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American Journal of Clinical Nutrition, Vol. 75, No. 2, 237-244, February 2002
© 2002 American Society for Clinical Nutrition

Original Research Communication

Triacylglycerol infusion improves exercise endurance in patients with mitochondrial myopathy due to complex I deficiency1,2,3

Mark J Roef, Kees de Meer, Dirk-Jan Reijngoud, Helma WHC Straver, Martina de Barse, Satish C Kalhan and Ruud Berger

1 From the Department of Pediatric Gastroenterology (MJR) and the Laboratory for Metabolic Diseases (HWHCS, MB, and RB), the University Children's Hospital, Utrecht, Netherlands; the Department of Clinical Chemistry, Vrije Universiteit Medical Center, Amsterdam (KM); the Laboratory for Metabolic Diseases, the Department of Pediatrics, the University Hospital Groningen, Groningen, Netherlands (D-JR); and the Robert Schwartz, MD, Center for Metabolism and Nutrition, MetroHealth Medical Center, Case Western Reserve University School of Medicine, Cleveland (SCK).

2 Supported by the Foundation Speurwerk in de Kindergeneeskunde (University Children's Hospital Het Wilhelmina Kinderziekenhuis) and by the Dutch Foundation De Drie Lichten.

3 Reprints not available. Address correspondence to K de Meer, Department of Clinical Chemistry, Reception K, Vrije Universiteit Medical Center, PO Box 7057, 1007 MB Amsterdam, Netherlands. E-mail: k.demeer{at}vumc.nl.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: A high-fat diet has been recommended for the treatment of patients with mitochondrial myopathy due to complex I (NADH dehydrogenase) deficiency (CID).

Objective: This study evaluated the effects of intravenous infusion of isoenergetic amounts of triacylglycerol or glucose on substrate oxidation, glycolytic carbohydrate metabolism, and exercise endurance time and energy state of muscle in CID patients.

Design: Four CID patients and 15 control subjects were infused with triacylglycerol (3.7 mg·kg-1·min-1) or glucose (10 mg·kg-1·min-1) during low-intensity leg exercise. Respiratory calorimetry was used to evaluate mitochondrial substrate oxidation. The concentration and rate of appearance of plasma lactate (from dilution of [1-13C]lactate) were used to evaluate glycolytic carbohydrate metabolism. 31P magnetic resonance spectroscopy was used to determine ratios of phosphocreatine to inorganic o-phosphate in forearm muscle during exercise.

Results: In 3 patients, leg exercise endurance time was better during the triacylglycerol infusion than during the glucose infusion. In all 4 patients, whole-body oxygen consumption rates during exercise were higher during triacylglycerol infusion than during the glucose infusion. In 3 patients, the concentration and rate of appearance of plasma lactate were lower during triacylglycerol infusion than during the glucose infusion. Ratios of phosphocreatine to inorganic o-phosphate during exercise were not significantly different between the 2 infusion studies or between the patients and control subjects.

Conclusions: Triacylglycerol infusion is associated with a greater oxidation of substrates, lower rates of appearance and concentrations of plasma lactate, and greater leg exercise endurance time in myopathic CID patients than is glucose infusion. The energy state of muscle during exercise, however, was not significantly different after infusion of triacylglycerol or glucose.

Key Words: Complex I deficiency • exercise • triacylglycerol infusion • mitochondrial myopathy • stable isotopes • 31P magnetic resonance spectroscopy


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In our preceding article in this issue, we proposed a potential role for high-fat, low-carbohydrate diets in the treatment of patients with mitochondrial myopathy due to complex I (NADH dehydrogenase; EC 1.6.99.3) deficiency (CID; 1). We hypothesized that supplementation with fatty acids (triacylglycerol) might improve the oxidation of substrates in CID patients by supplying FADH2-linked reducing equivalents (electrons) to the mitochondrial respiratory chain distal to complex I. This action would make these patients' muscle cells less dependent on glycolytic carbohydrate metabolism, ie, the rate of appearance (Ra) and concentration of plasma lactate might be lower during triacylglycerol infusion than during glucose infusion. We documented that substrate oxidation rates were stimulated rather than impaired in resting, myopathic CID patients during infusion of glucose or triacylglycerol. In addition, triacylglycerol infusion did not lower the Ra or concentration of plasma lactate to resting control values. The finding that these patients showed no clinical signs or symptoms of CID at rest indicates that in vivo cellular energy balance was maintained during the triacylglycerol or glucose infusion, despite in vitro mitochondrial impairment. During exercise, when energy requirements in muscle are elevated, in vivo mitochondrial oxidation of NADH may become limiting in CID. Triacylglycerol administration then may be of benefit to these patients because fatty acid oxidation increases the relative supply of FADH2, thereby improving oxygen consumption (O2), and as a result may lower the Ra and concentration of plasma lactate and improve exercise tolerance.

