I have watched the net over the past years, develop the term
BFS. It is not a diagnosis but only a descriptive term used for the symptoms.

During this time the studies on magnesium intake in the diet have paralleled the BFS term... up to 70% and maybe more do not get the RDA of magnesium from the diet. Or they lose it quickly thru lifestyle choices. Caffeine and alcohol increase magnesium excretion, and so does glucose intolerance and diabetes.

In fact heavy daily exercise may deplete it more, since magnesium is used by mitochondria to make the membranes of those energy factories more efficient. It is also used by muscles along with pyridoxal, to metabolize carbohydrates.

from http://www.ncbi.nlm.nih.gov/pubmed/17172008

A good resource to help athletes consume more magnesium from foods is:
http://www.slowmag.com/
This dosage form was developed with doctors in mind, when the oxide form that was historically used before was discovered to be useless as a supplement. However, this dosage form remains over the counter, and does not require a prescription. The average dose is one tablet twice a day.

In fact more and more discussion is available now on the net along with studies to show that heavy running habits, are not as healthy as people used to believe:
http://www.latimes.com/science/scien...202-story.html

This might be due in part to the heavy requirements the body has for magnesium, and since most people don't even get the RDA, the extra load needed for heavy exercise creates the potential for damage.

We have seen in an extended family member, who was a pilot in the Air Force, and is now retired and a commercial pilot.. who did his running as part of his fitness routine... he has had to have both hips replaced recently (and he is only in his 50's.)
The wear and tear he has experienced has this price for him.

There are many more articles like this on the net:
http://www.bbc.com/news/health-31095384

So moderation is the key, and also paying attention to the nutrients you need to exercise safely.