Lactate physiology and lactic acidosis

Contents

Normal Lactate Physiology

Lactate is produced continuously as a normal byproduct of anaerobic glycolysis — the conversion of pyruvate to lactate by lactate dehydrogenase (LDH). Despite producing approximately 1,500 mmol/day (0.8 mmol/kg/hour), blood lactate remains low (normal <2 mmol/L) because of the liver's enormous capacity for clearance.

Sources of lactate production (at rest):

Tissue Contribution
Skeletal muscle ~25%
Skin ~25%
Red blood cells ~20%
Brain ~20%
Gut ~10%

Lactate has a pKa of 3.86, meaning it exists almost entirely as the dissociated lactate anion at physiological pH. Lactate production does not cause acidosis. The LDH reaction (pyruvate + NADH + H⁺ → lactate⁻ + NAD⁺) consumes a proton. Lactate is a marker — and partly a buffer — of the underlying metabolic disturbance, not its cause.

The Cori Cycle

The Cori cycle is the metabolic pathway by which lactate produced in peripheral tissues is transported to the liver and converted back to glucose via gluconeogenesis.

Glucose → Pyruvate → Lactate    (peripheral tissues, anaerobic)
                ↓
Lactate → Pyruvate → Glucose    (liver, aerobic gluconeogenesis)
  • LDH catalyses the reversible interconversion: pyruvate⁻ + NADH + H⁺ ⇌ lactate⁻ + NAD⁺
  • The direction of this equilibrium is governed by the NAD⁺:NADH ratio: when NADH accumulates (e.g. hypoxia impairs mitochondrial re-oxidation), the ratio falls and the reaction shifts toward lactate production
  • Crucially, NAD⁺ regenerated by LDH is required by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) to sustain glycolysis — lactate production is therefore a survival mechanism, keeping ATP generation going when mitochondria are impaired
  • The intracellular lactate:pyruvate ratio is normally 10:1, reflecting the prevailing NAD⁺:NADH ratio
  • A raised lactate with a normal L:P ratio suggests increased glycolytic flux without mitochondrial dysfunction (e.g. beta-2 agonists, exercise)
  • A raised lactate with a raised L:P ratio (>25:1) suggests a reduced NAD⁺:NADH ratio — impaired mitochondrial NADH re-oxidation, as in hypoxia or PDH inhibition

Clearance organs:

  • Liver: primary site (~60%), via gluconeogenesis
  • Kidney: secondary (~30%), via direct excretion and gluconeogenesis
  • Heart: oxidises lactate directly as a fuel substrate

Lactate and pH

Why lactate does not cause acidosis

The traditional teaching — that each lactate ion is "accompanied by" a proton — is biochemically incorrect and has been overturned (Robergs et al., AJP, 2004). Understanding the correct mechanism matters for viva purposes.

The LDH reaction consumes a proton:

Pyruvate⁻ + NADH + H⁺ → Lactate⁻ + NAD⁺

Lactate and H⁺ are correlated, not causally linked. They are co-produced byproducts of the same high-flux glycolytic state, but lactate does not generate the H⁺.

Where the H⁺ actually comes from

The primary source of protons in lactic acidosis is ATP hydrolysis:

ATP⁴⁻ + H₂O → ADP³⁻ + HPO₄²⁻ + H⁺

In normal aerobic metabolism, these protons are recaptured by the mitochondria during oxidative phosphorylation (chemiosmotic coupling re-phosphorylates ADP, consuming the H⁺). When oxygen delivery is inadequate, the electron transport chain cannot re-oxidise NADH, oxidative ATP synthesis fails, and the protons from ATP hydrolysis accumulate — acidosis results.

Lactate accumulates in parallel as NADH builds up (NAD⁺:NADH ratio falls) and LDH is driven toward lactate production to regenerate the NAD⁺ that glycolysis requires. In this sense, lactate production is partially protective: it buys continued glycolytic ATP generation at the cost of pyruvate.

Glycolytic flux and pH feedback

The rate of glycolysis is sensitive to intracellular pH:

  • Acidosis suppresses glycolysis — a protective negative feedback
  • Alkalosis accelerates glycolysis — paradoxically, sodium bicarbonate therapy in hypoxic states can worsen lactate accumulation by driving more pyruvate into the LDH reaction

This underpins why correcting the underlying cause (restoring DO₂) matters more than buffering the pH.

Classification of Lactic Acidosis

Cohen and Woods Classification (1983)

The most widely used framework, despite its limitations:

Type Description Mechanism
Type A Tissue hypoperfusion/hypoxia ↓O₂ delivery → anaerobic glycolysis → ↑lactate production
Type B1 Associated with systemic disease Altered metabolism without frank hypoperfusion
Type B2 Drug- or toxin-induced Interference with mitochondrial function or glycolysis
Type B3 Inborn errors of metabolism Enzyme defects in pyruvate/lactate handling

Limitation: In clinical practice these categories frequently co-exist — septic shock, for example, involves both microvascular hypoperfusion (Type A) and mitochondrial dysfunction (Type B).

