Grapefruit juice contains furanocoumarins, which irreversibly inhibit CYP3A4. For this reason, when one is taking certain medications it is necessary to not eat grapefruits because the inhibition of CYP3A4 can reduce how much of the drug is metabolized before it can get used, resulting in an overdose. It apparently takes 24 hours after eating a grapefruit before the enzyme level is 50% and up to 72 hours for it to be completely normal. So, it seems that eating one grapefruit a day would substantially impair one's CYP3A4 levels indefinitely.
Wikipedia says, the purpose of CYP3A4 is "to oxidize small foreign organic molecules (xenobiotics), such as toxins or drugs, so that they can be removed from the body." It seems like inhibiting CYP3A4, from a biological standpoint, would reduce one's immune system or at least the ability to tolerate and eliminate certain toxins. Are there other enzymes that take up the slack, or is there something in grapefruit itself which does the job instead, or is a daily grapefruit just a bad idea? What are the effects of extended inhibition of CYP3A4? Will the body perhaps build up a tolerance or immunity, or (perhaps with negative effects) pump out more CYP3A4 to try to compensate?