present study investigated the test-retest reliability of glycogen measurements in patients with T1DM and matched controls (n = 10 each group) in quadriceps muscle and liver. Prior to the MR examination, participants followed a standardized diet and avoided strenuous exercise for two days. The average coefficient of variation (CV) of myocellular glycogen levels was 9.7% in patients with T1DM compared with 6.6% in controls after a 2 week period, while hepatic glycogen variability was 13.3% in patients with T1DM and 14.6% in controls. For comparison, a single-session test-retest variability in four healthy volunteers resulted in 9.5% for skeletal muscle and 14.3% for liver. Glycogen levels in muscle and liver were not statistically different between test and retest, except for hepatic glycogen, which decreased in T1DM patients in the retest examination, but without an increase of the group distribution. Since the CVs of glycogen levels determined in a "single session" versus "within weeks" are comparable, we conclude that the major source of uncertainty is the methodological error and that physiological variations can be minimized by a pre-study standardization. For hepatic glycogen examinations, familiarization sessions (MR and potentially strenuous interventions) are recommended. Copyright © 2016 John Wiley & Sons, Ltd.

Metabolic and hormonal response to intermittent high-intensity and continuous moderate intensity exercise in individuals with type 1 diabetes: a randomised crossover study

AIMS/HYPOTHESIS

To investigate exercise-related fuel metabolism in intermittent high-intensity (IHE) and continuous moderate intensity (CONT) exercise in individuals with type 1 diabetes mellitus.

METHODS

In a prospective randomised open-label cross-over trial twelve male individuals with well-controlled type 1 diabetes underwent a 90 min iso-energetic cycling session at 50% maximal oxygen consumption ([Formula: see text]), with (IHE) or without (CONT) interspersed 10 s sprints every 10 min without insulin adaptation. Euglycaemia was maintained using oral (13)C-labelled glucose. (13)C Magnetic resonance spectroscopy (MRS) served to quantify hepatocellular and intramyocellular glycogen. Measurements of glucose kinetics (stable isotopes), hormones and metabolites complemented the investigation.

RESULTS

Glucose and insulin levels were comparable between interventions. Exogenous glucose requirements during the last 30 min of exercise were significantly lower in IHE (p = 0.02). Hepatic glucose output did not differ significantly between interventions, but glucose disposal was significantly lower in IHE (p < 0.05). There was no significant difference in glycogen consumption. Growth hormone, catecholamine and lactate levels were significantly higher in IHE (p < 0.05).

CONCLUSIONS/INTERPRETATION

IHE in individuals with type 1 diabetes without insulin adaptation reduced exogenous glucose requirements compared with CONT. The difference was not related to increased hepatic glucose output, nor to enhanced muscle glycogen utilisation, but to decreased glucose uptake. The lower glucose disposal in IHE implies a shift towards consumption of alternative substrates. These findings indicate a high flexibility of exercise-related fuel metabolism in type 1 diabetes, and point towards a novel and potentially beneficial role of IHE in these individuals.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02068638 FUNDING: Swiss National Science Foundation (grant number 320030_149321/) and R&A Scherbarth Foundation (Switzerland).

Sucrose ingestion after exhaustive exercise accelerates liver, but not muscle glycogen repletion compared with glucose ingestion in trained athletes

The purpose of this study was to assess the effects of sucrose vs. glucose ingestion on postexercise liver and muscle glycogen repletion. Fifteen well-trained male cyclists completed two test days. Each test day started with glycogen-depleting exercise, followed by 5 h of recovery, during which subjects ingested 1.5 g·kg(-1)·h(-1) sucrose or glucose. Blood was sampled frequently and (13)C magnetic resonance spectroscopy and imaging were employed 0, 120, and 300 min postexercise to determine liver and muscle glycogen concentrations and liver volume. Results were as follows: Postexercise muscle glycogen concentrations increased significantly from 85 ± 27 (SD) vs. 86 ± 35 mmol/l to 140 ± 23 vs. 136 ± 26 mmol/l following sucrose and glucose ingestion, respectively (no differences between treatments: P = 0.673). Postexercise liver glycogen concentrations increased significantly from 183 ± 47 vs. 167 ± 65 mmol/l to 280 ± 72 vs. 234 ± 81 mmol/l following sucrose and glucose ingestion, respectively (time × treatment, P = 0.051). Liver volume increased significantly over the 300-min period after sucrose ingestion only (time × treatment, P = 0.001). As a result, total liver glycogen content increased during postexercise recovery to a greater extent in the sucrose treatment (from 53.6 ± 16.2 to 86.8 ± 29.0 g) compared with the glucose treatment (49.3 ± 25.5 to 65.7 ± 27.1 g; time × treatment, P < 0.001), equating to a 3.4 g/h (95% confidence interval: 1.6-5.1 g/h) greater repletion rate with sucrose vs. glucose ingestion. In conclusion, sucrose ingestion (1.5 g·kg(-1)·h(-1)) further accelerates postexercise liver, but not muscle glycogen repletion compared with glucose ingestion in trained athletes.

