Insulin and Glucose Metabolism
Glucose metabolism abnormalities in ME/CFS connect endocrine dysfunction directly to the cellular energy deficits discussed in Chapter Energy Metabolism and Mitochondrial Function. Insulin regulates glucose uptake into cells, coordinates switching between glucose and fat oxidation, affects mitochondrial function and ATP production, and modulates inflammatory responses and immune function. Dysregulation of insulin signaling and glucose metabolism therefore has cascading effects on multiple systems compromised in ME/CFS.
1 Metabolic Syndrome and Insulin Resistance
A population-based case-control study by Maloney et al. (2010) examined the relationship between ME/CFS and metabolic syndrome in Georgia (Maloney et al. 2010). The analysis revealed that ME/CFS patients were approximately 2-fold more likely to have metabolic syndrome compared to controls (odds ratio 2.12, 95% confidence interval 1.06–4.23). The key discriminating factors were increased waist circumference, elevated triglycerides, and higher fasting glucose. Notably, each additional metabolic syndrome component present associated with a 37% increase in the likelihood of having ME/CFS, demonstrating a dose-response relationship.
Metabolic syndrome comprises a cluster of abnormalities: abdominal obesity (increased waist circumference), elevated triglycerides, reduced HDL cholesterol, elevated blood pressure, and elevated fasting glucose or insulin resistance. The syndrome reflects underlying insulin resistance (reduced cellular responsiveness to insulin signaling) and predicts increased risk for type 2 diabetes, cardiovascular disease, and inflammatory conditions.
The association between ME/CFS and metabolic syndrome raises important mechanistic questions about causality. Does insulin resistance contribute to ME/CFS pathophysiology, or does ME/CFS-related inflammation, inactivity, and mitochondrial dysfunction lead to insulin resistance? Likely, bidirectional relationships exist, with each condition exacerbating the other in a self-reinforcing cycle.
2 Metabolic Phenotypes and Insulin Dynamics
Hoel et al. (2021) (Hoel et al. 2021) employed comprehensive metabolomics to identify distinct ME/CFS subtypes with different metabolic signatures. One subtype (ME-M2) demonstrated elevated triglyceride and insulin levels despite normal glucose, reflecting low-grade lipid-induced insulin resistance. ME/CFS patients overall showed slightly elevated insulin and leptin (an adipose tissue hormone signaling energy status) and lower high molecular weight adiponectin (an anti-inflammatory adipokine that enhances insulin sensitivity), confirming metabolic heterogeneity within ME/CFS.
3 Appetite Dysregulation and Hunger Signaling
Appetite control in ME/CFS is disrupted through multiple interacting pathways that connect energy sensing, metabolic state, and neuroendocrine signaling. Understanding these mechanisms is clinically important because abnormal appetite patterns can drive body composition changes (see Chapter Additional Symptoms and Manifestations) and complicate medication management.
AMPK as a Cellular Energy Sensor. AMP-activated protein kinase (AMPK) is a master cellular energy sensor that is activated when the cellular ATP:AMP ratio falls—precisely the condition present in ME/CFS due to mitochondrial dysfunction and impaired oxidative phosphorylation. AMPK activation suppresses anabolic processes and drives compensatory responses including hunger signaling (to seek more fuel) and metabolic switching. However, chronic AMPK activation in ME/CFS—reflecting persistent cellular energy deficit—may produce paradoxical downstream effects: while AMPK normally promotes fat oxidation, impaired downstream fatty acid oxidation enzymes (ACAD, CPT; see Chapter Energy Metabolism and Mitochondrial Function) prevent this compensation from succeeding (Maya et al. 2023). The cellular hunger signal is generated but cannot be satisfied by the intended metabolic response.
Leptin Resistance. Leptin is an adipokine secreted by fat cells that signals satiety to the hypothalamus, suppressing appetite and increasing energy expenditure proportional to fat stores. Multiple metabolic abnormalities in ME/CFS converge to impair leptin signaling:
- Elevated leptin levels have been documented in ME/CFS patients, particularly in the insulin-resistant metabolic subtype, consistent with leptin resistance (where high circulating leptin fails to produce adequate satiety signaling) (Hoel et al. 2021)
- Chronic low-grade inflammation, a feature of ME/CFS, impairs hypothalamic leptin receptor signaling through JAK-STAT pathway suppression
- The resulting leptin resistance may drive continued fat accumulation despite elevated leptin levels, contributing to the sarcopenic obesity pattern documented in ME/CFS
Phase-Dependent Appetite Changes. Clinical observation suggests that appetite in ME/CFS changes characteristically across disease phases, though formal prospective data are limited:
Appetite may follow a phase-dependent pattern in ME/CFS. During acute exacerbations and PEM episodes, appetite suppression is common—driven by elevated inflammatory cytokines (IL-6, TNF-\(\alpha\)) acting on hypothalamic appetite centers, nausea from autonomic dysfunction, and orthostatic symptoms that worsen with eating. During stable periods, appetite may normalize or increase, reflecting the body’s attempt to restore depleted energy stores. In severe ME/CFS, a distinct pattern of reduced appetite with weight loss may reflect hypermetabolism or severe mitochondrial dysfunction preventing adequate energy extraction from food. These phase-dependent changes mean that appetite complaints in ME/CFS require contextualization within current disease state rather than treatment as a fixed symptom.
Falsified if prospective dietary tracking across disease phases shows no consistent relationship between disease severity/phase and appetite/food intake patterns.
