Endurance exercise can cause loss of iron and anaemia

Most athletes are far less aware of the fact that iron is one of the most difficult minerals to absorb, and that they are especially vulnerable to iron depletion through training-induced losses, especially if their event involves endurance training. To make matters worse, the latest ways of measuring iron indicate that that it is perfectly possible to have a healthy blood Hb count while simultaneously suffering from depleted levels of tissue iron. And, if that weren’t enough, new research published this spring has demonstrated this tissue iron depletion impairs the ability of the body to adapt to endurance training.

Full article here.
Luckily, Guinness is a good source of iron.