¹³C MRS reveals a small diurnal variation in the glycogen content of human thigh muscle

There is marked diurnal variation in the glycogen content of skeletal muscles of animals, but few studies have addressed such variations in human muscles. (13)C MRS can be used to noninvasively measure the glycogen content of human skeletal muscle, but no study has explored the diurnal variations in this parameter. This study aimed to investigate whether a diurnal variation in glycogen content occurs in human muscles and, if so, to what extent it can be identified using (13)C MRS. Six male volunteers were instructed to maintain their normal diet and not to perform strenuous exercise for at least 3 days before and during the experiment. Muscle glycogen and blood glucose concentrations were measured six times in 24 h under normal conditions in these subjects. The glycogen content in the thigh muscle was determined noninvasively by natural abundance (13)C MRS using a clinical MR system at 3 T. Nutritional analysis revealed that the subjects' mean carbohydrate intake was 463 ± 137 g, being approximately 6.8 ± 2.4 g/kg body weight. The average sleeping time was 5.9 ± 1.0 h. The glycogen content in the thigh muscle at the starting point was 64.8 ± 20.6 mM. Although absolute and relative individual variations in muscle glycogen content were 7.0 ± 2.1 mM and 11.3 ± 4.6%, respectively, no significant difference in glycogen content was observed among the different time points. This study demonstrates that normal food intake (not fat and/or carbohydrate rich), sleep and other daily activities have a negligible influence on thigh muscle glycogen content, and that the diurnal variation of the glycogen content in human muscles is markedly smaller than that in animal muscles. Moreover, the present results also support the reproducibility and availability of (13)C MRS for the evaluation of the glycogen content in human muscles.

Reproducibility and absolute quantification of muscle glycogen in patients with glycogen storage disease by 13C NMR spectroscopy at 7 Tesla

Carbon-13 magnetic resonance spectroscopy (13C MRS) offers a noninvasive method to assess glycogen levels in skeletal muscle and to identify excess glycogen accumulation in patients with glycogen storage disease (GSD). Despite the clinical potential of the method, it is currently not widely used for diagnosis or for follow-up of treatment. While it is possible to perform acceptable 13C MRS at lower fields, the low natural abundance of 13C and the inherently low signal-to-noise ratio of 13C MRS makes it desirable to utilize the advantage of increased signal strength offered by ultra-high fields for more accurate measurements. Concomitant with this advantage, however, ultra-high fields present unique technical challenges that need to be addressed when studying glycogen. In particular, the question of measurement reproducibility needs to be answered so as to give investigators insight into meaningful inter-subject glycogen differences. We measured muscle glycogen levels in vivo in the calf muscle in three patients with McArdle disease (MD), one patient with phosphofructokinase deficiency (PFKD) and four healthy controls by performing 13C MRS at 7T. Absolute quantification of the MRS signal was achieved by using a reference phantom with known concentration of metabolites. Muscle glycogen concentration was increased in GSD patients (31.5±2.9 g/kg w. w.) compared with controls (12.4±2.2 g/kg w. w.). In three GSD patients glycogen was also determined biochemically in muscle homogenates from needle biopsies and showed a similar 2.5-fold increase in muscle glycogen concentration in GSD patients compared with controls. Repeated inter-subject glycogen measurements yield a coefficient of variability of 5.18%, while repeated phantom measurements yield a lower 3.2% system variability. We conclude that noninvasive ultra-high field 13C MRS provides a valuable, highly reproducible tool for quantitative assessment of glycogen levels in health and disease.

Validation of musculoskeletal ultrasound to assess and quantify muscle glycogen content. A novel approach

Glycogen storage is essential for exercise performance. The ability to assess muscle glycogen levels should be an important advantage for performance. However, skeletal muscle glycogen assessment has only been available and validated through muscle biopsy. We have developed a new methodology using high-frequency ultrasound to assess skeletal muscle glycogen content in a rapid, portable, and noninvasive way using MuscleSound (MuscleSound, LCC, Denver, CO) technology.

PURPOSE

To validate the utilization of high-frequency musculoskeletal ultrasound for muscle glycogen assessment and correlate it with histochemical glycogen quantification through muscle biopsy.

METHODS

Twenty-two male competitive cyclists (categories: Pro, 1-4; average height, 183.7 ± 4.9 cm; average weight, 76.8 ± 7.8 kg) performed a steady-state test on a cyclergometer for 90 minutes at a moderate to high exercise intensity, eliciting a carbohydrate oxidation of 2-3 g·min⁻¹ and a blood lactate concentration of 2 to 3 mM. Pre- and post-exercise glycogen content from rectus femoris muscle was measured using histochemical analysis through muscle biopsy and through high-frequency ultrasound scans using MuscleSound technology.

RESULTS

Correlations between muscle biopsy glycogen histochemical quantification (mmol·kg⁻¹) and high-frequency ultrasound methodology through MuscleSound technology were r = 0.93 (P < 0.0001) pre-exercise and r = 0.94 (P < 0.0001) post-exercise. The correlation between muscle biopsy glycogen quantification and high-frequency ultrasound methodology for the change in glycogen from pre- and post-exercise was r = 0.81 (P < 0.0001).

