Fasting for 72 h increases intramyocellular lipid content in nondiabetic, physically fit men

S. R. Stannard1, M. W. Thompson1, K. Fairbairn2, B. Huard3, T. Sachinwalla3, and C. H. Thompson2

1 School of Exercise and Sport Science, Faculty of Health Sciences, The University of Sydney, Lidcombe 1825; 2 Human Nutrition Unit, Department of Biochemistry, Faculty of Science, The University of Sydney 2006; 3 Rayscan Imaging, Liverpool 2170, New South Wales, Australia


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The purpose of this study was to determine changes in intramyocellular lipid (IMCL) content in the vastus lateralis of nondiabetic, physically fit males over 72 h of fasting. Six men, mean age 35 yr (range 23-55 yr), body mass index 23.7 kg/m2 (21.2-27.4 kg/m2), undertook a water-only fast for 84 h. Vastus lateralis IMCL content was determined using proton magnetic resonance spectroscopy after 12 and 84 h of fasting. Venous blood was sampled at 12-h intervals throughout the fast. IMCL-(CH2)n/water and IMCL-(CH2)n/total creatine ratios increased from 0.00623 ± 0.00065 to 0.0142 ± 0.0015 (P = 0.002) and 6.82 ± 0.87 to 14.96 ± 1.73 (P = 0.001), respectively. Plasma free fatty acid (FFA), serum triglyceride, and whole blood 3-hydroxybutyrate concentrations increased (P < 0.001, <0.05, <0.03, respectively), whereas plasma glucose and serum insulin concentrations decreased (both P < 0.001) during fasting. In conclusion, 72-h water-only fasting produces a large increase in plasma FFA concentration, a drop in serum insulin concentration, and accumulation of IMCL in the vastus lateralis muscle of nondiabetic, physically fit men.

muscle triglyceride; free fatty acids; insulin


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

TYPE 2 DIABETIC PATIENTS show an increase in resting intramyocellular lipid (IMCL) content (14). Correspondingly, insulin sensitivity alone in nondiabetic humans appears to be well predicted by IMCL content, as determined from muscle biopsy samples (24, 31) or proton magnetic resonance spectroscopy (1H-MRS) (23). It is not clear whether the increase in IMCL is a cause or an effect of insulin resistance, although it has been recently suggested that a very high content of IMCL creates a physical barrier to GLUT4 translocation (21).

In normal-weight nondiabetics, fasting plasma free fatty acid (FFA) concentration is correlated inversely with whole body insulin-stimulated glucose uptake (23) and positively with IMCL content (8). Indeed, an acute increase in plasma FFA concentration is associated with reduced insulin sensitivity and increased IMCL content (3, 4), whereas an acute reduction in FFA concentration improves insulin sensitivity (36).

Three to four days of fasting results in elevated plasma FFA concentration (15, 47) and impaired glucose uptake (25, 47). However, the glucose-"sparing" effect in this condition is understood to be mediated by the increase in circulating ketones, whose metabolism inhibits hexokinase via operation of the glucose-fatty acid cycle (32).

The effect of fasting on IMCL content is equivocal in animals (17) and unknown in humans. However, in 60-h-fasted nonobese men, whole body FFA esterification appears in excess of splanchnic FFA uptake, esterification, and release as triglyceride (20), suggesting an unidentified nonhepatic site of triglyceride accumulation, possibly muscle. Furthermore, 3-hydroxybutyrate infusion into the brachial artery arrests the release of glycerol and FFAs from deep forearm veins (49), suggesting cessation of intramuscular lipolysis. Arterial concentrations of 3-hydroxybutyrate substantially increase during fasting (30). Thus it is conceivable that the reduced insulin-mediated glucose uptake during fasting may be mediated, at least in part, by an increase in IMCL content.

It was, therefore, the objective of the present study to test the hypothesis that a prolonged period of fasting (72 h) would result in increased blood concentrations of FFAs and 3-hydroxybutyrate and an associated increase in IMCL. 1H-MRS was employed to determine changes in IMCL content, as this noninvasive procedure has been shown to be a valid method for determining IMCL content (5, 18, 39).


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Subjects. Six healthy, physically active men volunteered to participate in the study, which was approved by The University of Sydney Human Ethics Committee. Subject characteristics are presented in Table 1. None had any history of diabetes, and their overnight-fasted plasma glucose concentrations averaged 4.7 mmol/l (range 4.1-5.5 mmol/l). All subjects had extensive experience as subjects in human physiological studies in the laboratory.