Certainty: 0.35
Falsifiability: weakly — Falsified if glucose clamp studies show normal peripheral insulin sensitivity combined with normal cerebral glucose uptake on FDG-PET imaging
ME/CFS may involve a paradoxical state of peripheral insulin resistance (reduced glucose uptake in muscle and adipose tissue) combined with inadequate glucose delivery or utilization in the central nervous system. This would explain the constellation of elevated peripheral insulin levels reflecting compensatory hyperinsulinemia, cerebral glucose hypometabolism documented by PET imaging, symptoms resembling hypoglycemia without documented low blood glucose, and the energy deficit despite apparently adequate systemic glucose availability (Tirelli et al. 1998) (Siessmeier et al. 2003).
4 Cerebral Glucose Hypometabolism
Positron emission tomography (PET) studies using fluorodeoxyglucose (FDG) tracer have documented reduced cerebral glucose metabolism in ME/CFS patients. Tirelli et al. (1998) identified significant glucose hypometabolism in the right mediofrontal cortex (p=0.010) and brainstem (p=0.013) in ME/CFS patients compared to controls, with moderate hypometabolism in the pons (Tirelli et al. 1998). Siessmeier et al. (2003) employed observer-independent analysis in 26 ME/CFS patients and found that 12/26 (46%) showed hypometabolism bilaterally in the cingulate gyrus and adjacent mesial cortical areas, with 5 also demonstrating decreased orbitofrontal metabolism (Siessmeier et al. 2003). Importantly, hypometabolism correlated with anxiety and depression measures but not directly with fatigue severity, suggesting complex relationships between metabolic deficits and symptom expression.
Two independent PET studies (Tirelli 1998, n=18; Siessmeier 2003, n=26) both found cerebral hypometabolism, but in partially different brain regions. Both studies are old (pre-2005) with small samples and limited by PET methodology of that era. Modern replication with current PET technology and larger cohorts is needed.
The consistent documentation of reduced cerebral glucose uptake across independent studies using rigorous quantitative methods provides objective evidence of brain energy deficit in ME/CFS (Zhu et al. 2025). The affected regions—brainstem nuclei, cingulate cortex, prefrontal areas—overlap substantially with regions showing functional abnormalities in the NIH study and with brain networks subserving autonomic control, attention, emotion regulation, and effort-based decision-making (Walitt et al. 2024).
Several mechanisms could produce cerebral hypometabolism beyond simple insulin resistance:
Falsifiability: weakly — Falsified if cerebral perfusion is normal on SPECT, glucose transporters show normal expression, and neuronal mitochondria are intact in patients with documented hypometabolism
Multiple factors likely contribute to reduced brain glucose utilization. Cerebral hypoperfusion documented by SPECT imaging reduces glucose delivery to neurons. Glucose transporter dysfunction (GLUT1 at blood-brain barrier, GLUT3 in neurons) may impair glucose uptake even when delivery is adequate (Xu et al. 2026). Mitochondrial dysfunction in neurons reduces the capacity to metabolize glucose to ATP, causing glucose accumulation rather than utilization. Neuroinflammation with activated microglia alters brain energetics, as activated immune cells in the brain preferentially utilize glucose via glycolysis rather than oxidative phosphorylation. Finally, reduced neuronal activity secondary to other ME/CFS-related dysfunction may reduce metabolic demand, causing secondary hypometabolism as a consequence rather than cause of neurological symptoms.
5 Hypoglycemia Symptoms Versus Orthostatic Intolerance
Many ME/CFS patients report symptoms they attribute to “hypoglycemia”: nausea, lightheadedness and faintness, sweating and tremor, weakness and malaise, and cognitive impairment. However, studies examining ME/CFS patients during symptomatic episodes have shown that these symptoms frequently occur without documented hypoglycemia (blood glucose <70 mg/dL). Instead, the symptoms typically reflect orthostatic intolerance (inadequate blood pressure and cerebral perfusion upon standing or with prolonged upright posture, discussed extensively in Chapter Cardiovascular Dysfunction). Critically, when orthostatic intolerance is treated effectively, the “hypoglycemia” symptoms often improve despite no specific glucose intervention.
This misattribution has important clinical implications. ME/CFS patients may consume frequent snacks or high-carbohydrate meals attempting to prevent “hypoglycemia,” potentially exacerbating insulin resistance and weight gain. Additionally, focusing on blood sugar management diverts attention from the actual orthostatic problem requiring different interventions (increased salt and fluid intake, compression garments, medications affecting blood pressure or blood volume).
6 Integration With Energy Metabolism Dysfunction
The glucose metabolism abnormalities documented in this section connect directly to the mitochondrial dysfunction and cellular energy deficits discussed in Chapter Energy Metabolism and Mitochondrial Function. Insulin resistance reduces glucose uptake into cells, limiting substrate availability for ATP production. Impaired mitochondrial function reduces the capacity to oxidize glucose efficiently, causing metabolic bottlenecks. The resulting cellular energy deficit triggers compensatory responses including increased reliance on glycolysis (less efficient ATP production), activation of AMP-activated protein kinase (AMPK) signaling cellular energy stress, and metabolic shifts toward fat oxidation when glucose utilization fails.
These metabolic derangements help explain post-exertional malaise, as exertion depletes limited ATP stores that cannot be rapidly replenished due to impaired glucose metabolism and mitochondrial dysfunction. The delayed recovery characteristic of PEM reflects the slow restoration of cellular energy status when metabolic pathways remain compromised.