The definition of what is a "human body enhancement drug" is itself interesting: some are what people would obviously consider to be drugs, like steroids; and at the other end, the substances found in regular foods such as vitamins would not be, but what about dietary supplements and the various other types of "food optimisation" that people do? I'd say they certainly have an effect on performance too, so could they be considered a form of unfair advantage? This sort of reasoning leads to the conclusion that to eliminate all these factors the athletes' diets should be strictly regulated. Then you still have (as this article shows) their history and probably other factors down to how much they've trained, the climate they're in, etc... it's really quite a subtle issue to define what is "fair" in sports.