CONCLUSION

These results demonstrate that skeletal muscle glycogen can be measured quickly and noninvasively through high-frequency ultrasound using MuscleSound technology.

A low calorie morning meal prevents the decline of hepatic glycogen stores: a pilot in vivo13C magnetic resonance study

A low dose oral glucose challenge following an overnight fast inhibits further reduction in hepatic glycogen reserves without raising levels above baseline.

Variability in fasting lipid and glycogen contents in hepatic and skeletal muscle tissue in subjects with and without type 2 diabetes: a 1H and 13C MRS study

The measurement of tissue lipid and glycogen contents and the establishment of normal levels of variability are important when assessing changes caused by pathology or treatment. We measured hepatic and skeletal muscle lipid and glycogen levels using (1)H and (13)C MRS at 3 T in groups of subjects with and without type 2 diabetes. Within-visit reproducibility, due to repositioning and instrument errors was determined from repeat measurements made over 1 h. Natural variability was assessed from separate measurements made on three occasions over 1 month. Hepatic lipid content was greater in subjects with diabetes relative to healthy subjects (p = 0.03), whereas levels of hepatic and skeletal muscle glycogen, and of intra- and extra-myocellular lipid, were similar. The single-session reproducibility values (coefficient of variation, CV) for hepatic lipid content were 12% and 7% in groups of subjects with and without diabetes, respectively. The variability of hepatic lipid content over 1 month was greater than the reproducibility, with CV = 22% (p = 0.08) and CV = 44% (p = 0.004) in subjects with and without diabetes, respectively. Similarly, levels of variation in basal hepatic glycogen concentrations (subjects with diabetes, CV = 38%; healthy volunteers, CV = 35%) were significantly larger than single-session reproducibility values (CV = 17%, p = 0.02 and CV = 13%, p = 0.05, respectively), indicating substantial biological changes in basal concentrations over 1 month. There was a decreasing correlation in measurements of both hepatic lipid and glycogen content with increasing time between scans. Levels of variability in intra- and extra-myocellular lipid in the soleus muscle, and glycogen concentrations in the gastrocnemius muscle, tended to be larger than expected from single-session reproducibility, although these did not reach significance.

A dual-tuned transceive resonator for (13) C{(1) H} MRS: two open coils in one

Proton-decoupled, (13) C nuclear MRS experiments require a RF coil that operates at the Larmor frequencies of both (13) C and (1) H. In this work, we designed, built and tested a single-unit, dual-tuned coil based on a half-birdcage open coil design. It was constructed as a low-pass network with a resonant trap in series with each leg. Traps are tuned in alternate legs such that the two resonant modes arise from currents on alternate legs. The coil performance was compared with that of a dual-tuned coil consisting of two proton surface coils operating in quadrature and a single surface coil for (13) C transmission and reception. The half-birdcage coil was shown to produce a more homogeneous RF field at each frequency and was more sensitive to a (13) C signal arising from regions further from the coil surface. The applicability of the coil in vivo was demonstrated by acquiring a proton decoupled, natural abundance (13) C glycogen signal from the calf of a normal volunteer.

Banting Memorial lecture 2012: reversing the twin cycles of type 2 diabetes

It has become widely accepted that Type 2 diabetes is inevitably life-long, with irreversible and progressive beta cell damage. However, the restoration of normal glucose metabolism within days after bariatric surgery in the majority of people with Type 2 diabetes disproves this concept. There is now no doubt that this reversal of diabetes depends upon the sudden and profound decrease in food intake, and does not relate to any direct surgical effect. The Counterpoint study demonstrated that normal glucose levels and normal beta cell function could be restored by a very low calorie diet alone. Novel magnetic resonance methods were applied to measure intra-organ fat. The results showed two different time courses: a) resolution of hepatic insulin sensitivity within days along with a rapid fall in liver fat and normalisation of fasting glucose levels; and b) return of normal beta cell insulin secretion over weeks in step with a fall in pancreas fat. Now that it has been possible to observe the pathophysiological events during reversal of Type 2 diabetes, the reverse time course of events which determine the onset of the condition can be identified. The twin cycle hypothesis postulates that chronic calorie excess leads to accumulation of liver fat with eventual spill over into the pancreas. These self-reinforcing cycles between liver and pancreas eventually cause metabolic inhibition of insulin secretion after meals and onset of hyperglycaemia. It is now clear that Type 2 diabetes is a reversible condition of intra-organ fat excess to which some people are more susceptible than others.

Applications of multi-nuclear magnetic resonance spectroscopy at 7T

AIM: To discuss the advantages of ultra-high field (7T) for ¹H and ¹³C magnetic resonance spectroscopy (MRS) studies of metabolism.

METHODS: Measurements of brain metabolites were made at both 3 and 7T using ¹H MRS. Measurements of glycogen and lipids in muscle were measured using ¹³C and ¹H MRS respectively.

