Grapefruit is a CYP3A4 inhibitor, which gives it special properties. (drugs interactions for example). But I would like to talk about it's effect on oestrogens, which is particularly important for women generally and even more for CFS/ME women. In this article was discussed the effect of the cytochrome P450 polymorphisms on estrogen metabolism: What would happen to estradiol metabolism if there were polymorphisms in the detox enzymes? CYP3A4 AND CYP1A2: Known polymorphisms that lower the activity of these enzymes would decrease the fraction of estradiol that is metabolized by them in the liver. This would have the effect of increasing the fraction of estradiol that is metabolized in other organs by CYP1B1. CYP1B1: Known polymorphisms that raise its activity would cause a greater rate of production of 4-hydroxyestradiol, and would also cause more of this to be oxidized to form semiquinones and quinones (16). COMT: Known polymorphisms that lower its activity would decrease the fraction of 4-hydroxyestradiol that is methylated, leaving more to be oxidized to semiquinones and quinones. GST enzymes: Known polymorphisms that lower the activity of members of this superfamily of enzymes would decrease the rate of removal of semiquinones and quinones, leaving more of them to carry on redox cycling and to contribute to oxidative stress (25). Grapefruit not only inhibit CYP3A4, but also CYP1A2 (and CYP2C9, CYP2D6) (cf wiki). This action will affect estrogens metabolism : It has already been shown that Grapefruit consumption after menopause increase by 1.3 the risk of breast cancer... "relative risk=1.30, 95% confidence interval 1.06–1.58) for subjects in the highest category of intake, that is, one-quarter grapefruit or more per day, compared to non-consumers" (although It's interesting to notice that this study was not replicated by the european study) "Whole grapefruit intake had significant effects on endogenous Estrone 3 sulfate (a reserve form for Estradiol and Estrone). Peak effects were seen at 8 hr, increasing by 26% from baseline. No changes in mean Estrone or Estradiol with whole fruit intake were observed. In contrast, fresh juice, bottled juice, and soda intake all had significant lowering effects on Estradiol." I was myself taking grapefruit juice for 7/10 days in order to improve my Ubiquinol gut absorption, but I noticed a weight gain (about 2 kg). I don't know is this weight gain is caused by a general improvement in my fat and hormons metabolism, or if the grapefruit intake had any influence, but I suspect it had, so I wanted to share this warning.