Mechanism-Based Classification (Phypers & Pierce, 2006)

More clinically useful as it points toward treatment:

Mechanism Examples
↑ Glycolysis from ATP depletion Shock, hypoxia, severe anaemia, CO poisoning
↑ Glycolysis from exogenous stimulation Beta-2 agonists, catecholamines, malignancy
Unregulated substrate entry Xylitol, sorbitol, fructose infusions
Pyruvate dehydrogenase (PDH) inhibition Thiamine deficiency, sepsis, inborn errors
Oxidative phosphorylation defects Cyanide, metformin, propofol, NRTIs, salicylates
Decreased clearance Hepatic failure, renal failure, ethanol

Causes by Category

Type A — Tissue Hypoperfusion

  • Shock (septic, cardiogenic, haemorrhagic, obstructive)
  • Regional ischaemia (mesenteric, limb)
  • Severe hypoxaemia
  • Severe anaemia
  • Carbon monoxide poisoning

Type B1 — Systemic Disease

  • Malignancy (Warburg effect: aerobic glycolysis in tumour cells)
  • Hepatic failure (impaired gluconeogenesis and lactate clearance)
  • Renal failure
  • Septic shock (mitochondrial dysfunction even with adequate DO₂)
  • Thiamine deficiency (PDH requires thiamine as cofactor)
  • DKA / HHS (ketoacidosis shifts redox state)

Type B2 — Drugs and Toxins

Drug/Toxin Mechanism
Beta-2 agonists (salbutamol, adrenaline) ↑ glycolysis (β₂-mediated); Type B — not hypoperfusion
Metformin Inhibits complex I of mitochondrial electron transport chain
Propofol (infusion syndrome) Uncouples oxidative phosphorylation; >4 mg/kg/hr for >24h
Cyanide / nitroprusside Blocks cytochrome c oxidase (complex IV)
Isoniazid Depletes pyridoxine → impairs PDH
Paracetamol (massive overdose) Hepatotoxicity + direct mitochondrial dysfunction
Salicylates Uncouples oxidative phosphorylation
NRTIs (antiretrovirals) Mitochondrial DNA polymerase-γ inhibition
Toxic alcohols (methanol, ethylene glycol) Metabolite-mediated mitochondrial toxicity

Type B3 — Inborn Errors of Metabolism

  • Pyruvate dehydrogenase deficiency
  • Pyruvate carboxylase deficiency
  • Electron transport chain enzyme defects
  • G6PD deficiency (in crisis)

Clinical Approach

  1. Confirm lactate is genuinely raised — avoid arterial sample delay, tourniquet use, or fist clenching (all falsely elevate)
  2. Identify Type A causes first — treat shock, restore DO₂; this is the common and immediately reversible cause
  3. Consider Type B if lactate disproportionate to haemodynamic status:
    • Drug history (metformin, antiretrovirals, salbutamol infusion)
    • Thiamine level / empirical thiamine if deficiency possible (Wernicke risk, alcoholism, malnutrition)
    • Liver function, LDH, CK (malignancy, rhabdomyolysis)
    • Co-oximetry (CO poisoning)
    • Cyanide poisoning (smoke inhalation + refractory lactic acidosis)
  4. Interpret adrenaline-associated lactate carefully — β₂-stimulated glycolysis; does not imply worsening perfusion
  5. Lactate clearance (>10% per 2h or >20% per 6h) is a reasonable surrogate marker of response to resuscitation

Viva Questions

1. Describe normal lactate physiology and the Cori cycle.

Lactate is produced continuously from pyruvate by LDH as a normal byproduct of glycolysis. Around 1,500 mmol/day is produced at rest, predominantly by skeletal muscle, skin, RBCs, brain and gut. Blood levels remain normal because the liver (and to a lesser extent the kidney) has enormous capacity to reconvert lactate to glucose via gluconeogenesis — the Cori cycle. LDH catalyses the reversible equilibrium: pyruvate⁻ + NADH + H⁺ ⇌ lactate⁻ + NAD⁺; the intracellular ratio is normally 10:1, reflecting the prevailing NAD⁺:NADH ratio. Lactate has a pKa of 3.86 and is fully dissociated at physiological pH. Importantly, lactate production consumes a proton via the LDH reaction and does not cause the acidosis — the H⁺ load of lactic acidosis arises from ATP hydrolysis outstripping oxidative ATP regeneration. Raised lactate and raised H⁺ are co-markers of high glycolytic flux, not cause and effect.