RESULTS: In the brain, increased signal-to-noise ratio (SNR) and dispersion allows spectral separation of the amino-acids glutamate, glutamine and γ-aminobutyric acid (GABA), without the need for sophisticated editing sequences. Improved quantification of these metabolites is demonstrated at 7T relative to 3T. SNR was 36% higher, and measurement repeatability (% coefficients of variation) was 4%, 10% and 10% at 7T, vs 8%, 29% and 21% at 3T for glutamate, glutamine and GABA respectively. Measurements at 7T were used to compare metabolite levels in the anterior cingulate cortex (ACC) and insula. Creatine and glutamate levels were found to be significantly higher in the insula compared to the ACC (P < 0.05). In muscle, the increased SNR and spectral resolution at 7T enables interleaved studies of glycogen (¹³C) and intra-myocellular lipid (IMCL) and extra-myocellular lipid (EMCL) (¹H) following exercise and re-feeding. Glycogen levels were significantly decreased following exercise (-28% at 50% VO₂ max; -58% at 75% VO₂ max). Interestingly, levels of glycogen in the hamstrings followed those in the quadriceps, despite reduce exercise loading. No changes in IMCL and EMCL were found in the study.

CONCLUSION: The demonstrated improvements in brain and muscle MRS measurements at 7T will increase the potential for use in investigating human metabolism and changes due to pathologies.

Lipid-induced insulin resistance: unravelling the mechanism

Insulin resistance has long been associated with obesity. More than 40 years ago, Randle and colleagues postulated that lipids impaired insulin-stimulated glucose use by muscles through inhibition of glycolysis at key points. However, work over the past two decades has shown that lipid-induced insulin resistance in skeletal muscle stems from defects in insulin-stimulated glucose transport activity. The steatotic liver is also resistant to insulin in terms of inhibition of hepatic glucose production and stimulation of glycogen synthesis. In muscle and liver, the intracellular accumulation of lipids-namely, diacylglycerol-triggers activation of novel protein kinases C with subsequent impairments in insulin signalling. This unifying hypothesis accounts for the mechanism of insulin resistance in obesity, type 2 diabetes, lipodystrophy, and ageing; and the insulin-sensitising effects of thiazolidinediones.

The second-meal phenomenon is associated with enhanced muscle glycogen storage in humans

The rise in blood glucose after lunch is less if breakfast has been eaten. The metabolic basis of this second-meal phenomenon remains uncertain. We hypothesized that storage of ingested glucose as glycogen could be responsible during the post-meal suppression of plasma NEFAs (non-esterified fatty acids; 'free' fatty acids). In the present study we determined the metabolic basis of the second-meal phenomenon. Healthy subjects were studied on two separate days, with breakfast and without breakfast in a random order. We studied metabolic changes after a standardized test lunch labelled with 3 g of 13C-labelled (99%) glucose. Changes in post-prandial muscle glycogen storage were measured using 13C magnetic resonance spectroscopy. The rise in plasma glucose after lunch was significantly less if breakfast had been taken (0.9+/-0.3 compared with 3.2+/-0.3 mmol/l, with and without breakfast respectively; P<0.001), despite comparable insulin responses. Pre-lunch NEFAs were suppressed after breakfast (0.13+/-0.03 compared with 0.51+/-0.04 mmol/l) and levels correlated positively with the maximum glucose rise after lunch (r=0.62, P=0.001). The increase in muscle glycogen signal was greater 5 h after lunch on the breakfast day (103+/-21 compared with 48+/-12 units; P<0.007) and correlated negatively with plasma NEFA concentrations before lunch (r=-0.48, P<0.05). The second-meal effect is associated with priming of muscle glycogen synthesis consequent upon sustained suppression of plasma NEFA concentrations.

Role of fatty acids in the development of insulin resistance and type 2 diabetes mellitus

Insulin resistance is defined as the reduced responsiveness to normal circulating levels of insulin. It is the basic condition of type 2 diabetes mellitus, in which both experimental animals and humans accumulate lipids intracellularly in skeletal muscle and liver. Measurement of these lipids in humans, using nuclear magnetic resonance spectroscopy after lipid infusion, indicated they could cause inhibition of the glucose transporter GLUT4, thereby suppressing glucose entry into cells and inhibiting glucose oxidation and glycogen synthesis in muscle. Furthermore, it is known that the enzyme acetyl-CoA carboxylase2 (ACC2) suppresses the oxidation of fatty acids by inhibiting the entry of fatty acids into mitochondria. Further support for the lipocentric hypothesis of the pathogenesis of insulin resistance was provided by knocking out the gene coding for ACC2 in mice; this led to greater fatty acid oxidation, reduced fat mass and, in consequence, greatly enhanced insulin sensitivity. These studies suggest that a specific inhibitor of ACC2 would have therapeutic potential for type 2 diabetes mellitus.

Assessment of the mechanical properties of the musculoskeletal system using 2-D and 3-D very high frame rate ultrasound

One of the great challenges for understanding muscular diseases is to assess noninvasively the active and passive mechanical properties of the musculoskeletal system. In this paper we report the use of ultrafast ultrasound imaging to explore with a submillimeter resolution the behavior of the contracting tissues in vivo (biceps brachii). To image the contraction, which is a very brief phenomenon (100 ms), a recently designed ultrasound scanner prototype able to take up to 6000 frames/s was used. A very high frame rate from 1000 to 2500 frames/s was used to image the cross section plane of the muscle (transverse to fibers) enabling us to catch in real time the muscle contraction during a transient electrostimulation. Tissue velocities were obtained from radiofrequency based speckle tracking techniques and their profiles are discussed with respect to electrostimulation intensities and pulse repetition frequencies for different volunteers. Three-dimensional (3-D) very high frame rate movies were also acquired by repeating the experiment for different acquisition planes while triggering the imaging system with the electrostimulation device. The reconstructed 3-D velocity field allows the full localization of the contracting fibers bundle. This ultrasound technique, referred to as echo mechanomyography, offers new perspectives for in vivo and in situ noninvasive muscle diagnosis of an active contractile tissue.