In the present study, the effects of the triacylglycerol or glucose infusion on substrate oxidation and glycolytic metabolism were studied during low-intensity cycling in 4 CID patients and 15 healthy control subjects. Respiratory calorimetry was used as a measure of mitochondrial substrate oxidation. Glycolytic carbohydrate metabolism was evaluated on the basis of the Ra and concentration of plasma lactate (from dilution of [1-13C]lactate). In addition, the effects of submaximal exercise on the energy state of forearm muscle was measured by 31P magnetic resonance spectroscopy (31P-MRS).


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects
Details about the CID patients and healthy control subjects are provided in the preceding article (1). Briefly, the 4 CID patients had similar clinical histories, ie, easily fatiguable mild muscle weakness dating back to early childhood that remained stable over time. Therefore, exercise intolerance was the dominant clinical symptom at the time of the study. CID was diagnosed with microscopic and biochemical investigations in fresh biopsy specimens of the quadriceps (vastus lateralis) muscle, which showed markedly decreased activity of complex I in all patients (range: 5.6–23.8% of normal). Eighteen healthy control subjects matched for age ( age: 21 y), sex, and body weight were recruited: 15 for the tracer infusion and respiratory calorimetry studies and 3 (2 men and 1 woman aged 18–28 y) for the 31P-MRS studies. None of these subjects had a family history of diabetes mellitus or took medications. All subjects were studied after they had fasted overnight; no other dietary restrictions were imposed. Written, informed consent was obtained from all subjects. The experimental protocol was approved by the Medical Ethics Committee of the University Children's Hospital (Utrecht, Netherlands).

Preexperiment testing
All participants reported to the Laboratory for Metabolic Diseases (University Children's Hospital, Utrecht, Netherlands) >=3 d before the onset of the experiments to perform an incremental maximal exercise test on an electrically braked cycle ergometer (Lode Instruments, Groningen, Netherlands). Maximal workload (Wmax) and maximal O2 (O2max) were assessed as previously described (2). The Wmax ranged from 50 to 60 W in the CID patients, 25% of control values on average (: 210 W; Table 1Go). The results of this test were used to calculate a workload equal to 15% of each subject's Wmax. Thus, the calculated workloads ranged from 7 to 9 W in the CID patients. From our experience, these workloads can be sustained for a relatively prolonged period of time (>0.5 h) to test exercise endurance. Seven of the control subjects exercised at 15% of their respective Wmax (range: 24–37 W, corresponding to {approx}30% O2max). The remaining 8 control subjects were studied during stationary cycling at an absolute workload similar to that for the patients (7 W), to take into account possible confounding effects of the extreme low exercise intensity on exercise mechanics. Both workloads can be easily sustained by healthy subjects for hours.


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  		TABLE 1 Physical characteristics of individual patients with complex I deficiency (CID) and healthy control subjects at baseline1
 
Experimental protocol
The patients and control subjects reported to the Laboratory for Metabolic Diseases at 0800 on 2 occasions separated by >=7 d. Before exercise was initiated, all subjects were infused intravenously with either glucose (10% wt:vol, 5 mg·kg-1·min-1) or a triacylglycerol emulsion (Intralipid, 20% wt:vol, 1.85 mg·kg-1 ·min-1; Fresenius Kabi, ‘s-Hertogenbosch, Netherlands) and heparin (7.5 U·kg-1·h-1; prime 14 U/kg) for 120 min, during which time the subjects remained at rest. Next, the stationary cycling exercise began at 15% Wmax (all patients and 7 control subjects) or 7 W (8 control subjects). The control subjects exercised for a nominal period of 90 min, whereas the patients exercised until exhaustion ensued. During exercise, the triacylglycerol and glucose infusion rates were doubled to 3.7 and 10 mg·kg-1·min-1, respectively.

