Well I have to tell you that I have a bit of a knee-jerk reaction to the word "detox" used in conjunction with a diet. Our bodies don't need to "detox" in the sense that most dieters and diet plans use the word. And you're right - it doesn't really work like that.
However, it is possible to go through *withdrawal* from caffeine and sugar.
Withdrawals mean that your body is *missing* what you normally put into it, not that poisons are leaching out of your body.
There's a difference.
Many people who give up caffeine - not just in sodas, but in coffee, tea, even chocolate, suffer from withdrawal symptoms. Your body gets used to the boost from caffeine and not having it any more can give you headaches, make you feel drowsy, make you feel ill, and so forth. And when you feel like that, your immunity is lowered, so it's easier for you to catch colds or flu or whatever. So it's possible that you could get sick from going cold turkey - not because any kind of "poison" is draining from your body, but because your body is learning to adapt to not having the usual level of caffeine, sugar, etc. put in it.
I do have other issues with the Optifast diet - I don't think that kind of deprivation is healthy ... but I won't go there ... yet.
Good luck to you!
.