When it comes to performance, there’s no doubt that nutrition plays a significant role. In the past, I’ve really focused on acute nutrition: what you can do directly before or during your run to be faster (i.e. here).
An area I have neglected to focus on is what you should be doing from a nutritional standpoint on an on-going basis to stay healthy and perform at your best. One key area that I see as a recurring problem in my practice and athletes around me is iron deficiency anemia.
Iron has a number of roles in the human body. The most important function is how it is incorporated into hemoglobin and myglobin to facilitate oxygen transportation. If these proteins decline, our ability to transport oxygen to our working muscles also drops, and performance plummets along with it (such as here and here).