Chronic fatigue syndrome/myalgic encephalopathy (CFS/ME) as a syndrome was defined in consensus meetings by Fukuda et al [¹]. Fatigue was the major criterion in the definition. It was described as suddenly occurring, not explained, not caused by exercise, insufficiently relieved by rest and causing a major reduction in physical capacity. The additional symptoms of the syndrome were headache, pain in muscles and joints, unrefreshing sleep, postexertional malaise, sore throat, painful lymph glands and insufficient concentration. The combination of fatigue and four or more of the additional symptoms lasting at least for 6 months, sufficed for the diagnosis of CFS/ME. The main exclusion criterion for CFS/ME, was the presence of a disease that is generally accepted as an actual cause of fatigue.

Several groups of investigators assume that a defective oxidative phosphorylation and subsequent free radical production and oxidative stress play an important role in the pathophysiology of CFS/ME [^2-11]. A well accepted way to test ATP synthesis under increased work rate is the cardiopulmonary exercise test (CPET) [^12-16]. The ATP synthesis is measured indirectly by testing for oxygen uptake (V'O₂) as a measure for oxygen consumption (Q'O₂) in an exercise protocol. Q'O₂ can be restricted when mitochondria are insufficiently active, or by a restricted supply line of oxygen that consists of the lungs, both ventilation and perfusion, the heart pump, the blood vessels and the hemoglobin concentration in the blood. Modern equipment and algorisms suggested the exclusion of these latter causes of an inadequate Q'O₂, and supported the likelihood of inactivity of the mitochondrial oxidative phosphorylation. This was also suggested by the finding of a decreased anaerobic threshold in the CFS/ME patients, which is determined by CPET. The anaerobic threshold is the rate of oxygen consumption, when the work rate is reached at which blood lactic acid starts to accumulate, and is due to ATP synthesis from anaerobic glycolysis in muscles. In patients with defects in the oxidative phosphorylation, the anaerobic threshold is also reached at a lower oxygen consumption than in controls [¹⁷,¹⁸].

In this study we compared the CPET [¹⁹] with a direct assay of the oxidative phosphorylation in peripheral mononuclear cells (PBMC), attempting to prove an abnormality of this process in the patients. After 24 h these tests were repeated.

Patients who visited the CFS/ME Clinic Amsterdam and healthy sedentary controls were invited for the study. All patients fulfilled the criteria of Fukuda et al [¹] for CFS/ME and reported the start of symptoms after an infectious disease. Exclusion criteria were according to Fukuda et al [¹]. Contra indications for the CPET were mainly cardiac diseases, hypertension, or the inability to perform the exercise as in arthrosis of the knee. Medication was discontinued 2 weeks before the first test. All subjects performed a CPET on a cycle ergometer (Excalibur, Lode, Groningen, The Netherlands) according to our protocol: 3 min without activity, 3 min of unloaded pedaling, followed by pedaling against increasing resistance until exhaustion (RAMP protocol) and ended by 3 min pedaling with low resistance. The rate of work rate increase was estimated from history, physical examination, gender, weight and height. The participants performed symptom limited exercise tests as described by Wassermann et al. [¹³]. Verbal encouragement to perform maximally was used during the last phase of incremental exercise. Exhaustion of the leg muscles was the limiting symptom in all participants. The V'E, V'O₂, V'CO2 and oxygen saturation were continuously measured (Metasoft). The ECG was continuously recorded and blood pressure was measured every 2 min. The CPET was repeated after 24 h. The Respiratory Exchange Rate (RER) was used for validation of the repeated CPET.

The exercise ECG of the subjects was analyzed (by FCV). The anaerobic threshold was determined by the V-slope method.

The participants completed questionnaires among others (not shown) about additional symptoms of CFS/ME (Centers for Disease Control and Prevention Symptoms Inventory - Dutch Language Version (CDC Symptom Inventory-DLV)) [²⁰]. The criterion for fatigue was that at least 4 CFS/ME symptoms must be ≥ 7.5 [²⁰].

All subjects were seen and an ECG was approved by the internist (RMK).

The results of the tests were not available to the participants or the investigators until after the last test was performed by the participant.

Before entry into the study, the nature of the study was explained to the participants and written consent was obtained. The STEG independent ethics committee approved the study. The trial was conducted in accordance with the Declaration of Helsinki (1996 revision) and under the principals of good clinical practice, as laid out in the International Conference on Harmonization document Good Clinical Practice Consolidated Guideline.

At rest the cardiopulmonary exercise test 1 and 2 showed no difference between patients and controls. Increasing work rate made the differences obvious. The lower V'O2 at the anaerobic threshold indicated that the difference in V'O2 at maximal work rate was not due to a reduced willingness to perform in the CFS/ME group.