Isotope infusion
Primed, constant infusions of [6,6-2H2]glucose (98% enriched; Mass Trace, Woburn, MA) were administered in all patients and in 9 control subjects as previously described (1). An unprimed, constant infusion of [1-13C]lactate (98% enriched; Mass Trace) was administered throughout the nominal 90-min exercise period. The [1-13C]lactate tracer infusion rates for the CID patients (n = 4) ranged from 2.50 to 5.34 µmol·kg-1·min-1; control subjects (n = 9) received 0.56 µmol·kg-1·min-1. The remaining 6 control subjects received no lactate tracer, to examine the effects of triacylglycerol or glucose infusion on the background enrichment of lactate.

Blood sampling and urine collection
Blood samples were drawn at regular intervals during exercise and handled as described previously (1). All subjects voided before and after exercise, and the urine was collected for measurement of nitrogen excretion.

Respiratory calorimetry
Open-circuit indirect calorimetry with use of a face mask was performed continuously during the 90-min exercise period. Stable O2 and carbon dioxide production (CO2) values were reached within 5 min of recording. Computerized, continuous gas and air volume measurements were performed (Oxycon Champion; Jaeger, Breda, Netherlands) as previously described (1, 3).

31P-MRS studies ratios of phosphocreatine to inorganic o-phosphate at rest and during exercise
Three of the CID patients (patients 1, 2, and 3) and 3 control subjects reported to the MRS facility at 0800 for the triacylglycerol or glucose infusion on 2 separate days and received an intravenous infusion of either triacylglycerol plus heparin or glucose during a 100-min basal resting period as described above. The infusions were maintained at the same rate throughout the following 30-min study period. Peak ratios of phosphocreatine (PCr) to ß-ATP and of inorganic o-phosphate (Pi) to ß-ATP were measured at rest and during exercise at an average of 6 normalized power output levels; concentrations of Pi and PCr were calculated from each ratio, respectively, as described elsewhere (4).

The results obtained at rest were reported previously (1). Exercise consisted of bulb-squeezing at an audio signal frequency of 0.33 Hz (duration, 300 ms) with use of only the fourth and fifth digits at progressively increasing submaximal workloads in a ramp protocol with feedback of power output (details described previously; 5). Power output was measured and recorded as developed pressure x the displaced volume of air and was normalized to power output during maximal voluntary contraction (MVC). The highest implemented workload in the patients ranged from 30% MVC to 45% MVC and was 50% MVC in the control subjects. All subjects were right-handed.

Studies were conducted on the superficial mass of the flexor digitorum profundus (FDP) muscle of the right forearm, as described in detail elsewhere (5). The FDP muscle is affected in CID patients, as evidenced by observed decreases in MVC output of the muscle comparable with decreases in Wmax measured on a cycle ergometer (MJ Roef, K de Meer, unpublished observations, 1996). Radio frequency pulsing during exercise was gated to the audio signal that synchronized bulb-squeezing with a home-built audiotriggering device. To minimize motion artifacts and improve standardization of the measurements, the radio frequency pulse was set to occur 1250 ms after contraction.

Sample analysis
Blood and urine samples and MR spectra were analyzed as described previously (1).

Calculations
Whole-body glucose rate of appearance
In steady state experiments, the whole-body glucose Ra was calculated as follows (6):

(1)
where I is the infusion rate of the [6,6-2H2]glucose tracer (in µmol·kg-1·min-1) and TTR is the molar tracer-tracee ratio in plasma. Isotopic enrichments were considered to be at steady state when the TTRs of the final 4 consecutive samples (from time 175 to 205 min) had a CV < 10% and a slope not significantly different from zero. When the isotopic enrichments were not at steady state, the Ra was calculated with Steele's equations for non–steady state conditions (7), as previously described (1). Steady state [6,6-2H2]glucose TTRs were attained in 3 of the 4 patients and in 7 of the 9 control subjects during triacylglycerol infusion and in 3 of the 4 patients and in 6 of the 9 control subjects during the glucose infusion.