                              
View this table:
[in this window]
[in a new window]
 
Table 1.   Summary of subjects' physical characteristics

Study design. Subjects arrived at the laboratory on a Wednesday morning after an overnight (10-12 h) fast. Immediately after venous blood collection, 1H-MRS examination of the vastus lateralis muscle was conducted. No food or drink other than water was ingested until the completion of the fast the following Saturday morning. Venous blood (8 ml) was sampled every 12 h (7 AM and 7 PM) each day from the Wednesday morning until the fast was broken after a second MRS examination on Saturday morning. During the course of the fast, subjects were encouraged to participate in their normal daily routine but limit their exercise to walking required for essential activities. Twenty-four hour per day observer surveillance ensured compliance of subjects with the study protocol (frequent water ingestion only, with minimal physical activity). In total, the fasting period between the two MRS scans and first and last blood samples was 72 h and the total length of the fast ~84 h.

NMR spectroscopy. Image-guided, 1H-localized MRS and high-resolution T1-weighted imaging were performed on a 1.5 Tesla Gyroscan NT whole body system (Philips Medical Systems, Best, The Netherlands) by use of a combination of whole body and circular polarized standard extremity coil for radio frequency transmitting and signal receiving. Volumes of interest were centered within the vastus lateralis muscle at the level of midfemur, 3 cm medial to the axillary line. This position was chosen to avoid the vasculature and subcutaneous adipose depots and to ensure consistent orientation of the muscle fibers along the main magnetic field. Legs were fixed at the ankles, and subjects were instructed to lie as still as possible to prevent movement artifacts. A vitamin E capsule was taped to the skin to identify the area of interest on the MR images. During the initial scan, the leg was marked with indelible ink, and the mark was maintained until the second scan to ensure accurate repositioning of the 1H-MRS voxel. Image-guided spectra were acquired using the point resolved spectroscopy technique [repetition time (TR) = 5,000 ms, echo time (TE) = 32 ms, 32 measurements, 1,024 sample points, acquisition time 3 min]. Fully automated shimming was carried out on the voxel (5 × 1.5 × 1.5 cm) to ensure maximum field homogeneity, and excitation water suppression was used to suppress signal from water during data acquisition. The long repetition time was chosen to ensure a fully relaxed signal, minimizing T1 saturation effects. Unsuppressed water spectra were also acquired for use as an internal standard. Creatine was also used as a standard for quantitation, with signal measurements taken from the water-suppressed spectrum.

Spectral data processing. Spectral data were transferred offline for postprocessing with magnetic resonance user interface (MRUI) software [jMRUI version 1.1, EU Project "Advanced Signal Processing for Medical Magnetic Resonance Imaging and Spectroscopy," TMR, FMRX-CT97-0160 (27)]. After Fourier transformation and manual phasing of the spectra, the water peak was identified and nominated 4.7 ppm.

For the water-suppressed signal, the following steps were taken. 1) Residual water and any signals from 3.4 to 7.5 ppm were suppressed in the time domain with an HLSVD (Hankel Lanczos Singular Values Decomposition) filter; 2) an eight-resonance model was used, including carnitine (trimethyl: 3.2 ppm), total creatine (methyl: 3.0 ppm), lipid (C2 methylene: 2.25 ppm), lipid (allylic methylene: 2.15 ppm), extramyocellular lipid (EMCL) [(CH2)n: 1.45 ppm] and IMCL [(CH2)n: 1.35 ppm], and EMCL (CH3: 1.15) and IMCL (CH3: 0.85 ppm); and 3) the signal amplitude was obtained in absolute units for each resonance by using AMARES, a nonlinear least squares quantitation algorithm. This "prior knowledge" that was applied incorporated known factors from prior publications (33) with the addition of soft constraints on carnitine and creatine resonance frequencies and line widths. Fitting of lipid resonances at 2.15 and 2.25 ppm also allowed more reliable fitting of EMCL and IMCL peaks. The resonances were fitted assuming a Lorentzian line shape for carnitine and creatine and a Gaussian line shape for all other resonances. The zero-order phase correction was manually estimated, and the first-order phase correction was fixed at zero.

The unsuppressed water signal was calculated as follows. 1) The water resonance was identified at center frequency and nominated at 4.7 ppm. 2) The signal amplitude was obtained in absolute units with AMARES. The resonance was fitted to a Lorentzian line shape. As for the other resonances in the water-suppressed signal, zero-order phase correction was manually estimated with first-order correction fixed at zero.

