Cardiac Metabolism and Substrate Use
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Direct answer
The heart is an obligate aerobe with the metabolism of a hybrid engine: at rest it draws roughly 60-70% of its energy from fatty acid oxidation, about 20-30% from glucose and lactate, and the small remainder from ketones and amino acids, switching blend with circumstance — after a carbohydrate meal glucose rises, in prolonged fasting or diabetes fatty acids and ketones dominate, and in sepsis the myocardium burns unusually large amounts of lactate. It has no meaningful fuel store and no oxygen debt capacity, so it extracts about 70-75% of delivered oxygen at rest (coronary sinus PO2 near 20-25 mmHg) — near-maximal already, so any increase in work must be met by increased coronary flow, the five-fold reserve exercise uses. Fatty acids yield more ATP per molecule but more oxygen per ATP, which is why the ischaemic heart shifts toward glucose; the rate-pressure product remains the bedside proxy for oxygen demand, and matching metabolism to supply defines hibernating myocardium, ischaemic preconditioning and the remodelling of the failing heart.
What you must remember
- Substrate percentages at rest: fatty acids about 60-70%, glucose about 20-30% (with lactate), ketones and amino acids the remainder — quotable, and they shift with feeding, fasting, diabetes and adrenergic tone.
- Oxygen extraction reserve: about 70-75% extracted at rest against roughly 25% body-wide average, leaving almost no extraction reserve — flow must rise when work does, from roughly 250 mL per minute at baseline toward 1,000-1,200 mL in exercise.
- Oxygen demand determinants: heart rate, contractility, wall stress (pressure and radius per Laplace) — summarised bedside as the rate-pressure product; tachycardia hurts doubly by shortening diastolic filling time of the coronaries.
- Fatty acid machinery: carnitine palmitoyl transferase-1 shepherds long-chain acyl-CoA into mitochondria, inhibited by malonyl-CoA — the regulatory gate that shifts the heart between fat and carbohydrate burning.
- Ischaemic metabolism: within seconds of severe flow loss, oxidative phosphorylation stalls, glycolysis accelerates, intracellular acidosis and calcium overload follow, ATP falls to stunning levels and necrosis begins after roughly 20 minutes of total occlusion in classic teaching.
- Adaptive states: hibernating myocardium downregulates contraction to match reduced flow (reversible, distinguished from scar by positron emission tomography or dobutamine response); ischaemic preconditioning is the protection conferred by brief preceding ischaemic episodes through adenosine and K-ATP channel signalling.
- Insulin and thyroid hooks: insulin recruits myocardial GLUT4 transporters and favours glucose use; thyroid hormone shifts the balance and raises oxygen consumption — part of why thyrotoxicosis stresses the angina patient, and a favourite integrative question.
Where students slip
First, students give the heart a glycogen safety net like skeletal muscle; its store is trivial and its oxygen debt is not repayable — anoxia is contractile standstill within seconds. Second, the substrate numbers are recited without their oxygen logic: fatty acid oxidation yields more ATP per substrate molecule but consumes more oxygen per ATP, so glucose is the efficient choice precisely when oxygen is scarce — the reason enhancing glucose use is a legitimate anti-ischaemic strategy. Third, coronary flow reserve is attributed to extraction; extraction is already near-maximal at rest, and the reserve is flow — the sentence that explains why tachycardia and anaemia both cause angina with unobstructed arteries. Fourth, "the heart uses only glucose" survives from oversimplified teaching and must be actively replaced with the 60-70% fatty acid figure. Finally, hibernating myocardium is mislabelled as a weak but dead segment; it is alive, metabolically active and recoverable — the distinction that decides revascularisation.
Frequently asked questions
What is the resting substrate profile of the heart?
Roughly 60-70% fatty acids, 20-30% glucose and lactate, the balance from ketones and amino acids, shifting with feeding, fasting, diabetes and adrenergic state.
Why must coronary flow rise with cardiac work?
The heart already extracts about 70-75% of delivered oxygen at rest, leaving negligible extraction reserve, so increased demand can only be met by increased flow.
Which factors determine myocardial oxygen demand?
Heart rate, contractility and wall stress (pressure and cavity radius), summarised bedside as the rate-pressure product of heart rate and systolic pressure.
Why is glucose metabolism advantageous in ischaemia?
Glycolysis yields ATP without oxygen, and glucose oxidation consumes less oxygen per ATP than fatty acid oxidation, making it the efficient fuel when oxygen is limited.
What is hibernating myocardium?
Chronically underperfused myocardium that downregulates contraction to match reduced supply while remaining viable and metabolically active, recovering function after revascularisation.