Whole-body lactate+pyruvate Ra
A single common pool approach for lactate and pyruvate was used to calculate the whole-body lactate+pyruvate Ra (in µmol·kg-1·min-1). Wolfe et al (8, 9) reported that plasma pyruvate enrichment was 92% of plasma lactate enrichment at steady state in anesthesized dogs when [1-13C]lactate tracer was infused. Results of Large et al (10) suggest that this approach could also be applicable to humans. Therefore, we assumed that the turnover rates calculated from dilution of labeled lactate in plasma represent not only lactate but also pyruvate turnover:

(2)
where I is the infusion rate of [1-13C]lactate (in µmol·kg-1·min-1). Steady state conditions for lactate TTRs were defined as described for glucose. When isotopic enrichments were not at steady state, the Ra was calculated with Steele's equations for non–steady state conditions (7), as previously described (1). The volume of distribution for lactate+pyruvate was assumed to be 500 mL/kg (11, 12). During exercise, steady state lactate TTRs were attained in only 1 of the 4 patients during both the triacylglycerol and glucose infusions and in 7 of the 9 control subjects during the triacylglycerol infusion and in 6 of the 9 control subjects during the glucose infusion.

Respiratory calorimetry
Substrate oxidation rates were calculated as described previously (1). Calculated whole-body substrate oxidation rates during exercise were assumed to reflect mitochondrial oxidation, neglecting nonmitochondrial peroxisomal and microsomal O2.

Outcome parameters
We compared whole-body O2 rates, plasma lactate concentrations, lactate+pyruvate turnover rates, and leg exercise endurance time between the patients and the control subjects. The triacylglycerol or glucose infusion can be expected to affect outcome parameters differently, not only in the patients but also in the control subjects. Thus, the hypothesized effects in the patients should be additional to those in the control subjects. The effects of the triacylglycerol infusion (TG) compared with those of the glucose infusion (GL) on outcome parameters in the individual patients and the control subjects is described as follows:

(3)

(4)

(5)
When the values for {Delta}O2, {Delta}plasma lactate, and {Delta}lactate+pyruvate Ra in the individual patients exceed the upper limit of the 95% CI for the effect of the triacylglycerol infusion (compared with glucose infusion) in control subjects, the effect of the triacylglycerol substrate in the patients is considered additional compared with that of glucose.

Statistical analysis
The results are presented as means ± SDs, except for leg exercise endurance time, which are presented as absolute values. The data were analyzed by two-factor analysis of variance to identify main effects of group (CID patients compared with control subjects) and infusion condition (triacylglycerol compared with glucose infusion) and their interaction. When there was a significant (P < 0.05) interaction, post hoc Bonferroni correction was conducted. The outcome parameters O2, plasma lactate, and lactate+pyruvate Ra in the individual patients were compared with the 95% CIs of control subjects. SPSS for WINDOWS (version 7.5; SPSS Inc, Chicago) was used for the analyses.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Comparison between the CID patients and the control subjects at the group level: plasma concentrations, oxygen consumption, and substrate utilization
In control subjects exercising at 7 W, plasma glucose, plasma insulin, and blood pyruvate concentrations were significantly higher during the glucose infusion than during the triacylglycerol infusion, as expected (Table 2Go). Plasma fatty acid, triacylglycerol, and glycerol concentrations were significantly higher during the triacylglycerol infusion than during the glucose infusion. Plasma lactate, plasma cortisol, blood ß-hydroxybutyrate, and blood acetoacetate concentrations and lactate-pyruvate ratios were not significantly different between the 2 infusion conditions.


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  		TABLE 2 Plasma substrate concentrations during cycling exercise and infusion of triacylglycerol or glucose in patients with complex I deficiency (CID) and healthy control subjects1
 
Plasma glucose, insulin, and lactate concentrations and lactate-pyruvate ratios were significantly lower during the glucose infusion in control subjects exercising at 15% Wmax than in control subjects exercising at 7 W (data not shown); however, plasma substrate concentrations during the triacylglycerol infusion were not significantly different between the 2 exercise conditions.