Absolute quantification of carnosine in human calf muscle by proton magnetic resonance spectroscopy

Carnosine has been shown to be present in the skeletal muscle and in the brain of a variety of animals and humans. Despite the various physiological functions assigned to this metabolite, its exact role remains unclear. It has been suggested that carnosine plays a role in buffering in the intracellular physiological pHi range in skeletal muscle as a result of accepting hydrogen ions released in the development of fatigue during intensive exercise. It is thus postulated that the concentration of carnosine is an indicator for the extent of the buffering capacity. However, the determination of the concentration of this metabolite has only been performed by means of muscle biopsy, which is an invasive procedure. In this paper, we utilized proton magnetic resonance spectroscopy (1H MRS) in order to perform absolute quantification of carnosine in vivo non-invasively. The method was verified by phantom experiments and in vivo measurements in the calf muscles of athletes and untrained volunteers. The measured mean concentrations in the soleus and the gastrocnemius muscles were found to be 2.81 +/- 0.57/4.8 +/- 1.59 mM (mean +/- SD) for athletes and 2.58 +/- 0.65/3.3 +/- 0.32 mM for untrained volunteers, respectively. These values are in agreement with previously reported biopsy-based results. Our results suggest that 1H MRS can provide an alternative method for non-invasively determining carnosine concentration in human calf muscle in vivo.

Use of imaging to assess normal and adaptive muscle function

Physical therapists must be able to determine the activity and passive properties of the musculoskeletal system in order to accurately plan and evaluate therapeutic measures. Discussed in this article are imaging methods that not only allow for the measurement of muscle activity but also allow for the measurement of cellular processes and passive mechanical properties noninvasively and in vivo. The techniques reviewed are T1- and T2-weighted magnetic resonance (MR) imaging, MR spectroscopy, cine-phase-contrast MR imaging, MR elastography, and ultrasonography. At present, many of these approaches are expensive and not readily available in physical therapy clinics but can be found at medical centers. However, there are ways of using these techniques to provide important knowledge about muscle function. This article proposes creative ways in which to use these techniques as evaluative tools.

Musculoskeletal spectroscopy

Magnetic resonance spectroscopy (MRS) of skeletal muscle has been successfully applied by physiologists over several decades, particularly for studies of high-energy phosphates (by (31)P-MRS) and glycogen (by (13)C-MRS). Unfortunately, the observation of these heteronuclei requires equipment that is typically not available on clinical MR scanners, such as broadband capability and a second channel for decoupling and nuclear Overhauser enhancement (NOE). On the other hand, (1)H-MR spectra of skeletal muscle can be acquired on many routine MR systems and also provide a wealth of physiological information. In particular, studies of intramyocellular lipids (IMCL) attract physiologists and endocrinologists because IMCL levels are related to insulin resistance and thus can lead to a better understanding of major health problems in industrial countries. The combination of (1)H-, (13)C-, and (31)P-MRS gives access to the major long- and short-term energy sources of skeletal muscle. This review summarizes the technical aspects and unique MR-methodological features of the different nuclei. It reviews clinical studies that employed MRS of one or more nuclei, or combinations of MRS with other MR modalities. It also illustrates that MR spectra contain additional physiological information that is not yet used in routine clinical applications.

MRI detection of glycogen in vivo by using chemical exchange saturation transfer imaging (glycoCEST)

Detection of glycogen in vivo would have utility in the study of normal physiology and many disorders. Presently, the only magnetic resonance (MR) method available to study glycogen metabolism in vivo is (13)C MR spectroscopy, but this technology is not routinely available on standard clinical scanners. Here, we show that glycogen can be detected indirectly through the water signal by using selective radio frequency (RF) saturation of the hydroxyl protons in the 0.5- to 1.5-ppm frequency range downfield from water. The resulting saturated spins are rapidly transferred to water protons via chemical exchange, leading to partial saturation of the water signal, a process now known as chemical exchange saturation transfer. This effect is demonstrated in glycogen phantoms at magnetic field strengths of 4.7 and 9.4 T, showing improved detection at higher field in adherence with MR exchange theory. Difference images obtained during RF irradiation at 1.0 ppm upfield and downfield of the water signal showed that glycogen metabolism could be followed in isolated, perfused mouse livers at 4.7 T before and after administration of glucagon. Glycogen breakdown was confirmed by measuring effluent glucose and, in separate experiments, by (13)C NMR spectroscopy. This approach opens the way to image the distribution of tissue glycogen in vivo and to monitor its metabolism rapidly and noninvasively with MRI.

