Just read that:

Ryan Kwanten’s Diet ("Jason Stackhouse")

When Ryan Kwanten needs to prepare for a shirtless scene (or go to beach), he follows a Paleolithic diet (or paleo diet or paleodiet), which is also known as the caveman diet, the Stone Age diet, and the hunter-gatherer diet. The Paleolithic diet is based on the presumed ancient diet of wild plants and animals that various human species habitually consumed during the Paletolithic period (an era from about 2.5 million years ago until about 10,000 years ago).

The (modern) Paleolithic diet consists mainly of meat, fish, vegetables, fruit, roots, and nuts. It excludes grains, legumes, dairy products, salt, refined sugar, and processed oils.

Ryan claims that he only needs to do this for about 2 or 3 days before the event. The diet helps to burn fat because insulin levels are kept in check due to the low carb intake (which would otherwise increase insulin and decrease your fat-burning ability).

However, Ryan admits that he doesn’t follow a strict diet anymore, and isn’t afraid to have a beer on occasion, but still prefers water or a protein shake.