T2 relaxation times were measured in a subset of the subjects during a separate but identical 84-h fast. This was done to ensure 1) that there was no difference in T2 relaxation times of the metabolites considered under 12- and 84-h fast conditions and 2) to allow for T2 relaxation effect correction in quantitation under the study conditions. T2 relaxation times were measured for unsuppressed water, total creatine-CH3, EMCL-(CH2)n, and IMCL-(CH2)n resonances by use of the same positioning and volume of the region of interest (TR 1,500 ms, TE 40 ms). Ten measurements were made at 40, 65, 90, 120, 150, 175, 205, 235, 265, and 295 ms. Rates of signal decay were plotted as ln amplitude vs. time in milliseconds, and the linear best line of fit was used to estimate the rate of signal decay.

Seventy-two-hour fasting induced no apparent changes in T2 (Table 2). Furthermore, the measured values are similar to those previously reported (37, 39).

                              
View this table:
[in this window]
[in a new window]
 
Table 2.   T2 relaxation time of various resonances in muscle

Absolute quantification of metabolites. Signal amplitude of water, carnitine-N(CH3)3, creatine-CH3, EMCL-(CH2)n, and IMCL-(CH2)n were corrected using appropriate T1 relaxation times available in the literature for leg skeletal muscle [H2O, 1,100 ms (37); IMCL-(CH2)n, 300 ms; EMCL-(CH2)n, 300 ms; creatine-CH3, 1,350 ms (10)] and the T2 relaxation times obtained under study conditions. The (CH2)n resonance was used for measurement of the intramuscular triglyceride content, given the higher signal intensity and narrower line width compared with the CH3 resonance (39). IMCL-(CH2)n values were then expressed as the ratio of both IMCL-(CH2)n to water and IMCL-(CH2)n to creatine. Absolute concentrations of IMCL and total creatine were also calculated in millimoles per kilogram wet weight by use of the internal reference of muscle water concentration. Total creatine was calculated as follows (7): 1) total creatine CH3 signal, measured at 3.00 ppm resonance, and unsuppressed water resonance, measured at 4.7 ppm corrected for T1 and T2 relaxation effects; 2) corrected signal amplitude, expressed as the ratio of total creatine to unsuppressed water; 3) the ratio corrected for the number of protons contributing to signal, three for total creatine and two for water; 4) the ratio converted to units of millimoles per kilogram wet weight, assuming a water concentration of 55 mmol/kg wet wt and a tissue water fraction of 0.81 kg/kg (39). IMCL and EMCL concentrations were similarly calculated using muscle water as the internal reference with the added considerations of 1) IMCL structure similar to trioleate (61.0 mmol 1H/ml triglyceride) and 2) tissue water fraction of 0.81 kg/kg and tissue density of 1.05 g/ml (39).

Although prior knowledge in the use of the MRUI peak-picking routine is intended to separate IMCL-(CH2)n and EMCL-(CH2)n content, the presence of EMCL within the volume of interest can cross-contaminate the IMCL signal, reducing the precision of IMCL estimation. Careful placement of the voxel wholly within the muscle ensured the exclusion of subcutaneous adipose tissue (Fig. 1). We also report the EMCL-(CH2)n signals, corrected for T1 and T2 effects, both in relation to total creatine and water.


View larger version (183K):
[in this window]
[in a new window]
 
Fig. 1.   Image showing voxel positioning. Position of leg in scan: A, anterior; L, left.

Blood analysis. Plasma glucose was determined immediately upon collection by use of an EML 105 autoanalyzer (Radiometer, Copenhagen, Denmark). Plasma FFA concentration was determined using a commercially available assay kit (Wako Pure Chemicals, Osaka, Japan), scaled down for the microplate. Serum insulin was determined with a microparticle enzyme immunoassay by means of a fully automated procedure (AxSYM; Abbott Laboratories, Sydney, Australia). Deproteinized supernatant removed from a 1:2 blood-0.6 M perchloric acid mix was used for determination of both 3-hydroxybutyrate and glycerol by the use of methods previously described (6, 35). Plasma total triglycerides were determined from a commercially available kit (ThermoTrace, Noble Park, Australia).

Statistical analysis. Student's paired t-test (two tailed) was used to compare anthropometric and MRS data at 12 and 84 h. One-way, repeated-measures ANOVAs were used to determine the effect of fasting time on blood biochemical variables. Pairwise comparisons between means were performed by post hoc contrasts. All statistical analysis was performed using a specialized statistical package (SPSS for Windows, version 10.0.1).