Dynamic MRS and MRI of skeletal muscle function and biomechanics

MR is a powerful technique for studying the biomechanical and functional properties of skeletal muscle in vivo in health and disease. This review focuses on 31P, 1H and 13C MR spectroscopy for assessment of the dynamics of muscle metabolism and on dynamic 1H MRI methods for non-invasive measurement of the biomechanical and functional properties of skeletal muscle. The information thus obtained ranges from the microscopic level of the metabolism of the myocyte to the macroscopic level of the contractile function of muscle complexes. The MR technology presented plays a vital role in achieving a better understanding of many basic aspects of muscle function, including the regulation of mitochondrial activity and the intricate interplay between muscle fiber organization and contractile function. In addition, these tools are increasingly being employed to establish novel diagnostic procedures as well as to monitor the effects of therapeutic and lifestyle interventions for muscle disorders that have an increasing impact in modern society.

Glycogen turnover forms lactate during exercise

The power needs of muscle are supplied by rapid anaerobic glycogenolytic ATP generation. Lactate is built up by the mismatch between steady state energy needs and short-term power demands and the increased concentration drives the lactate shuttle. In this model, contributions to fatigue should be looked for in the flux of glucose through glycogen rather than in the concentrations of fuel.

Glycogen synthesis in human gastrocnemius muscle is not representative of whole-body muscle glycogen synthesis

The introduction of 13C magnetic resonance spectroscopy (MRS) has enabled noninvasive measurement of muscle glycogen synthesis in humans. Conclusions based on measurements by the MRS technique assume that glucose metabolism in gastrocnemius muscle is representative for all skeletal muscles and thus can be extrapolated to whole-body muscle glucose metabolism. An alternative method to assess whole-body muscle glycogen synthesis is the use of [3-(3)H]glucose. In the present study, we compared this method to the MRS technique, which is a well-validated technique for measuring muscle glycogen synthesis. Muscle glycogen synthesis was measured in the gastrocnemius muscle of six lean healthy subjects by MRS and by the isotope method during a hyperinsulinemic-euglycemic clamp. Mean muscle glycogen synthesis as measured by the isotope method was 115 +/- 26 micromol x kg(-1) muscle x min(-1) vs. 178 +/- 72 micromol x kg(-1) muscle x min(-1) (P = 0.03) measured by MRS. Glycogen synthesis rates measured by MRS exceeded 100% of glucose uptake in three of the six subjects. We conclude that glycogen synthesis rates measured in gastrocnemius muscle cannot be extrapolated to whole-body muscle glycogen synthesis.

Effects of depletion exercise and light training on muscle glycogen supercompensation in men

Supercompensated muscle glycogen can be achieved by using several carbohydrate (CHO)-loading protocols. This study compared the effectiveness of two "modified" CHO-loading protocols. Additionally, we determined the effect of light cycle training on muscle glycogen. Subjects completed a depletion (D, n = 15) or nondepletion (ND, n = 10) CHO-loading protocol. After a 2-day adaptation period in a metabolic ward, the D group performed a 120-min cycle exercise at 65% peak oxygen uptake (VO2 peak) followed by 1-min sprints at 120% VO2 peak to exhaustion. The ND group performed only 20-min cycle exercise at 65% VO2 peak. For the next 6 days, both groups ate the same high-CHO diets and performed 20-min daily cycle exercise at 65% VO2 peak followed by a CHO beverage (105 g of CHO). Muscle glycogen concentrations of the vastus lateralis were measured daily with 13C magnetic resonance spectroscopy. On the morning of day 5, muscle glycogen concentrations had increased 1.45 (D) and 1.24 (ND) times baseline (P < 0.001) but did not differ significantly between groups. However, on day 7, muscle glycogen of the D group was significantly greater (p < 0.01) than that of the ND group (130 +/- 7 vs. 104 +/- 5 mmol/l). Daily cycle exercise decreased muscle glycogen by 10 +/- 2 (D) and 14 +/- 5 mmol/l (ND), but muscle glycogen was equal to or greater than preexercise values 24 h later. In conclusion, a CHO-loading protocol that begins with a glycogen-depleting exercise results in significantly greater muscle glycogen that persists longer than a CHO-loading protocol using only an exercise taper. Daily exercise at 65% VO2 peak for 20 min can be performed throughout the CHO-loading protocol without negatively affecting muscle glycogen supercompensation.

Evaluation of muscle glycogen content by 13C NMR spectroscopy in adult-onset acid maltase deficiency

Muscle glycogen storage was measured by in vivo, natural abundance 13C nuclear magnetic resonance spectroscopy in distal and proximal lower limb segments of patients suffering from adult-onset acid maltase deficiency. Interleaved T1-weighted acquisitions of glycogen and creatine served to quantify glycogen excess. For acid maltase deficient patients (n=11), glycogen:creatine was higher than controls (n=12), (1.20+/-0.39 vs. 0.83+/-0.18, P=0.0005). Glycogen storage was above the normal 95% confidence limits in at least one site for 7/11 patients. The intra-individual coefficient of reproducibility was 12%. This totally atraumatic measurement of glycogen allows repeated measurement at different muscle sites of acid maltase deficient patients, despite selective fatty replacement of tissue. This could provide an additional parameter to follow the development of disease in individual patients, including in the perspective of forthcoming therapeutic trials. It may also offer an appropriate tool to study the role of glycogen accumulation in progression of the pathology.