Plasma glucose and insulin concentrations were not significantly different between the CID patients and the control subjects exercising at either 7 W or 15% Wmax during infusion of either substrate. Plasma lactate concentrations were significantly higher in the patients than in the control subjects during the triacylglycerol and glucose infusions. Blood pyruvate concentrations and lactate-pyruvate ratios were significantly higher in the patients than in the control subjects during infusion of both substrates. Plasma fatty acid concentrations were significantly lower in the patients than in control subjects during the triacylglycerol infusion, but were not significantly different between the 2 subject groups during the glucose infusion. During the triacylglycerol infusion, the ratio of ß-hydroxybutyrate to acetoacetate in blood was significantly higher in the patients than in the control subjects exercising at 15% Wmax (data not shown for control subjects).

In control subjects, whole-body O2 was not significantly different between the 2 infusion conditions, but the respiratory exchange ratio and the total carbohydrate oxidation rate were significantly lower during the triacylglycerol infusion than during the glucose infusion at exercise intensities of 7 W and 15% Wmax (data not shown for control subjects at 15% Wmax) (Table 3Go). In the 6 control subjects who received no lactate tracer, the background enrichment of lactate was not significantly different during the triacylglycerol and glucose infusions (data not shown).


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  		TABLE 3 Respiratory calorimetry and the rate of appearance (Ra) of whole-body glucose during cycling exercise and infusion of triacylglycerol or glucose in patients with complex I deficiency (CID) and healthy control subjects1
 
Although there was no significant main effect of infusion condition, group, or condition x group interaction on whole-body O2 rates (Table 3Go), whole-body O2 values in the CID patients were higher during the triacylglycerol infusion than during the glucose infusion (Table 4Go). During the triacylglycerol infusion, respiratory exchange ratios were significantly higher in the patients than in the control subjects at exercise intensities of 7 W and 15% Wmax (data not shown for control subjects at 15% Wmax in Table 3Go); total carbohydrate oxidation rates were significantly higher in the patients than in the control subjects at the exercise intensity of 7 W. Plasma glucose turnover rates in the patients were not significantly different from those in the control subjects during infusion of either substrate.


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  		TABLE 4 Whole-body oxygen consumption (O2), plasma lactate concentrations, and rate of appearance (Ra) of lactate+pyruvate during cycling exercise and infusion of triacylglycerol or glucose in individual patients with complex I deficiency (CID) and healthy control subjects1
 
Comparison of outcome parameters between the individual CID patients and the control subjects
Whole-body oxygen consumption, plasma lactate, and lactate+pyruvate Ra
Changes in O2, plasma lactate, and lactate+pyruvate Ra in the individual patients and the respective 95% CIs in the control subjects are shown in Table 4Go and Figure 1Go. The triacylglycerol infusion was associated with substantially higher O2 rates than was the glucose infusion in all 4 CID patients. In each of the patients, this effect of triacylglycerol infusion on O2 was additional to that in the control group. In comparison with control subjects exercising at an exercise intensity of 7 W, absolute rates of whole-body O2 were lower (on average by 14%) in patients 1 and 2 but were higher (on average by 40%) in patients 3 and 4.



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  		FIGURE 1. Mean differences ({Delta}) in oxygen consumption (O2), plasma lactate, and rate of appearance (Ra) of lactate+pyruvate between the triacylglycerol and glucose infusion studies in the individual patients and the respective mean 95% CIs for differences in the control subjects (gray-shaded area). P5, lower limit of 95% CI; P95, upper limit of 95% CI. Values greater than the P95 in the individual patients indicate an effect of triacylglycerol additional to that in the control subjects according to the following hypothesis: {Delta}O2 in patients > {Delta}O2 in control subjects, and {Delta}plasma lactate and {Delta}lactate+pyruvate Ra in patients < {Delta}plasma lactate and {Delta}lactate+pyruvate Ra in control subjects.

 
Additional effects of the triacylglycerol infusion on plasma lactate concentrations and lactate+pyruvate Ra were shown in 2 and 3 of the 4 patients, respectively. In patient 2, the triacylglycerol infusion had an additional effect on plasma lactate concentrations and lactate+pyruvate Ra.

Leg exercise endurance time
The control subjects completed the leg exercise protocol with ease, irrespective of the substrate being infused (Figure 2Go). Patients 1, 2, and 3 had to stop exercising prematurely (between 48 and 65 min) during the glucose infusion because of muscle fatigue and exhaustion, whereas patient 4 completed the nominal 90-min study period. All patients completed the 90-min cycling exercise during the triacylglycerol infusion.



