An altitude block is mostly a bet on the athlete's iron stores

You cannot manufacture red cells out of thin air, and the raw material runs out quietly weeks before anyone notices.

The whole point of going up is to make more red cells. Red cells need iron. If the athlete arrives with depleted stores, the hypoxic signal fires, the body tries to respond, and nothing much happens except three weeks of poor sleep at altitude and a hard reset of training quality.

This is the least glamorous part of endurance sport and it is the part I would check first. Not the training plan. Not the tent. The bloods.

The mechanism has a nasty feature: an athlete can look fine on the basic numbers and still have nothing in the tank. Haemoglobin, the thing carrying oxygen today, holds up for a long while by robbing the reserve. Ferritin, roughly the reserve itself, drains first. So a runner can post an unremarkable full blood count and be functionally unable to build anything new. Endurance athletes are also unusually good at losing iron. Sweat, gut losses, the mechanical destruction of red cells in the feet, and in female athletes menstrual losses that nobody in a training group wants to bring up. Distance running is an iron leak with a race schedule attached.

The response in most programmes is a supplement bought at a pharmacy and taken every morning with coffee, which is close to the worst available protocol. Absorption is regulated by a hormone that rises after a hard session and after each dose, so daily dosing with a large tablet can deliver less total iron than alternate day dosing with a smaller one. Coffee and tea cut uptake further. Timing matters more than dose here, and almost nobody times it.

There is also an argument I will make and defend: intravenous iron has drifted from a medical treatment into a performance shortcut in parts of the sport, and the line is genuinely blurred. If a runner is anaemic and symptomatic, treat them. If a runner sits comfortably in range and gets an infusion three weeks out because a camp is coming, that is not medicine, whatever the paperwork says.

Screen twelve weeks before the camp, not two. Twelve weeks gives you time to fix a low result with food and oral dosing. Two weeks gives you a choice between an expensive intervention and an expensive holiday.