Oxygen delivery & nitric oxide
One of 4 mechanistic pathways to 🏃 Endurance & work capacity · 11 options
You can only use the oxygen that arrives. Nitric-oxide-mediated vasodilation improves perfusion and — separately and more interestingly — reduces the oxygen cost of a given workload.
The highest-yield test on this whole page. Ferritin under 30 impairs endurance well before haemoglobin drops, so the full blood count reads normal while performance falls off a cliff — and it is disproportionately common in women and plant-based athletes.
Complete Blood Count (CBC) with DifferentialFerritinIron Panel (Iron, TIBC, Transferrin Saturation)Vitamin B12ADMA & SDMA🩸 Am I Iron Deficient? covers these in one panel →
What engages this pathway
Ordered by how directly each one acts on the mechanism above — never by how much trial evidence exists. Each links to its full breakdown with dosing, half-life and vendor.
🧬 Beetroot (Nitrates)
Dietary nitrate converts to nitrite then NO, reducing the oxygen cost of submaximal exercise by around 5%. One of the very few ergogenic aids with consistently replicated field results. Antibacterial mouthwash abolishes the effect, because oral bacteria do the first conversion step.
🧬 Citrulline Malate
Raises plasma arginine better than arginine itself, since citrulline escapes intestinal and hepatic arginase. Improves repetitions to failure and reduces soreness.
🧬 L-Arginine
The direct NOS substrate, undermined by heavy first-pass metabolism. The sustained-release form partially addresses this.
🧬 Nitrosigine
Inositol-stabilised arginine silicate with markedly better bioavailability and human data for blood flow lasting hours rather than minutes.
🧬 Pycnogenol
Increases eNOS expression and improves endothelial function in trials. Also used with arginine, where it appears synergistic.
🧬 Cocoa Flavanols
Epicatechin-driven NO improvement with strong trial support for endothelial function and blood-flow endpoints.
🧬 Epicatechin
The isolated active from cocoa. Both a NO and a myostatin argument in the same molecule.
🧬 Iron
Haemoglobin's oxygen-carrying capacity depends on it. Deficiency is the most common genuine cause of unexplained endurance decline, especially in menstruating and plant-based athletes — and supplementing without testing is a real risk, because excess iron is pro-oxidant.
🧬 Iron Complex
Iron with the B12, folate and C cofactors that erythropoiesis actually requires. Test first, always.
🧬 Methylcobalamin (B12)
Required for red-cell maturation. B12 deficiency produces fatigue that no amount of training addresses.
🧬 D-Ribose
A pentose sugar in the ATP salvage pathway. The argument is faster ATP repletion after depleting work; trials in healthy athletes are largely null, with better signal in cardiac patients.
The other 3 routes to endurance & work capacity
Pick the pathway that matches where you are actually stuck. An appetite drug does nothing for someone who already undereats.
Want the protocols behind these?
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Frequently asked questions
You can only use the oxygen that arrives. Nitric-oxide-mediated vasodilation improves perfusion and — separately and more interestingly — reduces the oxygen cost of a given workload.
11 options are mapped to this pathway in the Vault, including Beetroot (Nitrates), Citrulline Malate, L-Arginine, Nitrosigine. They are grouped by the mechanism they act through rather than ranked by how much trial evidence exists — 10 carry clinical validation and 1 are mechanistic predictions.
The highest-yield test on this whole page. Ferritin under 30 impairs endurance well before haemoglobin drops, so the full blood count reads normal while performance falls off a cliff — and it is disproportionately common in women and plant-based athletes. The markers worth checking are Complete Blood Count (CBC) with Differential, Ferritin, Iron Panel (Iron, TIBC, Transferrin Saturation), Vitamin B12.
Unproven is not the same as ineffective. Of the 11 options on this pathway, 10 have clinical validation and 1 are graded theoretical — meaning the mechanism is sound but the specific human trial has not been run, which is true of a great deal of what works in this space. Nothing on this page is ordered by evidence tier, because sorting by trial count would bury the compounds you came looking for.