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She wore an itsy-bitsy hot-pink bikini...

Last week, Jennifer Aniston shot another scene for Just Go with It on the Hawaiian island of Kauai -- wearing close to nothing.

Shooting near a tropical waterfall, the 41-year-old actress wore a barely-there, hot-pink bikini. The tan star's super-sleek physique is no accident. "She's obsessed with staying fit and young," a source tells Us Weekly.

Adds a source, "She very rarely misses a workout, even when she's traveling." Indeed, between filming scenes her latest flick, the 5-foot-6 actress hit the treadmill or StairMaster for an hour daily, sipped pureed papaya, pineapple and mango and dined on veggies and fish.

And Aniston has another incentive to look good on the Just Go With It set: 23-year-old costar and Sports Illustrated cover girl Brooklyn Decker. "Jen's intimidated," the source says of Decker. "She's very conscious of her body."

Back home, the fitness fiend has added Tracy Anderson's dance cardio to her routine as well.