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

MRS. The IMCL-(CH2)n-to-water ratio increased from 0.00623 ± 0.00065 to 0.0142 ± 0.0015 (mean ± SE, P = 0.002; Fig. 2A). The IMCL-(CH2)n-to-creatine-CH3 ratio increased from 6.82 ± 0.87 to 14.96 ± 1.73 (P = 0.001; Fig. 2B). Calculated absolute IMCL concentration increased from 8.95 ± 0.92 to 20.10 ± 2.05 mmol/kg wet wt (P = 0.002). Mean corrected unsuppressed water signal did not change significantly during fasting (6.54 × 10-1 ± 4.11 × 10-2 vs. 6.73 × 10-1 ± 6.18 × 10-2 for 12- and 84-h fasting, respectively, P = 0.713). Similarly, there was no significant change in total creatine concentration (29.6 ± 4.3 vs. 30.6 ± 4.0 mmol/kg wet wt, P = 0.817). Also, neither the EMCL-(CH2)n-to-water nor the EMCL-(CH2)n-to-creatine-CH3 ratio was significantly changed by fasting (0.00633 ± 0.00123 vs. 0.00654 ± 0.00134, and 7.3 ± 1.63 vs. 7.52 ± 2.14, P = 0.851 and P = 0.885; Fig. 2, A and B, respectively). Calculated absolute EMCL-(CH2)n concentration was also unchanged (9.08 ± 1.76 to 9.38 ± 1.91 mmol/kg, P = 0.853).


View larger version (25K):
[in this window]
[in a new window]
 
Fig. 2.   Effect of 72 h of water-only fasting on vastus lateralis intra- and extramyocellular lipid (IMCL and EMCL, respectively) content expressed relative to water (A) and relative to total creatine (Cr; B). Values are means ± SE. * P < 0.01 compared with 12-h fasting.

Biochemistry. Plasma FFA, serum triglyceride, and whole blood 3-hydroxybutyrate concentrations all significantly increased (Fig. 3) during the fasting period (P < 0.001, P < 0.05, P < 0.001, respectively), whereas plasma glucose (Fig. 4A) and serum insulin (Fig. 4B) concentrations decreased (P < 0.001 and P < 0.001, respectively).


View larger version (12K):
[in this window]
[in a new window]
 
Fig. 3.   Effect of 72 h of water-only fasting on plasma free fatty acid (FFA; A), serum triglyceride (B), and whole blood 3-hydroxybutyrate (C) concentrations. Values are means ± SE. * P < 0.05 compared with 12-h fasting.



View larger version (13K):
[in this window]
[in a new window]
 
Fig. 4.   Effect of 72 h of water-only fasting on plasma glucose (A) and serum insulin (B) concentrations. Values are means ± SE. * P < 0.05 compared with 12-h fasting.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The present investigation was conducted to test the hypothesis that a prolonged period of fasting would result in an increase in vastus lateralis IMCL content. Our results show a greater than twofold increase in IMCL, expressed in relation to either water or creatine or as calculated absolute concentration. This increase was associated with significant reductions in venous blood concentrations of glucose and insulin and concomitant significant elevations in concentrations of FFAs, triglycerides, and 3-hydroxybutyrate.

Greater adiposity will result in a large EMCL signal with the potential for greater contamination of the IMCL signal, making accurate IMCL quantification more difficult using the MRUI curve-fitting technique. Subjects in the present study, however, were relatively lean (Table 1), and careful voxel placement was able to exclude subcutaneous fat and minimize the possibility of EMCL contamination. Also, EMCL and IMCL peaks in most subjects were quite distinct, and changes were easily observed (Fig. 5). In addition, cross-contamination was unlikely, because there was no significant change in EMCL-(CH2):Cr/water ratios (Fig. 2).


View larger version (15K):
[in this window]
[in a new window]
 
Fig. 5.   Fasting spectra [12 (A) and 84 h (B)] from 1 subject.