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  		FIGURE 2. Leg exercise endurance time during stationary cycling exercise in 4 patients with complex I deficiency (CID) during infusion of glucose ({square}) or triacylglycerol ({blacksquare}).

 
31P MRS measurements of PCr-Pi ratios at rest and during exercise
PCr-Pi ratios measured by 31P-MRS at rest and during exercise in one typical CID patient and in one typical control subject are shown in Figure 3Go. Relative exercise levels in the forearm exercise ramp protocol during the triacylglycerol infusion were not significantly different from those during the glucose infusion in both the patients and control subjects. At rest, PCr-Pi ratios were lower in all patients than in the control subjects during infusion of both triacylglycerol and glucose. During exercise, PCr-Pi ratios were not significantly different during the triacylglycerol infusion than during the glucose infusion in either the patients or control subjects.




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  		FIGURE 3. Ratios of phosphocreatine (PCr) to inorganic o-phosphate (Pi) at rest and during forearm bulb-squeeze exercise in a typical patient with complex I deficiency (A) and in a typical control subject (B) during the triacylglycerol ({blacksquare}) and glucose ({circ}) infusions. MVC, maximal voluntary contraction.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Oxidative phosphorylation in the muscle of patients with mitochondrial CID is stimulated during exercise, whereas the ability of the muscle cells' mitochondria to oxidize NADH is limited. We hypothesized that fatty acid oxidation, by providing more FADH2-linked reducing equivalents to the respiratory chain distal of complex I, would enable the affected cells to bypass the metabolic defect. The finding of beneficial effects of triacylglycerol infusion in 4 myopathic patients with isolated CID provides new insights into the nutritional metabolism and cellular pathophysiology of CID patients. Results were not unequivocal in all patients however; therefore, the limitations of the study need to be discussed.

Whole-body oxygen consumption
Our assumption that substrate oxidation rates are impaired during exercise was true in only 2 of the 4 patients exercising at 7 W on the basis that absolute whole-body O2 rates during the triacylglycerol infusion were lower in patients 1 and 2 but higher in patients 3 and 4 than in control subjects. In our previous study (1), we suggested that the higher whole-body O2 rates after infusion of both substrates in all 4 CID patients is explained by a compensatory mechanism necessary to maintain the resting ATP synthetic rate when mitochondrial oxidative phosphorylation is less efficient because of the respiratory chain deficit (1). The increased O2 rates during exercise in the present study suggest that this putative mechanism may also partly compensate for the lower efficacy of ATP synthesis during exercise in 2 of the patients.

Plasma lactate and lactate+pyruvate Ra
In 3 of the 4 CID patients, our finding of a significant additional diminishing effect of triacylglycerol infusion on plasma lactate concentrations and the lactate+pyruvate Ra was as expected, ie, supported our hypothesis. In the fourth patient (patient 2), however, our finding was contrary to our hypothesis; this patient had a higher plasma lactate concentration and higher lactate+pyruvate Ra during the triacylglycerol infusion than during the glucose infusion. Because Steele's equation was used to calculate lactate+pyruvate Ra (see Methods), the lactate concentration and Ra in this patient were not independently measured. We have no explanation for this contrary finding, which is even more remarkable because this patient's leg exercise endurance time during triacylglycerol infusion was significantly better than that of the other 3 patients.

Leg exercise endurance time and muscle bioenergetics
The leg exercise endurance time was clearly lower during the glucose infusion than during the triacylglycerol infusion in 3 of the 4 CID patients because these patients terminated their low-intensity cycling exercise prematurely during the glucose infusion. The remaining patient (patient 4) completed the 90-min low-intensity exercise trial irrespective of the substrate infused and had the lowest plasma lactate concentration of the 4 patients during infusion of both substrates. These findings suggest that leg exercise endurance time may be related to circulating plasma lactate concentrations. Unfortunately, patient 4 was not available for the 31P-MRS studies. The finding that leg exercise endurance time in patient 2 was lower during the glucose infusion, despite lower plasma lactate concentrations during the glucose infusion than during the triacylglycerol infusion, suggests that leg exercise endurance time is not just a simple function of plasma lactate concentrations (13).

