Chris seem vaguely cognizant that he is unhealthy. Probably not aware enough to realize the extent of his horrifying condition, but he does seem to notice it. Either by himself or through reading something online, he probably found a connection between health and libido and realized he has no libido, and made a connection with his obviously ailing condition.
Then, of course, being the utter dipshit he is, took the idiom "an apple a day keeps the doctor away" as literally as possible and genuinely thinks eating exactly one (1) apple daily will restore your health and vigor. This ties in with his previous beliefs that if you eat grilled chicken then you'll be super healthy, because he doesn't understand a healthy diet is only one aspect to a healthy lifestyle and not the entire lifestyle.