Our results are in support of previous findings in animals, where triglyceride content, determined by biochemical methods, increased by 60% in the gastrocnemius muscle of rats (26) and 255% in the pectoralis muscles of pigeons (44) after 72 h of fasting. They are, however, in contrast with the findings of others (19), who observed a 50% decrease in three rat leg muscles during a fast of the same time period. Interestingly, in both these and other (28) rodent studies, diaphragm triglyceride content decreased significantly during fasting. The discrepancy between studies, and indeed between muscle groups in one study (26), may be a function of the level of contractile activity of the different muscle groups during the fasting period; IMCL is thought to be an important fuel during submaximal exercise (13). During the fasting intervention, the diaphragm is in continual use, but the legs may have undergone different levels of activity. The subjects in our study were habitual cyclists or runners, but during the fast their level of activity was severely curtailed. An alternative explanation of the discrepancies among the aforementioned animal studies could be that, in some cases, there was error due to the methodological difficulties in separating the contribution of intracellular and extracellular triglyceride from a muscle biopsy sample (48).

The observed increase in IMCL with fasting helps to explain the observations of Jensen et al. (20), which suggest a nonhepatic site of net fatty acid esterification over a 60-h fast. Such a rapid adaptation may serve to provide an immediate fuel supply to the muscle as endogenous carbohydrate stores are challenged. In an evolutionary context, a 72-h period of food deprivation is not long and has probably relatively often been experienced. Physical adeptness would possibly be more important when food was scarce, and thus preservation of the muscle and more immediate energy stores would be vital.

During the fasting period, serum insulin significantly decreased below basal levels. These findings refute others (8), who suggest that significant storage of IMCL occurs only with the combination of high concentrations of circulating FFAs and raised insulin levels.

Much of the fatty acid taken up by resting skeletal muscle is not immediately oxidized but is reesterified (29) for later lipolysis and oxidation (11). This may thus limit cytoplasmic fatty acyl concentration, high levels of which have the potential to disrupt cellular integrity (45). Thus we can think of IMCL as a buffer between the rate of fatty acid entry into the myocyte and the rate of fatty acid oxidation in the mitochondria.

The rate at which fatty acids enter the muscle cell is largely a function of the plasma/cytoplasmic gradient (46). Thus, if cytoplasmic concentrations are kept low, uptake is driven primarily by plasma FFA concentration. However, uptakes at high plasma concentrations appear limited by saturation kinetics (40), i.e., at lower concentrations in the untrained than in the trained (41). Indeed, the perfused muscles of endurance-trained rats have a greater rate of uptake for the same plasma concentration and a greater content of plasma membrane fatty acid-binding protein (FABPPM) (43). Furthermore, 48-h fasting has been shown to increase the FABPPM content in rat red skeletal muscle by 60% (42).

Therefore, in a situation where both the plasma/cytoplasmic FFA gradient and transporter activity are high, such as in endurance-trained, fasted muscle, uptake will be near maximal. If, at the same time, the muscle cell is in a relative state of rest, the rate of oxidation is reduced so that, for a time at least, uptake is greater than oxidation and an increase in IMCL must ensue. Thus our well-trained subjects may have accumulated IMCLs during the fast when the untrained would not have. However, there is the paradox that endurance-trained (22) and insulin-resistant, obese persons (16, 24) exhibit higher levels of IMCLs than normals. In both situations, for a period of time at least, there is a mismatch, so that the rate of muscle fiber fatty acid uptake is greater than the rate of oxidation. In athletes at rest, it may be because the rate of cellular uptake is elevated compared with normals because of increased FABP and improved capillarization. In the insulin-resistant obese, it is more likely a function of reduced mitochondrial uptake (12) and oxidation (38), because muscle FABPPM is lower in type 2 diabetics compared with normals (2).

Although we did not compare glucose sensitivity before and after the fast, it is known that fasting inhibits insulin-mediated whole body oxidative glucose disposal (25, 47), presumably via operation of the glucose-fatty acid cycle (32). However, when FFAs are available in excess after lipid-heparin infusion, glucose 6-phosphate levels fall below control levels, suggesting inhibition of glucose transport/phosphorylation rather than via elevation of acetyl-CoA/CoA and NADH/NAD+ (34). Hence, it is also unlikely that the glucose-fatty acid cycle operates when sufficient acetyl-CoA is available from ketone sources. Accordingly, Beylot et al. (1) observed no change in insulin-stimulated glucose disposal during euglycemic hyperinsulinemic clamp, when acetoacetate was infused into normal subjects. These observations, taken together with the results of the present study and those of others who have shown an association between IMCL content and insulin resistance in normals (23, 24), suggest that the insulin resistance seen during fasting may be mediated, at least in part, by an increase in IMCL.

In conclusion, 72 h of water-only fasting results in accumulation of IMCL in the vastus lateralis of nondiabetic, nonobese, physically fit men. This accumulation of IMCL occurs in the presence of elevated circulating FFA, triglyceride, and 3-hydroxybutyrate but declining glucose and insulin concentrations. These results suggest that, in physically fit, nondiabetic men, it is the relationship between the rate of fatty acid availability and oxidation, not circulating insulin levels, that determines IMCL content.