The question arises as to why 3 of the 4 patients were unable to complete the 90-min exercise trial. Although our infusion experiments were not double-blind, we believe that the patients stopped exercising because of total exhaustion. Therefore, we are confident that the observed difference in leg exercise endurance time between the 2 infusion periods has a physiologic rather than a psychological basis. Because lactate production in muscle is usually accompanied by release of [H+], differences in the release of [H+] could explain the observed differences in leg exercise endurance time. However, none of the patients became acidotic during infusion of either substrate during exercise at this low intensity level, ie, 15% Wmax (data not shown), suggesting that these patients can effectively compensate for an excess release of [H+] from their exercising muscles.

Blood bicarbonate concentrations decreased only mildly and to a comparable extent during exercise and infusion of either substrate (data not shown). These findings do not rule out a relation between exercise endurance and a decrease in intramuscular pH. The MRS data obtained from forearm muscle during exercise showed no significant differences in chemical shift of Pi resonance (reflecting differences in intramuscular pH; data not shown) between the glucose and triacylglycerol infusions, however. Thus, we can only speculate that the mechanism responsible for the early termination of leg exercise during the glucose infusion in these patients was the early depletion of muscle glycogen stores. In healthy subjects, carbohydrate administration during exercise does not spare muscle glycogen stores (14–16); in contrast, triacylglycerol infusion is associated with significant sparing of muscle glycogen during exercise (17–21). Most of these studies, however, involved exercise at moderate-to-intense exercise levels ({approx}70% O2max, corresponding to {approx}60% Wmax in healthy individuals). Although we considered the exercise intensity of 15% Wmax to be low and sustainable for a prolonged period in our myopathic patients, their relatively high O2 rates (55% of O2max on average) and respiratory exchange ratios during exercise suggest that this exercise level was relatively intense for our patients. (In the control subjects, exercise intensity levels correlated with carbohydrate oxidation rates, and thus with respiratory exchange ratios). If we then assume, on the basis of observations in healthy subjects at higher intensity levels (70% O2max), that the effects of glucose and triacylglycerol infusion on muscle glycogen stores during exercise were similar in our patients, the possibility that glycogen stores are depleted faster during glucose infusion than during triacylglycerol infusion seems likely.

31P MRS measurements of PCr-Pi ratios during forearm exercise
The finding in CID patients of no significant differences in PCr-Pi ratios during exercise between the triacylglycerol and glucose infusion studies—despite improvements in endurance time, O2, and lactate metabolism in the patients during the triacylglycerol infusion (compared with glucose infusion)—is explained by the different exercise protocols used. The tracer infusion and respiratory calorimetry studies involved steady state cycling at a low intensity level chosen to be endurable for a prolonged period of time (>0.5 h). The 31P-MRS measurements were made in forearm muscle during submaximal exercise at increasing workloads designed to fatigue the muscle in a relatively short period of time.