Smoking impairs muscle recovery from exercise

Cigarette smoking is a leading cause of many adverse health consequences. Chronic nicotine exposure leads to insulin resistance and may increase the risk of developing non-insulin-dependent diabetes mellitus in young otherwise healthy smokers. To evaluate smoking-induced effects on carbohydrate metabolism, we studied muscle glycogen recovery from exercise in a young healthy population of smokers. The study used 31P-13C NMR spectroscopy to compare muscle glycogen and glucose 6-phosphate levels during recovery in exercised gastrocnemius muscles of randomized cohorts of healthy male smokers (S) and controls (C). Data for the two groups were as follows: S, > or =20 cigarettes/day (n = 8), 24 +/- 2 yr, 173 +/- 3 cm, 70 +/- 4 kg and age- and weight-matched nonsmoking C (n = 10), 23 +/- 1 yr, 175 +/- 3 cm, 67 +/- 3 kg. Subjects performed single-leg toe raises to deplete glycogen to approximately 20 mmol/l, and glycogen resynthesis was measured during the first 4 h of recovery. Plasma samples were assayed for glucose and insulin at rest and during recovery. Test subjects were recruited from the general community surrounding Yale University. Glycogen was depleted to similar levels in the two groups [23.5 +/- 1.2 (S) and 19.1 +/- 1.3 (C) mmol/l]. During the 1st h of recovery, glycogen synthesis rates were similar [13.8 +/- 1.1 (S) and 15.3 +/- 1.3 (C) mmol x l-1 x h-1]. Between hours 1 and 4, glycogen synthesis was impaired in smokers [0.8 +/- 0.2 (S) and 4.5 +/- 0.5 (C) mmol x l-1 x h-1, P = 0.0002] compared with controls. Glucose 6-phosphate was reduced in smokers during hours 1-4 [0.105 +/- 0.006 (S) and 0.217 +/- 0.019 (C) mmol/l, P = 0.0212]. We conclude that cigarette smoking impairs the insulin-dependent portion of muscle recovery from glycogen-depleting exercise. This impairment likely results from a reduction in glucose uptake.

Direct assessment of muscle glycogen storage after mixed meals in normal and type 2 diabetic subjects

To understand the day-to-day pathophysiology of impaired muscle glycogen storage in type 2 diabetes, glycogen concentrations were measured before and after the consumption of sequential mixed meals (breakfast: 190.5 g carbohydrate, 41.0 g fat, 28.8 g protein, 1253 kcal; lunch: 203.3 g carbohydrate, 48.1 g fat, 44.0 g protein, 1497.5 kcal) by use of natural abundance (13)C magnetic resonance spectroscopy. Subjects with diet-controlled type 2 diabetes (n = 9) and age- and body mass index-matched nondiabetic controls (n = 9) were studied. Mean fasting gastrocnemius glycogen concentration was significantly lower in the diabetic group (57.1 +/- 3.6 vs. 68.9 +/- 4.1 mmol/l; P < 0.05). After the first meal, mean glycogen concentration in the control group rose significantly from basal (97.1 +/- 7.0 mmol/l at 240 min; P = 0.005). After the second meal, the high level of muscle glycogen concentration in the control group was maintained, with a further rise to 108.0 +/- 11.6 mmol/l by 480 min. In the diabetic group, the postprandial rise was markedly lower than that of the control group (65.9 +/- 5.2 mmol/l at 240 min, P < 0.005, and 70.8 +/- 6.7 mmol/l at 480 min, P = 0.01) despite considerably greater serum insulin levels (752.0 +/- 109.0 vs. 372.3 +/- 78.2 pmol/l at 300 min, P = 0.013). This was associated with a significantly greater postprandial hyperglycemia (10.8 +/- 1.3 vs. 5.3 +/- 0.2 mmol/l at 240 min, P < 0.005). Basal muscle glycogen concentration correlated inversely with fasting blood glucose (r = -0.55, P < 0.02) and fasting serum insulin (r = -0.57, P < 0.02). The increment in muscle glycogen correlated with initial increment in serum insulin only in the control group (r = 0.87, P < 0.002). This study quantitates for the first time the subnormal basal muscle glycogen concentration and the inadequate glycogen storage after meals in type 2 diabetes.