    FOOTNOTES

Address for reprint requests and other correspondence: S. Stannard, School of Exercise and Sport Science, Faculty of Health Sciences, The Univ. of Sydney, Lidcombe 1825, NSW, Australia (E-mail: stevestannard{at}ozemail.com.au).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

July 30, 2002;10.1152/ajpendo.00108.2002

Received 8 March 2002; accepted in final form 18 July 2002.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Beylot, M, Khalfallah Y, Riou JP, Cohen R, Normand S, and Mornex R. Effects of ketone bodies on basal and insulin-stimulated glucose utilization in man. J Clin Endocrinol Metab 63: 9-15, 1986[Abstract].

2.   Blaak, EE, Wagenmakers AJM, Glatz JFC, Wolffenbuttel BHR, Kemerink GJ, Langenberg CJM, Heidendal GAK, and Saris WHM Plasma FFA utilization and fatty acid-binding protein content are diminished in type 2 diabetic muscle. Am J Physiol Endocrinol Metab 279: E146-E154, 2000[Abstract/Free Full Text].

3.   Boden, G, Jadali F, White J, Liang Y, Mozzoli M, Chen X, Coleman E, and Smith C. Effects of fat on insulin-stimulated carbohydrate metabolism in normal men. J Clin Invest 88: 960-966, 1991[ISI][Medline].

4.   Boden, G, Lebed B, Schatz M, Homko C, and Lemieux S. Effects of acute changes of plasma free fatty acids on intramyocellular fat content and insulin resistance in healthy subjects. Diabetes 50: 1612-1617, 2001[Abstract/Free Full Text].

5.   Boesch, C, Slotboom J, Hoppeler H, and Kreis R. In vivo determination of intra-myocellular lipids in human muscle by means of localized 1H-MR-spectroscopy. Magn Reson Med 37: 484-493, 1997[ISI][Medline].

6.   Boobis, LH, and Maughan RJ. A simple one-step enzymatic fluorometric method for the determination of glycerol in 20 µl of plasma. Clin Chim Acta 132: 173-179, 1983[ISI][Medline].

7.   Bottomley, PA, Lee Y, and Weiss RG. Total creatine in muscle: imaging and quantification with proton MR spectroscopy. Radiology 204: 403-410, 1997[Abstract].

8.   Brechtel, K, Dahl DB, Machann J, Bachman OP, Wenzel I, Maier T, Claussen CD, Haring HU, Jacob S, and Schick F. Fast elevation of the intramyocellular lipid content in the presence of circulating free fatty acids and hyperinsulinaemia: a dynamic 1H-MRS study. Magn Reson Med 45: 179-183, 2001[ISI][Medline].

9.   Brozek, J, Grande F, Anderson JT, and Keys A. Densiometric analysis of body composition: revision of some quantitative assumptions. Ann NY Acad Sci 110: 113-140, 1963[ISI].

10.   Bruhn, H, Frahm J, Gyngell ML, Merboldt KD, Hanicke W, and Sauter R. Localized proton NMR spectroscopy using stimulated echoes: applications to human skeletal muscle. Magn Reson Med 17: 82-91, 1991[ISI][Medline].

11.   Dagenais, GR, Tancredi RG, and Zierler KL. Free fatty acid oxidation by forearm muscle at rest, and evidence for an intramuscular lipid pool in the human forearm. J Clin Invest 58: 421-431, 1976[ISI][Medline].

12.   Dobbins, RL, Szczepaniak LS, Bentley B, Esser V, Myhill J, and McGarry JD. Prolonged inhibition of muscle carnitine palmitoyltransferase-1 promotes intramyocellular lipid accumulation and insulin resistance in rats. Diabetes 50: 123-130, 2001[Abstract/Free Full Text].

13.   Essen, B. Intramuscular substrate utilization during prolonged exercise. Ann NY Acad Sci 301: 30-44, 1977[Abstract].

14.   Falholt, K, Jensen I, Lindkaer Jensen S, Mortensen H, Volund A, Hedling LG, Noerskov Peterse P, and Falholt W. Carbohydrate and lipid metabolism of skeletal muscle in type 2 diabetic patients. Diabet Med 5: 27-31, 1988[ISI][Medline].

