A decade or so ago, the idea arose that athletes might gain a competitive edge by drinking water that contained extra dissolved oxygen (O₂). The notion stems from observations that O₂ breathing during exercise enhances athletic performance,^1,2 but the connection of O₂ breathing during exercise with drinking “hyperoxygenated” water before exercise conflates physics and physiology in a struthonian visit to placebo land. Fuelled by bottled-water mavens, who collect testimonials for oxygenated waters, claims abound of ergogenic benefits of water advertised to hold up to 40 times more O₂ than plain water.

The issue of hydration aside, such claims have a flimsy rationale and no rigorous experimental support. On close inspection, three scientific problems immediately arise. Firstly, for all practical purposes, supplemental O₂ improves performance only during exercise, not before or between bouts.³ With normal lungs, breathing of even pure O₂ at sea level increases maximal O₂ uptake (o₂max) by only 5–10% because air breathing alone, except at high intensity work, keeps the arterial O₂ saturation (Sao₂) and content (Cao₂) very high.²

The next problem is that the solubility of O₂ in water (and plasma) is low. The solubility constant obeys Henry’s Law: it is directly proportional to O_{2 …}