"I don’t want to be vegan," David Carter once said to his wife. "That’s for weaklings."

At the time, Carter was an NFL defensive end, weighing in at more than 300 pounds. Growing up in Los Angeles, he was raised on barbecue at his family’s restaurant and felt that meat was his source of strength, so there was no way he’d ever adopt his wife’s vegan diet.

That is, until he started to experience health problems that affected his career, which ended in 2015.

"I had been big and strong," Carter says. "But ... I was taking medication for high blood pressure and suffering from nerve damage," and as a result, he had a hard time doing bench presses and pushups.

On Feb. 14, 2014, his attitude suddenly changed: "I was drinking a milkshake and watching (animal rights documentary) ‘Forks Over Knives.’ And I just thought to myself, ‘That’s it, I’m done.’ I got up, threw out the milkshake and went vegan."

For many athletes who have switched to a vegan lifestyle, the change was prompted by reasons similar to Carter’s.

Poor health plagued Rich Roll before he adopted a plant-based diet and began a career as a world-class ultramarathoner in his 40s. And tennis star Venus Williams famously chose a raw vegan regimen five years ago after being diagnosed with Sjogren’s syndrome, an immune-system disorder that caused her joint pain, fatigue and shortness of breath.

Along with these stories comes a common query from incredulous nonvegans: "How do you get enough protein?"

"It’s always the first question," says Torre Washington, a bodybuilder who has been following a vegan diet for 18 years, laughing. "But you really don’t need protein to get shredded and lean. I’m looking for nutrients and focused on variety."

Indeed, vegan athletes including Washington and 10-time Olympic medalist Carl Lewis – who has said that athletes have "the worst diets in the world and compete in spite of it" – tend to agree that athletes should put an emphasis on increased calories when in training, as opposed to more protein.

"I’ve never met a patient who has had a protein deficiency," said sports dietitian Susan Levin, who is also a vegan and recreational runner. "It’s really a nonissue."

Ashley Koff, a registered dietitian, agrees that "we tend to overemphasize the amount of protein that we need," but says a plant-based diet of vegetables, fruit, grains and legumes may not be enough for many athletes. Williams, for instance, calls herself a "cheagan" because she occasionally cheats by indulging in cheese, yogurt or even fish.

"Plants play a primary role," Koff says, "but … all of our digestive systems and our bodies work differently. I like to say that you are not what you eat; you are what you digest and absorb."

That means she might build organic pasteurized eggs into an athlete’s diet because it’s a highly absorbable protein, but not, say, cheese.

Susan Levin is head of nutrition education for the nonprofit Physicians Committee on Responsible Medicine, which promotes nonmeat and nondairy nutrition. She cites the federal government’s Dietary Guidelines for Americans as an appropriate starting point. It recommends that about 10 percent of total daily calories be protein, meaning roughly 200 calories in an ideal diet.

"Protein is actually a fairly small percentage of what goes into a healthy diet," Levin says. "The emphasis really is on having a wide variety of nutrient-dense foods throughout the day, and, because protein is found in varying amounts in plants, legumes, grains and nuts, it’s pretty easy to get to the recommended amount. Most athletes don’t need a different diet, they just need more calories."

When Carter, who is 28, was in intense physical training, he was consuming 8,000 to 10,000 calories a day and looking to get about 300 grams of protein. "You’ve got to be focused to get that many calories in a day," Carter says.

Once he made the switch to a vegan diet, he realized that all the beans, grains, fruit and vegetables he was eating were helping him be big and strong, bench-pressing 465 pounds on a diet of bananas, lentils and cashew cheese.

"The biggest and strongest animals on the planet – elephants, gorillas, rhinos – are herbivores," Carter says.

James Loomis, medical director of the PCRM’s Barnard Medical Center in Washington, admits packing on calories with a vegan diet is still not easy for athletes.

"Some people struggle with it," he says. "Try eating 1,000 calories of quinoa or blueberries."

Koff says, "The quality of what you put into your body is really critical for athletes – you want to get the most bang for every bite."

Loomis, who served as team internist for football’s St. Louis (now Los Angeles) Rams and baseball’s St. Louis Cardinals before he got interested in plant-based medicine, saw professional athletes develop serious health problems at an early age by focusing only on eating a lot of calories.

"I call it ‘bro science,’ " he says. "They’re thinking about getting big and strong without thinking about the long-term impact. But if we help develop healthy habits in younger athletes, it helps them in post-career."

Inflammation caused by dietary choices, which can lead to a host of chronic diseases, is a common concern for Loomis, Levin and Koff, even if they don’t agree on the need for a completely vegan diet. Eating meat and animal products, Koff says, has been associated with inflammation, "so it’s all the more reason to have a lot of plants in the diet to balance (that) out."

Matt Frazier, 35, an ultramarathoner and author of "No Meat Athlete," used a gradual transition from omnivore to vegan for his own diet.

"I spent a whole year without eating four-legged animals before I took the next step and cut the two-legged animals out of my diet, and then spent a month there before I stopped eating fish," he says. At that point, Frazier found that his speed and endurance were increasing, leading him to shave 10 minutes off his marathon time to ultimately qualify for the Boston Marathon. Over the next two years, he gradually cut out eggs and dairy.

"I think short-term challenges are a great way to dive in and see what it’s like," Frazier says. "Try a 10-day vegetarian or vegan challenge, and if it goes well, make it 30 days. Having that finish line at the end keeps you from having to deal with those ‘I can never eat a cheeseburger again?’ thoughts that aren’t helpful at all when you’re trying to transition."

The feared loss of indulgent food is a topic that seems to come up often among both vegan athletes and curious omnivores.

"I like to make junk food that’s not really junk food," Carter says. "My favorite is nachos with couscous, beans and cashew-cheese queso. I’ve converted so many athletes with that cashew cheese – they eat it and say, ‘Man, that tastes just like real cheese, bro!’ "