WHEN SUSAN LEVIN hears what Michael Phelps stuffs into his mouth — like entire pepperoni pizzas — she worries that his Olympic career could be headed for a dive. That’s because she’s the director of nutrition education at the Physicians Committee for Responsible Medicine, which promotes a vegan diet. And along with benefits such as losing weight, lowering your risk of certain diseases and being environmentally sound, she says switching to solely plant-based food sources makes you a better athlete.

It worked for Levin, who never liked running until she went vegan 15 years ago. “I was suddenly better at it, enjoyed it more and was faster. At the end of the day, I had all of this energy,” says Levin, who’s seen the same level of activity in her colleagues at their Friendship Heights office. “People are bouncing off the walls. They run, bike, do yoga.” And contrary to the stereotype of the vegan waif, and thanks to rich sources of protein such as grains and beans, they have real muscles.

They also have real role models, including former NBA star John Salley and professional triathlete Brendan Brazier, who are two of the celebs backing PCRM’s 21-Day Vegan Kickstart. The program walks users through a diet makeover with a meal plan, cooking demos and other resources, which makes the process simpler than what both Salley and Brazier went through on their own.

It was high blood pressure and family members’ health history that convinced Salley to drop meat five years into his career, a move his teammates thought was bonkers. While he devoured books about nutrition, they were grabbing grub at fast-food joints. The difference, he believes, is why he managed to stay in the NBA for 15 years (and score four championship rings) and some of the other guys barely had the mojo to practice. “Literally, I was 36 and rookies couldn’t keep up with me,” he remembers.

Now he’s working on launching a line of organic, kosher and vegan foods and consulting with a dozen current professional athletes on the advantages of a vegan diet.

He could start by handing over a copy of Brazier’s new book, “Thrive Fitness: The Vegan-Based Training Program for Maximum Strength, Health, and Fitness” ($15.95, Da Capo Press). It’s the result of 20 years of experimenting with food to figure out which ones would best help his body recover from grueling endurance workouts.

“I tried all different kinds of diets. I would eat whatever anybody told me if it would help me race,” says Brazier, who initially kept quiet about how he was fueling. “I figured if I do start improving, they’ll ask me about it.”

Now that they do, they find some of his favorite foods aren’t common on most people’s shopping lists: chlorella, buckwheat, kelp, salba. But if those items are too hard to find, Brazier says, you’ll see a difference in your performance by any increase in the veggie content of your diet. “Eat a salad at least five times a week with leafy greens, sprouts, avocado. And a smoothie is great after a workout. It gets it in your body right away,” he promises.

Brazier’s followers aren’t all strict vegans. He says many of them have just shifted their views on meat and dairy, and cut back. “They thought they needed it for iron and protein, and now it’s because of the taste,” he says.

But if you do decide to try an entirely plant-based approach to food to improve your sports performance, Levin says it’s not as complicated as you might think. Just be sure to take a vitamin B12 supplement and eat sensibly. “Don’t think of it in terms of what you’re not eating. Think about what you are — fruits, vegetables, beans, whole grains,” she says. “When you see the results, you won’t want to go back.”

Photo courtesy PCRM