13C NMR of intermediary metabolism: implications for systemic physiology

The study of intermediary metabolism in biomolecules has been given new directions by recent experiments in human muscle and brain by 13C NMR. Labeled substrates, generally glucose, have enabled the fluxes to be determined in vivo, whereas the naturally abundant 13C has enabled concentrations to be measured. In muscle the glycogen synthesis pathway has been measured and the flux control determined by metabolic control analysis of data, which shows that this pathway is mainly responsible for insulin-stimulated glucose disposal and that a deficiency in the glucose transporter in the pathway is responsible for hyperglycemia in non-insulin-dependent diabetics. From a physiological point of view the most surprising result was that the heavily regulated allosteric enzyme, glycogen synthase, does not control flux but is needed to maintain homeostasis during flux changes. This novel role for a phosphorylated allosteric enzyme is proposed to be a general phenomenon in metabolic and signaling pathways, which physiologically link different cellular activities. In human and rat brains 13C NMR measurements of the flow of labeled glucose into glutamate and glutamine simultaneously determine the rate of glucose oxidation and glutamate neurotransmitter cycling and reveal a 1:1 stoichiometry between the two fluxes. Implications for the interpretation of functional imaging studies and for psychology are discussed. These results demonstrate how intermediary metabolism serves to connect biochemistry with systemic physiology when measured and analyzed by in vivo NMR methods.

Functional and metabolic early changes in calf muscle occurring during nutritional repletion in malnourished elderly patients

BACKGROUND

Metabolic alterations in skeletal muscle associated with malnutrition and the potential reversibility of such alterations during refeeding are not fully understood.

OBJECTIVE

We characterized early changes in muscle during refeeding in malnourished, hospitalized elderly subjects.

DESIGN

Muscle function, metabolism, and mass were evaluated in 24 clinically stable patients (11 were malnourished) by using isokinetic plantar flexor torque measurements and nuclear magnetic resonance (NMR) imaging for medial gastrocnemius mass assessment and 31P and 13C NMR spectroscopy for inorganic phosphate (Pi), phosphocreatine, and glycogen quantitation.

RESULTS

Malnourished subjects had lower muscle mass (P < 0.02) and tended to have lower strength than did control subjects. In malnourished subjects, muscle strength increased after refeeding (P < 0.01) whereas muscle mass was unchanged. The ratio of Pi to ATP was lower in malnourished than in control subjects (P < 0.001) and increased during refeeding (P < 0.01). The mean ratio of phosphocreatine to ATP was lower in malnourished than in control subjects (P < 0.01) and increased to control values after refeeding. Muscle glycogen showed a scattered distribution for malnourished subjects; the mean value did not differ significantly from that of control subjects, either at baseline or after refeeding.

CONCLUSIONS

The lower ratio of phosphocreatine to ATP in malnourished subjects could have resulted from either lower total muscle creatine or reduced oxidative capacities. High or normal glycogen associated with a low Pi-to-ATP ratio in malnourished subjects suggested preferential use of lipid over carbohydrate for energy supply, which is known to reduce muscle performance. The data suggest that normalization of muscle metabolite content after refeeding improves muscle strength in malnourished subjects.

The "glycogen shunt" in exercising muscle: A role for glycogen in muscle energetics and fatigue

Stimulated by recent (13)C and (31)P NMR studies of exercising muscle, we propose a model of the energetics of contraction. Previous studies of energetics have followed energy consumption. However, the rapidity of contraction, in 10-40 msec, requires that energy be delivered rapidly, so that the muscle has power requirements of rapid energy expenditure that are ultimately met by the slower averaged consumption of carbon and oxygen from blood. We propose that energy is supplied in milliseconds by glycogenolysis and that between contractions, glycogenesis refills the pools. The energy for glycogenesis is supplied by oxidative phosphorylation. This mechanism utilizes the rapid conversion of glycogen phosphorylase, the "fight-or-flight" enzyme, to its active form. Lactate is necessarily generated by this pathway to serve as a time buffer between fast and slow energy needs, which resolves the paradoxical generation of lactate in well oxygenated tissue. Consequences of the glycogen shunt are compatible with numerous biochemical and physiological experiments. The model provides a possible mechanism for muscle fatigue, suggesting that at low but nonzero glycogen concentrations, there is not enough glycogen to supply millisecond energy needs.

Muscle glycogen recovery after exercise measured by 13C-magnetic resonance spectroscopy in humans: effect of nutritional solutions

The rate of glycogen resynthesis in human skeletal muscle after glycogen-depleting exercise is known to depend on carbohydrate intake and is reported to reach a plateau after an adequate amount of carbohydrate (CHO) consumption. Efforts to maximize the rate of glycogen storage by changing the type and form of CHO, as well as by adding proteins or lipids have yielded inconsistent results. The objective of this study was to assess whether isocaloric addition of proteins and arginine to a CHO diet in the first 4 h after an endurance exercise would increase the rate of glycogen synthesis. The CHO solution, given twice at a 2 h interval according to earlier optimized protocols, contained 1.7 g CHO/kg(body weieght) The effects of this solution were compared to those of an isocaloric solution containing 1.2 g CHO/kg(body weight) plus 0.5 g protein/kg(body weight) (including 5 g arginine). Glycogen was measured in quadriceps muscle in vivo with natural abundance 13C-magnetic resonance spectroscopy before exercise and twice after exercise, before and at the end of a 4-h period following the intake of one of the solutions. Eight subjects took part in a randomized cross-over trial separated by at least 1 week. Glycogen synthesis was found to be significantly increased with both regimes compared to a zero-caloric placebo diet, but no significant difference in glycogen resynthesis was found between the CHO-only diet and the one supplemented by proteins and arginine. It is estimated that significance would have been reached for an increase of 34%, while the effectively measured synthesis rates only differed by 5%.