Conclusion
In the present study, the triacylglycerol infusion was associated with higher mitochondrial substrate oxidation rates and lower plasma lactate concentrations and lactate+pyruvate Ra in myopathic CID patients during exercise than was the glucose infusion. These findings were associated with improved exercise endurance times during triacylglycerol infusion in most of the patients. However, PCr-Pi ratios measured in a forearm muscle during submaximal exercise were not significantly different in the patients during the 2 infusion studies. Thus, despite the beneficial effect of triacylglycerol on mitochondrial oxidative phosphorylation and muscle function during exercise, this macronutrient does not seem to change the muscle's energy state.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Roef MJ, de Meer K, Reijngoud D-J, et al. Triacylglycerol infusion does not improve hyperlactemia in resting patients with mitochondrial myopathy due to complex I deficiency. Am J Clin Nutr 2002; 75:228–36.[Abstract/Free Full Text]
  2. Gulmans VAM, de Meer K, Brackel HJL, Helders PJM. Maximal work capacity in relation to nutritional status in children with cystic fibrosis. Eur Respir J 1997;10:2014–7.[Abstract]
  3. Knops N, Wulffraat N, Lodder S, Houwen R, de Meer K. Resting energy expenditure and nutritional status in children with juvenile rheumatoid arthritis. J Rheumatol 1999;26:2039–43.[Medline]
  4. Jeneson JAL, Westerhoff HV, Brown TR, van Echteld CJA, Berger R. Quasi-linear relationship between Gibbs free energy of ATP hydrolysis and power output in human forearm muscle. Am J Physiol 1995;268:C1474–84.[Abstract/Free Full Text]
  5. Jeneson JAL, van Dobbenburgh JO, van Echteld CJA, et al. Experimental design of 31P MRS assessment of human forearm function: restrictions imposed by functional anatomy. Magn Reson Med 1993; 30:634–40.[Medline]
  6. Tserng K, Kalhan SC. Calculation of substrate turnover rate in stable isotope tracer studies. Am J Physiol 1983;245:E308–11.[Abstract/Free Full Text]
  7. Steele R. Influence of glucose loading and of injected insulin on hepatic glucose output. Ann N Y Acad Med Sci 1959;82:420–30.
  8. Wolfe RR, Jahoor F, Miyoshi H. Evaluation of the isotopic equilibration between lactate and pyruvate. Am J Physiol 1988;254:E532–5.[Abstract/Free Full Text]
  9. Zhang X, Baba H, Wolfe RR. Further evaluation of isotopic equilibration between labeled pyruvate and lactate. J Nutr Biochem 1993;4:218–21.
  10. Large V, Soloviev M, Brunengraber H, Beylot M. Lactate and pyruvate isotopic enrichments in plasma and tissues of postabsorptive and starved rats. Am J Physiol 1995;268:E880–8.[Abstract/Free Full Text]
  11. Searle GL, Cavalieri RR. Determination of lactate kinetics in the human analysis of data from single injection vs. continuous infusion methods. Proc Soc Exp Biol Med 1972;139:1002–6.[Medline]
  12. Foster DM, Hetenyi G Jr, Berman M. A model for carbon kinetics among plasma alanine, lactate, and glucose. Am J Physiol 1980; 239:E30–8.[Abstract/Free Full Text]
  13. Vollestad NK, Sejersted OM. Biochemical correlates of fatigue. Eur J Appl Physiol 1988;57:336–47.
  14. Coyle EF, Coggan AR, Hemmert MK, Ivy JL. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J Appl Physiol 1986;61:165–72.[Abstract/Free Full Text]
  15. Hargreaves M, Briggs CA. Effect of carbohydrate ingestion on exercise metabolism. J Appl Physiol 1988;65:1553–5.[Abstract/Free Full Text]
  16. Bosch AN, Dennis SC, Noakes TD. Influence of carbohydrate ingestion on fuel substrate turnover and oxidation during prolonged exercise. J Appl Physiol 1994;76:2364–72.[Abstract/Free Full Text]
  17. Dyck DJ, Putman CT, Heigenhauser GJF, Hultman E, Spriet LL. Regulation of fat-carbohydrate interaction in skeletal muscle during intense aerobic cycling. Am J Physiol 1993;265:E852–9.[Abstract/Free Full Text]
  18. Vukovich MD, Costill DL, Hickey MS, Trappe SW, Cole KJ, Fink WJ. Effect of fat emulsion infusion and fat feeding on muscle glycogen utilization during cycle exercise. J Appl Physiol 1993;75:1513–8.[Abstract/Free Full Text]
  19. Romijn JA, Coyle EF, Sidossis LS, Zhang X-J, Wolfe RR. Relationship between fatty acid delivery and fatty acid oxidation during strenuous exercise. J Appl Physiol 1995;79:1939–45.[Abstract/Free Full Text]
  20. Dyck DJ, Peters SJ, Wendling PS, Chesley A, Hultman E, Spriet LL. Regulation of muscle glycogen phosphorylase activity during intense aerobic cycling with elevated FFA. Am J Physiol 1996;270:E116–25.[Abstract/Free Full Text]
  21. Odland LM, Heigenhauser GJF, Wong D, Hollidge-Horvat MG, Spriet LL. Effects of increased fat availability on fat-carbohydrate interaction during prolonged exercise in men. Am J Physiol 1998; 274:R894–902.
Received for publication December 18, 2000. Accepted for publication April 6, 2001.




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M. J. Roef, K. de Meer, S. C. Kalhan, H. Straver, R. Berger, and D.-J. Reijngoud
Gluconeogenesis in humans with induced hyperlactatemia during low-intensity exercise
Am J Physiol Endocrinol Metab, June 1, 2003; 284(6): E1162 - E1171.
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