I'm doing a slow carb diet and it says you should avoid fruit (unless its your offday). I'm curious as to why exactly this is the case?
Many fruits are high in carbohydrate, specifically fructose, which is not "slow" and has a number of effects that may be considered deleterious to one's metabolism.
At the risk of being a bit off-topic I have to say: Don't cut fruits off you diet to lower your carb intake. That's like putting sunflower oil in your engine, instead of the regular machine grade engine oil, because it's cheaper.
You'll have seriously more benefits of consuming fruits over pretty much any other source of carbs, which you DO need do function properly. Besides clumping all fruits into one category in terms of their fructose levels is severely flawed.
I suggest that you keep your fruits as a central part of your diet, no matter if you are trying to go down in weight or "beef up". The reasoning behind this is that you DO need carbs, and better get them of natural sources, with all the vitamins and fibers etc that come with it, instead of using some labtech formula which might in 20 years prove to be cancerogenic or something. That's of course my two cents...
Fructose is the main reason, just keep them under control. A banana a day isn't bad, 30 a day is not the way to go for low carb.