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Grapefruit juice is talked about exclusively in the context of damage control on a cheat day. Re-read the section in 4HB for the science behind all of it's effects. By and large it is used to keep your blood sugar in check.
That said, grapefruit juice is a cheat day only item. There's a great deal of sugar in it and you would most likely see your losses stall out during the week if you drank it outside of cheat day. Hope this helped!
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