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Kim, Khloe and Kourtney may bust their butts at the gym, but we've uncovered another reason they stay so fit and fab -- Diet Pepsi Skinny!

The girls were caught sharing a few Diet Pepsi Skinny cans and a laugh at the opening party for their new DASH NYC boutique earlier this week.

I may not join them at boot camp, but this is one diet tip I just may be able to follow along with them!