Four months after Arian Foster tweeted that he had stopped eating meat, eggs, and all other animal foods, the Houston Texans running back was grinding out more yards on the field than nearly any other player in the NFL. By early November, he had amassed 168 carries – for 659 yards – and nine touchdowns.

Not bad for a leaf eater. And a nice in-your-face rejoinder to the fans and cable pundits who reacted to Foster's switch to a vegan diet with stupefaction and mockery. How would he get enough protein? Why would he jeopardize the team's Super Bowl chances? Why – in the middle of cattle country – would he start eating only vegetables and grains? "I told him, 'If this doesn't work, I'm going to kick your ass,'" his teammate Brian Cushing told Yahoo! Sports. Another blogger wrote on a fan site: "I have no problem with a vegan diet, but attempting such a drastic lifestyle change weeks before training camp when you play one of the most physically demanding contact sports in the world seems foolish to me."

Foster told the press he became a vegan because he didn't feel good when he ate big meals with meat. At 25, he'd been cutting meat out of his diet for six months before turning vegan and considers the unwanted attention, as his management put it, "a distraction." Both he and his team refuse to discuss it.

The NFL is one of the least hospitable places for vegans and vegetarians, not only because of social pushback from teammates, fans, and the media, but also because of the game's physical demands: Most players eat around 6,000 calories and 200 grams of protein daily, which is difficult to get from a diet without high-calorie, protein-dense animal foods. In recent years, a handful of players have tried being vegan, cutting out animal products, or vegetarian, eating some dairy and eggs. And most of them have had to counteract a consequential drop in performance and have had to add small amounts of fish or chicken back to their diets.

"It's not compatible with the lifestyle of a professional football player on so many levels," says former NFL running back Ricky Williams, who became one of the league's first vegetarians before his 2005 season with the Miami Dolphins. He made the change because he felt strongly it was a healthier way to eat. "People get weird about it. They don't understand it. It goes against what they believe. And it takes a lot of work."

Many athletes used to think a plant-based diet couldn't deliver enough calories and protein for high-level performance. Nutritionists now know that's not entirely true. Players can eat enough vegetables and quinoa, hemp, beans, seeds, nuts, and vegan shakes to get the calories and the protein they need, but they have to remain vigilant about what and how much they can eat. For some, it's worth the effort. As a vegetarian, Williams racked up five successful seasons, carrying the ball 1,121 yards in 2009, an impressive feat for any NFL player. "It changed my game, and it changed my body," he says. "I had tons of energy."

There's also evidence that a plant-based diet can help reduce the risk of diabetes, cancer, and heart disease. The Centers for Disease Control says NFL linemen in particular – guys who spend their lives gorging on cheeseburgers and fried chicken – have a 52 percent greater risk of dying from heart disease than the general population. Eating plant-based foods has also been shown to improve blood and oxygen flow, helping repair torn muscles and tendons for faster recovery from training and injuries. High-fiber fruits and vegetables, loaded with antioxidant vitamins and minerals, have been shown to limit inflammation in arteries, while processed foods can cause it. The complex carbs in whole grains, legumes, fruits, and vegetables also "break down more easily [than meat] into fuel," says Mitzi Dulan, an NFL nutritionist who helped Atlanta Falcons tight end Tony Gonzalez become a vegan. Turning red meat into energy is much harder. "Taking that fat and that density, it's a lot more work. It makes your blood acidic, and your body has to work to get your body in pH balance again."

"On average, I play 65 downs a game," says Gonzalez, who switched to a plant-based diet (mostly vegetarian with only small amounts of white meat daily) five years ago, after eating full vegan for two months. "Let's say I catch five [balls] a game. I get tackled five times. My body takes a pounding, no doubt. Why wouldn't I want to be putting healing stuff in my body?"

With 16 years in the NFL, Gonzalez is the league's elder statesman when it comes to vegetarianism. When Foster was considering his dietary switch, he called Gonzalez, who told him: "Get enough protein or you'll lose strength." Gonzalez first went vegan in 2007 after reading The China Study, which outlines the link between animal protein and the Western world's rise in diseases like cancer and heart disease, and he was concerned about his health. He quickly slimmed down on the diet. "I lost a lot of weight, 10 pounds down to 235," he says. "I looked good, I felt great." But at training camp the next season, he could feel the loss of strength. "I can usually handle 100-pound dumbbells pretty easily," he says. "But I was just crushed by them. I was like, 'I can't do this.' I felt the best I ever felt in my career, as far as energy and endurance, but there's no way I could go out there and do my job. I've got to block 290-pound charging linemen. For my position, I need to be more bulky."