15.   Fery, F, Melot C, Bosson D, and Balasse EO. Effect of short term fasting on glucose tolerance and insulin secretion: influence of the initial glucose level. Diabetes Metab 16: 77-85, 1990[ISI].

16.   Forouhi, NG, Jenkinson G, Thomas EL, Mullick S, Mierisova S, Bhonsli U, McKeigue PM, and Bell JD. Relation of triglyceride stores in skeletal muscle cells to central obesity and insulin sensitivity in European and South Asian men. Diabetologia 42: 932-935, 1999[ISI][Medline].

17.   Gorski, J. Muscle triglyceride metabolism during exercise. Can J Physiol Pharmacol 70: 123-131, 1990.

18.   Howald, H, Boesch C, Kreis R, Matter S, Billeter R, Essen-Gustavsson B, and Hoppeler H. Content of intramyocellular lipids derived by electron microscopy, biochemical assays, and 1H-MR spectroscopy. J Appl Physiol 92: 2264-2272, 2002[Abstract/Free Full Text].

19.   Jaromowska, M, and Gorski J. Effect of fasting on skeletal muscle triglyceride content. Experientia 41: 357-358, 1985[ISI][Medline].

20.   Jensen, MD, Ekberg K, and Landau BR. Lipid metabolism during fasting. Am J Physiol Endocrinol Metab 281: E789-E793, 2001[Abstract/Free Full Text].

21.   Keizer, HA, Hesselink M, Schaart G, Borghouts L, and van Kranenburg G. Triglyceride content and fatty acid translocase (FAT/CD36) expression in individual muscle fibres of vastus lateralis muscles in type II diabetic patients. In: 6th Ann Congr Eur College Sports Sci, edited by Mester J, King G, Struder H, Tsolakidis E, and Osterburg A.. Cologne, Germany: Sport und Buch Strauss, 2001, p. 573.

22.   Kiens, B, Essen-Gustavsson B, Christensen NJ, and Saltin B. Skeletal muscle substrate utilization during submaximal exercise in man: effect of endurance training. J Physiol 469: 459-478, 1993[Abstract].

23.   Krssak, M, Falk Petersen K, DiPietro L, Vogel SM, Rothman DL, Shulman GI, and Roden M. Intramyocellular lipid concentrations are correlated with insulin sensitivity in humans: a 1H NMR study. Diabetologia 42: 113-116, 1999[ISI][Medline].

24.   Levin, K, Daa Schroeder H, Alford FP, and Beck-Nielsen H. Morphometric documentation of abnormal intramyocellular fat storage and reduced glycogen in obese patients with Type II diabetes. Diabetologia 44: 824-833, 2001[ISI][Medline].

25.   Mansell, PI, and Macdonald IA. The effect of starvation on insulin-induced glucose disposal and thermogenesis in humans. Metabolism 39: 502-10, 1990[ISI][Medline].

26.   Masoro, EJ. Skeletal muscle lipids. III. Analysis of the functioning of skeletal muscle lipids during fasting. J Biol Chem 242: 1111-1114, 1967[Abstract/Free Full Text].

27.   Naressi, A, Couturier C, Devos JM, Janssen M, Mangeat C, De Beer C, and Graveron-Demilly D. Java-based graphical user interface for the MRUI quantitation package. MAGMA 12: 141-152, 2001[Medline].

28.   Neptune, EM, Sudduth HC, Foreman DR, and Fash FJ. Phospholipid and triglyceride metabolism of excised rat diaphragm and the role of these lipids in fatty acid uptake and oxidation. J Lipid Res 1: 229-235, 1959[ISI].

29.   Oscai, LB, Essig DA, and Palmer WK. Lipase regulation of muscle triglyceride synthesis. J Appl Physiol 69: 1571-1577, 1990[Abstract/Free Full Text].

30.   Owen, OE, and Reichard GA. Human forearm metabolism during progressive starvation. J Clin Nutr 50: 1536-1545, 1971.

31.   Pan, DA, Lillioja S, Kriketos AD, Milner MR, Baur LA, Bogardus C, Jenkins AB, and Storlein LH. Skeletal muscle triglyceride levels are inversely related to insulin action. Diabetes 46: 983-988, 1997[Abstract].

32.   Randle, PJ, Garland PB, Hales CN, and Newsholme EA. The glucose fatty-acid cycle. Its role in insulin sensitivity and the metabolic disturbances of diabetes mellitus. Lancet 1: 785-789, 1963[ISI].

