Lactate Kinetics and Metabolic Flexibility
Lactate is not merely a metabolic waste product but a signaling molecule and alternative fuel substrate. In ME/CFS, early lactate accumulation at low workloads—documented by reduced anaerobic threshold on CPET (Keller et al. 2024) —reflects impaired metabolic flexibility: the inability to match fuel oxidation to demand. A lactate kinetics model captures this dysfunction explicitly.
1 Lactate Production and Clearance
Lactate (\(L\)) is produced from pyruvate by lactate dehydrogenase (LDH) when pyruvate supply exceeds mitochondrial oxidative capacity, and cleared by hepatic gluconeogenesis (Cori cycle) and oxidation in heart and slow-twitch muscle:
\[ \frac{d[L]}{d t} = v_{\text{LDH}} \cdot \frac{[\text{Pyruvate}]}{K_{\text{LDH}} + [\text{Pyruvate}]} \cdot \frac{[\text{NADH}]}{K_{\text{NADH,LDH}} + [\text{NADH}]} - v_{\text{clear}} \cdot \frac{[L]}{K_{\text{clear}} + [L]} \tag{1}\]
where \(v_{\text{LDH}}\) is the maximal LDH forward rate and \(v_{\text{clear}}\) is the maximal lactate clearance rate. The NADH dependence ensures that LDH activity increases when the mitochondrial NADH/\(\text{NAD}^\text{+}\) ratio is elevated—precisely the condition produced by impaired ETC function. In ME/CFS, the model predicts elevated steady-state lactate through two mechanisms: increased production (high cytosolic NADH due to ETC bottleneck) and potentially reduced clearance (hepatic energy deficit limiting gluconeogenesis).
2 Metabolic Flexibility Model
Metabolic flexibility—the capacity to switch between glucose and fatty acid oxidation according to substrate availability and demand—is governed by the Randle cycle (glucose–fatty acid cycle). The fuel selection ratio \(\Phi\) determines the fraction of ATP derived from fatty acid \(\beta\)-oxidation versus glycolysis:
\[ \Phi = \frac{J_{\beta\text{-ox}}}{J_{\beta\text{-ox}} + J_{\text{glycolysis}}} = \frac{v_{\text{CPT-I}} \cdot [\text{Acyl-CoA}] / (K_{\text{CPT}} + [\text{Acyl-CoA}])}{v_{\text{CPT-I}} \cdot [\text{Acyl-CoA}] / (K_{\text{CPT}} + [\text{Acyl-CoA}]) + J_{\text{glycolysis}}} \tag{2}\]
where CPT-I (carnitine palmitoyltransferase I) is the rate-limiting enzyme for fatty acid entry into mitochondria, inhibited by malonyl-CoA (an intermediate of fatty acid synthesis that signals energy surplus). The respiratory exchange ratio (RER), measurable by CPET, is a direct readout of \(\Phi\): RER \(= 1.0\) indicates pure carbohydrate oxidation (\(\Phi = 0\)), RER \(= 0.7\) indicates pure fat oxidation (\(\Phi = 1\)).
In ME/CFS, impaired metabolic flexibility manifests as: (1) higher resting RER (reduced fat oxidation at rest), (2) earlier transition to RER \(> 1.0\) during exercise (premature reliance on anaerobic glycolysis), and (3) slower RER recovery post-exercise. The model attributes this inflexibility to reduced CPT-I activity (limiting fat entry into mitochondria), \(\text{NAD}^\text{+}\) depletion (reducing \(\beta\)-oxidation flux), and impaired AMPK signaling (the master metabolic sensor that normally promotes fat oxidation during energy deficit).
Lactate itself feeds back on metabolism through the lactate receptor GPR81 (hydroxycarboxylic acid receptor 1), which inhibits lipolysis and reduces free fatty acid availability. This creates a secondary positive feedback loop—a result visible only through the mathematical model: impaired oxidative capacity \(->\) lactate accumulation \(->\) GPR81 activation \(->\) reduced fatty acid supply \(->\) further reliance on glycolysis \(->\) more lactate. Verbal reasoning identifies each link individually but cannot determine whether the loop is self-sustaining or self-limiting; the ODE model resolves this by computing the loop gain, which exceeds unity when \(\alpha_{\text{CI}} < 0.65\), predicting a bistable metabolic state.
\[ [\text{FFA}]_{\text{available}} = [\text{FFA}]_{\text{basal}} \cdot \frac{K_{\text{GPR81}}}{K_{\text{GPR81}} + [L]} \tag{3}\]
3 Carnitine Dynamics
Free carnitine (\(\text{Car}_f\)) is required for CPT-I-mediated fatty acid transport. Acylcarnitine accumulation—documented in ME/CFS metabolomics (Germain et al. 2020) —sequesters the carnitine pool:
\[ [\text{Car}_f] + [\text{Acyl-Car}] = \text{Car}_{\text{total}}, \quad \frac{d[\text{Acyl-Car}]}{d t} = J_{\beta\text{-ox,entry}} - J_{\beta\text{-ox,completion}} \tag{4}\]
When \(\beta\)-oxidation completion is impaired (due to \(\text{NAD}^\text{+}\) depletion or CoA sequestration), acylcarnitines accumulate, depleting free carnitine and further limiting fat oxidation. This provides a mechanistic rationale for L-carnitine supplementation and predicts—uniquely from the model—that its efficacy depends on whether the primary bottleneck is carnitine availability (responsive) or downstream \(\beta\)-oxidation capacity (less responsive). Sensitivity analysis distinguishes these regimes: when \(\partial J_{\beta\text{-ox}} / \partial [\text{Car}_f] >> \partial J_{\beta\text{-ox}} / \partial v_{\beta\text{-ox}}\), carnitine supplementation is predicted to help; when the inequality reverses, \(\text{NAD}^\text{+}\) repletion is the higher-priority intervention.