Rather than risk performance, Gonzalez started adding some meat back to his food plan. Today, he eats a largely vegetarian diet, with small amounts of fish or chicken and large quantities of vegan protein powder. He maintains what nutritionists call an 80/20 approach – 80 percent plant-based foods and 20 percent animal protein, mostly fish. "I hardly eat red meat," he says.

Gonzalez credits his best performance to his diet. "Just look at my stats over the past five years," he says, noting especially his 2008 season of 1,000-yard gains, 96 receptions, and double-digit touchdowns. "I have more energy, better focus, and more endurance. I don't get tired. I hardly ever come out of the game. And I'm strong as ever."

Montell Owens, a Jacksonville Jaguars running back, has a similar story. Owens, 28, went vegan at the end of his 2010 season after his wife read The Thrive Diet, by vegan triathlete Brendan Brazier. "I was looking for a change, for an advantage on the field and off," says Owens. As a vegan, he lost 15 pounds and, like Gonzalez, felt great – lighter, faster, "cleaner." But when he returned to camp, a strength coach took one look at him and said, "You're too small. You look like a wide receiver," meaning he might be able to run the ball, but he couldn't block the Big Boys.

Owens considered tweaking his diet to get more vegan protein, but the team didn't want to wait while he experimented. "It was pretty much, 'We can't wait on you, we'll replace you,'" says Owens. So he started eating fish. "It's more calorie-dense," he says. "You're eating the same size of salmon or shrimp as you would vegetables, but you're getting twice the calories and protein."

In the past few years, teams have added salad bars, juicers, and plant-based superfoods like kale, quinoa, almond butter, and whole-wheat pasta to their cafeterias, alongside the bacon cheeseburgers and meatball subs. While this makes eating healthy easier at home, it's still difficult on the road, where players spend half the season. Before travel days, Gonzalez and Owens plan ahead, asking for specific foods and packing vegan bars and powders. "It's 365 days a year to be a pro football player," says Sue A. James, a nutritionist for the Ravens who worked with Williams. "Whether they're training or it's the off-season, they know they have to take care of their bodies and their health. The career doesn't last forever, and [eating right] is healthier over the long term."

Evan Marcus, a former NFL strength and conditioning trainer, says coaches always get worried when an athlete goes vegan or vegetarian. "You can only show your concern by saying, 'Make sure you get enough protein. Make sure your muscles get fed and that you get complete proteins with amino acids.'" Those who are vegan or vegetarian – Williams, Gonzalez, Foster – are "very special athletes," he adds. "They seek out every advantage. They've researched this, read about it, thought about it." The bigger problem for trainers, he says, is junk-food eaters. "I've seen guys with boxes of Count Chocula in their lockers. How do you reach these fast-food junkies?"

Yet fast food and meat are so ingrained in NFL culture that players and fans can get hostile when athletes change diets. "My teammates said things like, 'You're gonna get your ass kicked,'" says Gonzalez. After Tennessee Titans guard Deuce Lutui went vegan last spring to lose weight – he dropped from 400 pounds to 340 – Ryan Kalil, a center for the Carolina Panthers, texted him pictures of cheeseburgers and roast pig. Williams says teammates would pass him sticking plates of wings under his nose, asking if he wanted "just a taste."

A big reason for the hazing is that there's a lot at stake when a player begins to tinker with his diet: his performance, and consequently, his entire team's season. But NFL contracts say nothing about diet, and Gonzalez says he and other plant-fueled players are only acting responsibly. "I feel more of an obligation to eat this way because of the money they've invested in us. The evidence is there that this is healthy – I have more energy right through the fourth. It can be a matter of seconds if you catch the ball or miss it. The guy who's eating the steak is sluggish in the fourth quarter. I want to be 100 percent. That's what puts more money in your pocket."