The FEV1 and the FVC were not different, but the higher ventilatory equivalent for CO₂ at the anaerobic threshold indicated the possibility of a ventilation-perfusion mismatch in the patient group. The reproducibility of CPET was high, relatively poor performers at the first test ranked low in the second test too. There were significant differences between the patients and controls. Based on the oxygen uptake test, the patients not only performed worse than controls in the first test, but the recovery after 24 h was not completed in this group as well. This indicates an impaired recovery [²⁵], as expressed in the criterion "postexertional malaise" of the CDC Symptom score.

A limited mitochondrial ATP synthesis was the working hypothesis for this investigation. This is probably not true, as the energy production can be limited by other mechanisms as well. The exercise tests with increased work load suggested the possibility that the mitochondrial ATP synthesis was decreased, because the anaerobic threshold was reached. Then the mitochondria were not longer able to produce sufficient ATP to sustain the exercise, and the anaerobic glycolysis in muscle had to produce the extra ATP needed, which is reflected by the lactate production. This is also the case in patients with defects in the oxidative phosphorylation. Peripheral blood mononuclear cells are commonly used to assess the gene expression in CFS/ME [^26-34], and the expressions of various genes involved in mitochondrial protein synthesis, energy metabolism, and in free radical metabolism were found to be changed. The results of the present study do not support a physiological effect of these changes, and demonstrated that the oxidative phosphorylation in PCMB of CFS/ME patients is fully normal. And it is likely that also their muscle mitochondria are normal, since 24 h after strenuous exercise CK did not leak to the blood, as is the case in patients with defective oxidative phosphorylation.

A recent publication [⁶] claimed to have found a defective oxidative phosphorylation in neutrophils of CFS/ME patients, but the flux through this process had not been measured. These investigators performed a so called "ATP profile" test, and determined ATP under five different conditions, and the sum of these was found to be abnormal in 70 of 71 patients. One of us (WS) was involved in an investigation that clearly showed that neutrophils do not catalyze oxidative phosphorylation and the remaining complexes of the respiratory chain maintain the mitochondrial membrane potential [³⁵]. Their mitochondria are only active in apoptosis [³⁶].

In line with the present work, Mathew et al [³⁷] discovered by proton magnetic resonance spectroscopy imaging that ventricular cerebrospinal lactate was 3.5-fold increased in CFS/ME patients. McCully and Natelson [³⁸] demonstrated by combined near-infrared spectroscopy and ³¹P magnetic resonance spectroscopy that during exercise the oxygen delivery to skeletal muscle was delayed. Neary et al [³⁹] established by near-infrared spectrometry that the oxygenation of the prefrontal brain lobe was decreased in exercising patient to 67% of that in controls, confirming the results of a study by Streeten and Bell [⁴⁰] and Hurwitz et al [⁴¹] and in line with a decreased blood volume in CFS patients.

The decrease in mitochondrial ATP production at increasing work rate, detected by the CPET tests in the present well-characterized though small group of CFS/ME patients, is a secondary phenomenon. This was shown by the normality of the oxidative phosphorylation in peripheral blood mononuclear cells. The chain of mechanisms that couple (external) pulmonary to (internal) cellular respiration showed no abnormal differences in this study between CFS patients and healthy controls at the pulmonary, cardiac and circulatory level. Two possible explanations for the insufficient energy production in CFS remained: a lower transport capacity of oxygen as in anemia or a mitochondrial insufficiency. We showed that the mitochondrial ATP production shows no defect. Then the conclusion must be that the transport capacity of oxygen is limited in CFS patients.

CDC: US Centers for Disease Control and Prevention; CFS/ME: chronic fatigue syndrome/myalgic encephalopathy; CK: creatine kinase; CPET: cardiopulmonary exercise test; CS: citrate synthase; FEV_1: forced expiratory volume in the first second; FVC: forced vital capacity; PBMC: peripheral blood mononuclear cells; V'CO₂: carbondioxyde output; V'E: minute ventilation; V'O₂: oxygen uptake; WR work rate

The authors declare that they have no competing interests.

All authors were involved in the set up of the experiments. The clinicians (RCWV, RMK, FCV) performed the clinical experiments. The blood separation and the biochemical assays were done by WS. The first drafts of the paper were written by RCWV and HRS. The final manuscript was read and approved by all authors.

This study was supported by the William Dircken grant from the "ME/CVS-Stichting Nederland". We thank Elly de Wit for expert biochemical assistance, Prof dr J Lindemans for the CK test and Otto Bauermann for fruitful discussions and support.