Not quiet sure on exact figures,
But i follow the rule of each dog indavidual .
Iron which is a must for good blood which in returns feeds oxegan to the blood which is vital for the dogs wind and a clear flow of air.

I do how ever feel you are more likely to leave a run on the track threw training.
Only when a dog is been known to lack of iron would i feel it was the main cause and would only be able to veify a out come like this by a blood group count.

Every thing in moderation and blood counts should help you in your next step of this question.