33.   Rico-Sanz, J, Hajnal JV, Thomas EL, Mierisova S, Ala-Korpela M, and Bell JD. Intracellular and extracellular skeletal muscle triglyceride metabolism during alternating intensity exercise in humans. J Physiol 510: 615-622, 1998[Abstract/Free Full Text].

34.   Roden, M, Price TB, Perseghin G, Falk Petersen K, Rothman DL, and Cline GW. Mechanism of free-fatty acid-induced insulin resistance in humans. J Clin Invest 97: 2859-2865, 1996[Abstract/Free Full Text].

35.   Ruell, PA, and Gass GC. Enzymatic measurement of 3-hydroxybutyrate in extracts of blood without neutralization. Ann Clin Biochem 28: 183-184, 1991[ISI][Medline].

36.   Santomauro, AT, Boden G, Silva ME, Rocha DM, Santos RF, Ursich MJ, Strassmann PG, and Wajchenberg BL. Overnight lowering of free fatty acids with Acipimox improves insulin resistance and glucose tolerance in obese diabetic and nondiabetic subjects. Diabetes 48: 1836-1841, 1999[Abstract].

37.   Schick, F, Eismann B, Jung WI, Bongers H, Bunse M, and Lutz O. Comparison of localized proton NMR signals of skeletal muscle and fat tissue in vivo: two lipid compartments in muscle tissue. Magn Reson Med 29: 158-167, 1993[ISI][Medline].

38.   Simoneau, J, and Kelley DE. Altered glycolytic and oxidative capacities of skeletal muscle contribute to insulin resistance in NIDDM. J Appl Physiol 83: 166-171, 1997[Abstract/Free Full Text].

39.   Szczepaniak, LS, Babcock EE, Schick F, Dobbins RL, Garg A, Burns DK, McGarry JD, and Stein DT. Measurement of intracellular triglyceride stores by 1H spectroscopy: validation in vivo. Am J Physiol Endocrinol Metab 276: E977-E989, 1999[Abstract/Free Full Text].

40.   Turcotte, LP, Kiens B, and Richter EA. Saturation kinetics of palmitate uptake in perfused skeletal muscles. FEBS Lett 279: 327-329, 1991[ISI][Medline].

41.   Turcotte, LP, Richter EA, and Kiens B. Increased plasma FFA uptake and oxidation during prolonged exercise in trained vs. untrained humans. Am J Physiol Endocrinol Metab 262: E791-E799, 1992[Abstract/Free Full Text].

42.   Turcotte, LP, Srivastava AK, and Chiasson JL. Fasting increases plasma membrane fatty-acid binding protein (FABPPM) in red skeletal muscle. Mol Cell Biochem 166: 153-158, 1997[ISI][Medline].

43.   Turcotte, LP, Swenberger JR, Tucker MZ, and Yee AJ. Training-induced elevation in FABP is associated with increased palmitate use in contracting muscle. J Appl Physiol 87: 285-293, 1999[Abstract/Free Full Text].

44.   Vallyathan, NV, Grinyer I, and George JC. Effect of fasting and exercise on lipid levels in muscle. Can J Zool 48: 377-383, 1970[Medline].

45.   Van der Vusse, GJ, Glatz JFC, Stam HCG, and Reneman RS. Fatty acid homeostasis in the normoxic and ischemic heart. Physiol Rev 72: 881-940, 1992[Free Full Text].

46.   Van der Vusse, GJ, and Roemen THM Gradient of fatty acids from blood plasma to skeletal muscle in dogs. J Appl Physiol 78: 1839-1843, 1995[Abstract/Free Full Text].

47.   Webber, J, Taylor J, Greathead H, Dawson J, Buttery PJ, and Macdonald IA. Effects of fasting on fatty acid kinetics and on the cardiovascular, thermogenic and metabolic responses to the glucose clamp. Clin Sci (Colch) 87: 697-706, 1994[ISI][Medline].

48.   Wendling, PS, Peters SJ, Heigenhauser GJF, and Spriet LL. Variability of triacylglycerol content in human skeletal muscle biopsy samples. J Appl Physiol 81: 1150-1155, 1996[Abstract/Free Full Text].

49.   Wicklmayr, M, Rett K, Dietze G, and Menhnert H. Inhibition of muscular triglyceride lipolysis by ketone bodies: a mechanism for energy-preservation in starvation. Horm Metab Res 18: 476-478, 1986[ISI][Medline].


Am J Physiol Endocrinol Metab 283(6):E1185-E1191
0193-1849/02 $5.00 Copyright © 2002 the American Physiological Society