He is "channeling his energy into working out," a source told Us Weekly in its most recent issue.
In other words, the 28-year-old actor didn't want to look teeny in tiny red shorts while playing lifeguard Matt Brody.
"He is so regimented about everything he does," the source said.
Zac's ridiculously ripped body has been talked about ever since she shed his shirt while filming the movie, a remake of the highly-popular TV series of the same name that ran in the late 80s and early 90s.
How Zac got that chiseled body is interesting… and dehydrating, too!
One trick he uses for a more toned and defined appearance on camera is a common bodybuilder technique called "flushing."
The source said, "Before they shoot a big shirtless sequence, Zac won't have any water for 12 hours to make his muscles look as defined as humanly possible."
The "We Are Your Friends" star even takes it up a notch when he films scenes with Dwayne Johnson, a man who is always in shape, thanks to his football and professional wrestling backgrounds.
"Zac jokes that he doesn't want to look tiny next to 'The Rock,'" the source said.
He recently took to Twitter to reveal his strict Baywatch diet.
"9 days of absolutely zero carbs & sugar. Only organic grass feed/free range protein and organic leafy greens. This leads too…" he wrote, before following up with another tweet showing off an array of dirty dishes from his day off. "#Cheatday! Devoured a steak, 4 side potatoes/veggies, macncheese, 3 desserts bread/butter, brick o cheese & caviar."
Zac and The Rock seem to be having a great time razzing each other.
"Special delivery for The Rock," Zac captioned a March 15 shirtless Instagram snap.
On the first day of shooting the remake, which will feature David Hasselhoff, The Rock posted a snap of the two and described a scene. "Zac's ad-libbed responses were brilliant! Talented and great dude. World get ready to have some fun, 'cause the beach will never be the same again."
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