And who is the author of the study that proves a little bit of pineapple will keep you from dropping bodyfat?
If you are eating five whole pineapples a day, yeah - that's not going to happen. Hell, even an overdose of LOW-GI fruit will keep you from your goals.
But if you eat 1-2 SERVINGS of pineapple a week (call it a "cheat" meal or whatever, if it makes you feel better psychologically), and it fits into your calories/macros for your diet, for that week, it is almost certainly not going to be detrimental to your goals.
Like I said, different foods work differently for everyone -- so it's upsetting when people who are starting out, are made to fear eating a little fruit. I encountered it last year when I was cutting. Everyone was warning me, "Don't eat bananas!" (not that i would - i hate them). But a reasonable serving of banana or pineapple or any other higher sugar fruit is not likely going to kill a week's worth of dieting. And if it keeps you from going insane and bingeing on something sweet because your diet is otherwise so damn strict, then hey, you know what? Hell yeah, go a little wild - have a few ounces of pineapple on friday night.