First of all dont listen to him- you are not depriving your brain of oxygen. They have done many studies on this topic and they found you will pass out long before you start to incur any brain damage. The main consequences I can think of are overstretching your muscles or the o2 debt taking a toll on your body, just like if you were doing too many anaerobic workouts. Just try not to cram too much air in their when your breathing up because you can pull a muscle or screw up length tension relationshiips if you do it alot. To accomplish the goal of your body dealing with oxygen debt better and/or yourself getting tuffer its not just about how long you hold your breath- like if you breath out half your air youll accomplish the same goal twice as fast as a full breath. If you go into o2 debt long enough youll feel your muscles fill with lactic acid and youll also start to get very sweaty, oh yeah, and the best part is your diaphram spasming uncontrollably. If you keep on pushing past this point i believe it definitly will make you tuffer. I have no idea what kind of an effect, if at all, it would have on your running. To find out everything you need to know about the physiology of it you should go to deeperblue.net I recommend doing c02 tables ,which you can at deeperblue.net,to accomplish your goal. that is if you actually want to go through with it. By the way i held my breath for 4minutes, but i wasnt doing interval training at this time so i dont know if I would have been able to push harder. I do know after a few weeks of practicing it I became alot more accustomed to dealing with the o2 debt and I was able to deal with alot more pain.