2. A patient in the ICU has a lactate of 8 mmol/L. How do you classify and approach the causes?

I would use the Cohen and Woods framework as a starting point: Type A (tissue hypoperfusion) is the most important and common cause — shock, hypoxaemia, CO poisoning, severe anaemia — and should be addressed immediately by restoring oxygen delivery. If the lactate is disproportionate to the haemodynamic picture, I would consider Type B causes: Type B1 (hepatic failure impairing clearance, sepsis-driven mitochondrial dysfunction, malignancy via the Warburg effect, thiamine deficiency inhibiting pyruvate dehydrogenase); Type B2 (drugs — metformin, adrenaline infusion, propofol infusion syndrome, cyanide, NRTIs); Type B3 (inborn errors, rare in adults). In practice, multiple mechanisms often co-exist, particularly in septic shock.

3. Why does adrenaline cause a lactic acidosis, and how should you interpret it clinically?

Adrenaline stimulates β₂ receptors in skeletal muscle, markedly increasing glycolytic flux and thus the rate of ATP turnover. As glycolytic throughput rises, NADH production increases faster than mitochondria can re-oxidise it, the NAD⁺:NADH ratio falls, and LDH is driven toward lactate production to regenerate NAD⁺ for continued glycolysis — a Type B2 mechanism independent of tissue hypoperfusion. Adrenaline also inhibits pyruvate dehydrogenase, further diverting pyruvate toward lactate and impairing clearance. The H⁺ accumulation (if present) reflects the increased rate of ATP hydrolysis from the high glycolytic flux state, not lactate toxicity. The lactate is therefore a marker of stimulated metabolism rather than inadequate oxygen delivery. Clinically, a rising lactate on adrenaline should prompt consideration of whether it is drug-driven: stable haemodynamics, improving organ function, and absence of other signs of hypoperfusion all support a benign interpretation. ScvO₂, urine output, and clinical examination should guide management rather than lactate in isolation.

4. How does metformin cause lactic acidosis, and in what circumstances is this clinically relevant?

Metformin inhibits complex I of the mitochondrial electron transport chain in hepatocytes, reducing oxidative phosphorylation and impairing hepatic gluconeogenesis from lactate. Under normal conditions, this is mild and inconsequential. It becomes clinically significant when additional stressors impair lactate clearance or increase production: acute kidney injury (metformin accumulates as it is renally cleared), hepatic impairment, sepsis, or iodinated contrast exposure (historically associated with AKI, though the risk is lower than previously thought). MALA (metformin-associated lactic acidosis) can be severe and is associated with high mortality. Treatment is supportive; haemodialysis can enhance metformin clearance and correct acidaemia simultaneously.

5. Does lactate cause the acidosis in lactic acidosis? Where do the protons actually come from?

No. This is a common misconception. The LDH reaction (pyruvate⁻ + NADH + H⁺ → lactate⁻ + NAD⁺) consumes a proton, so lactate production is proton-neutral or slightly alkalinising at the cellular level. Lactate and H⁺ are co-markers of the same high-glycolytic-flux state, but lactate does not generate the H⁺.

The protons arise from ATP hydrolysis (ATP⁴⁻ + H₂O → ADP³⁻ + HPO₄²⁻ + H⁺). In aerobic conditions, mitochondria recapture these protons via chemiosmotic coupling during oxidative phosphorylation — no net acid accumulates. When oxygen delivery is insufficient, the electron transport chain cannot re-oxidise NADH; the mitochondria stop re-capturing protons from ATP hydrolysis, and H⁺ accumulates.

Lactate rises in parallel because the same NADH accumulation (falling NAD⁺:NADH ratio) drives LDH toward lactate production — regenerating NAD⁺ to sustain continued glycolytic ATP synthesis via GAPDH. Lactate production is therefore a survival mechanism, partially buffering the acidosis while keeping glycolysis running. The villain is impaired oxidative phosphorylation and uncompensated ATP hydrolysis, not lactate itself.

6. What is the clinical significance of the lactate:pyruvate ratio?

The L:P ratio directly reflects the intracellular NAD⁺:NADH ratio, since LDH equilibrium (pyruvate⁻ + NADH + H⁺ ⇌ lactate⁻ + NAD⁺) is governed by the relative concentrations of these cofactors. A low NAD⁺:NADH ratio (NADH accumulation) shifts the equilibrium toward lactate — so the L:P ratio is a window onto mitochondrial redox state.

A normal L:P ratio (~10:1) with elevated absolute lactate suggests increased glycolytic flux with intact mitochondrial NADH re-oxidation — seen with beta-2 agonist excess, intense exercise, or alkalosis. A raised L:P ratio (>25:1) indicates a reduced NAD⁺:NADH ratio: mitochondria cannot re-oxidise NADH normally, pointing toward hypoxia, PDH inhibition (thiamine deficiency, sepsis), or direct electron transport chain dysfunction. In practice, pyruvate levels are rarely measured in clinical ICM, but the concept underpins empirical thiamine administration in unexplained lactic acidosis and the confident interpretation of adrenaline-associated hyperlactataemia as a high-flux state rather than a hypoxic one.