Even as little as 300 ml of blood increases your performance in time trials. So in order to remove suspicion from blood doping we have to forbid being at altitude. Being a good GC is all about being good at time trials and being good at climbing. The pros take lots of supplements and some respond better than others- just as with doping. 39% respond positive to coffee before races, but not the rest. About half the population gets better with baking powder etc.