Reduced bioenergetic function in immune cells in ME/CFS
The fatigue in ME/CFS is not a metaphor or a mood state. A 2020 study by Tomas and colleagues published in PLOS ONE demonstrates it with direct cellular measurement: immune cells taken from ME/CFS patients produce less energy than immune cells from healthy controls, measurably and reproducibly, when their mitochondrial function is tested in real time. The finding establishes that cellular energy impairment exists at a level that is detectable in blood. But the more important clinical question — what is causing these cells to operate at reduced energy output? — points not toward primary mitochondrial disease but toward the upstream environment in which these cells are functioning. Impaired oxygen delivery from chronic circulatory and orthostatic dysfunction is a leading candidate, and the Tomas data should be read in that context.
The Measurement: Directly Assessing Cellular Energy Production
The study isolated peripheral blood mononuclear cells — PBMCs, a mixed population of immune cells including lymphocytes and monocytes — from ME/CFS patients and healthy controls. PBMCs can be obtained from standard blood draws, making them accessible for research without requiring tissue biopsy or specialized collection procedures.
Mitochondrial function in the isolated cells was measured using a Seahorse XF analyzer, an instrument that quantifies real-time oxygen consumption rates and extracellular acidification rates in live cells under controlled conditions. By sequentially adding agents that block or stimulate specific parts of the mitochondrial respiratory chain, the Seahorse protocol can distinguish basal respiration, ATP production, maximal respiratory capacity, and the spare respiratory capacity that cells hold in reserve for high-demand situations. This is a direct measurement of cellular energy production — not inferred from how patients feel, not estimated from indirect markers, but measured from how much oxygen the cells consume when asked to work.
The results: ME/CFS patient PBMCs showed significantly reduced basal respiration compared to controls — the cells were consuming less oxygen under baseline conditions. Maximal respiration capacity was reduced — the cells could not generate as much energy when driven to their maximum rate. ATP production was reduced — the functional output of the mitochondrial process was lower. Spare respiratory capacity, the buffer that cells draw on under physiological stress, was also reduced. Across every measured parameter of mitochondrial performance, ME/CFS cells were operating below control values.
What Reduced Mitochondrial Function in Immune Cells Actually Means
The finding that immune cells specifically show reduced energy production is notable because immune cell function is energetically expensive. Mounting an immune response, maintaining immune surveillance, and managing inflammatory signaling all require substantial ATP. Immune cells with reduced mitochondrial capacity are not just less energetically efficient in an abstract sense — they are less capable of performing the functions that require energy.
This connects to the fatigue and cognitive impairment experienced in ME/CFS through multiple pathways. The immune system and the brain are in continuous communication through cytokine signaling. Immune cells with reduced energy production generate different patterns of cytokine release — they are biologically different from normally-functioning immune cells, and the signals they send to the brain and nervous system reflect that difference. The sickness behavior that accompanies immune activation — fatigue, cognitive slowing, motivational reduction, social withdrawal — is a normal response to the cytokine signals healthy immune cells send during an infection. What happens when immune cells are chronically functioning in a reduced-energy state, sending abnormal cytokine profiles, is less well characterized but is mechanistically plausible as a contributor to the neurological symptom burden in ME/CFS.
The Upstream Question: What Is Forcing the Low-Energy State?
Mitochondrial respiration is exquisitely sensitive to cellular oxygen levels. Mitochondria produce ATP through oxidative phosphorylation, which requires oxygen as the terminal electron acceptor. When oxygen delivery to cells is reduced — when circulating blood is not delivering adequate oxygen to tissues — mitochondria cannot sustain their normal respiration rate and down-regulate their aerobic capacity in response. This is a normal adaptive response to hypoxic conditions: the cell shifts toward less oxygen-dependent energy production (glycolysis) and reduces its overall energy output to match the reduced oxygen availability.
The orthostatic and perfusion impairment documented extensively in ME/CFS provides a physiological mechanism for chronic cellular oxygen underdelivery. CPET studies establish that ME/CFS involves impaired cardiac output augmentation during exertion — the cardiovascular system fails to adequately expand oxygen delivery when metabolic demand increases. Systematic review data establish that cerebral blood flow is significantly reduced in ME/CFS and is further reduced in patients with comorbid orthostatic intolerance. The cells of patients living with chronic positional cerebral hypoperfusion and exercise-induced delivery failure are repeatedly experiencing oxygen delivery that falls below metabolic demand.
Cells that experience repeated hypoxia adapt over time by down-regulating their mitochondrial capacity — a process that involves epigenetic changes in mitochondrial gene expression and structural remodeling of the respiratory chain. This adaptation may explain why the mitochondrial impairment in ME/CFS PBMCs is measurable even when those cells are taken out of the body and placed in a controlled in vitro environment with normal oxygen levels: the cells have adapted their energy machinery to the chronically low-oxygen delivery they have been experiencing in vivo. The Seahorse measurement is capturing the cellular memory of that adaptation, not just the in vitro oxygen conditions at the time of measurement.
The Metabolic Fingerprint Connection
The mitochondrial findings in PBMCs connect directly to the metabolomic findings in the Naviaux 2016 metabolomics study. The 20 metabolic pathway abnormalities identified in ME/CFS plasma included disruptions in sphingolipid metabolism, fatty acid oxidation, and nucleotide synthesis — pathways that are directly regulated by mitochondrial function and that shift characteristically when cells move toward reduced aerobic, high-glycolytic metabolic states. The two findings are consistent with each other: a population of cells with reduced mitochondrial capacity would produce exactly the kind of metabolic signature that the Naviaux untargeted analysis found.
Both findings — metabolomics and mitochondrial bioenergetics — describe the same biological state from different angles. They are not independent evidence of two separate problems. They are converging measurements of a single problem: cells operating in a reduced-energy, reduced-aerobic, stress-adapted metabolic mode. The question both findings point toward is the same: what upstream conditions are forcing that adaptation?
What This Means for the Patient and Clinician
For patients who have been told their fatigue is not measurable or not biological: the Tomas data provide direct cellular evidence that it is. Mitochondria in immune cells taken from a blood draw are producing less energy, measured in real time with instruments that do not respond to subjective report. This is not inferred from how tired you feel. It is measured from how much oxygen your cells consume.
For clinicians and researchers, the important direction this paper points is upstream. Treating the mitochondrial impairment in isolation — with mitochondrial supplements, for instance — without addressing the oxygen delivery environment in which those mitochondria are functioning is addressing the adaptation rather than what the cells are adapting to. If the cells are in a reduced-energy state because they have adapted to chronic oxygen underdelivery from orthostatic and circulatory dysfunction, restoring adequate oxygen delivery may be what allows mitochondrial function to normalize. The cellular energy finding should direct attention to the circulatory and autonomic systems that regulate oxygen delivery — the same systems that the orthostatic physiology literature has been documenting as impaired in ME/CFS for more than two decades.