Long before Chip Kelly arrived in Philadelphia and banned junk food in favor of personalized smoothies, fitness experts have been helping NFL players get bigger, stronger and faster by changing their eating habits.

Players often transform their bodies during the offseason, but it requires more than pumping iron. Whether they're trying to bulk up or slim down, proper nutrition is essential.

"We are not eating for pleasure, we are eating with a defined purpose to fill each athlete with rocket fuel," said Kevin Dunn, owner and CEO of TEST Football Academy.

And what they eat isn't for the average person. Training guru Steve Saunders has an "earn your carbs" diet plan for athletes who want to gain lean muscle mass and burn body fat. It's popular among linebackers.

"They're only allowed carbs after a workout and the meal after a workout," said Saunders, president of the fast-growing Power Train franchise.

Saunders has trained numerous Eagles, Steelers and other NFL players, including two-time All-Pro linebacker James Harrison. A sample lunch from the "earn your carbs" diet includes: 16-20 ounces of chicken breast, beef, steak, or ground turkey or turkey sausage; one cup of rice, two potatoes and one cup cooked pasta with green vegetables or salad and two tablespoons of olive oil.

Saunders says wide receivers present the biggest challenges. Considering their diva reputation, that's no surprise.

"Receivers are flaky at the beginning because they automatically associate weight gain with getting slower, which is wrong, so they don't want to gain or lose a pound," Saunders said. "Once you get their trust, they'll do it. I remember Antwaan Randle El was 184 pounds and it took me three, four months to convince him that he'd be better at 190, that he'd be more durable, he'd feel better, he'd take hits better."

Some players — think linemen — want to slim down and lose body fat. Saunders has a "zero tolerance" diet for them that consists of no carbohydrates other than greens for three weeks. Colts offensive lineman Todd Herremans is one of many players who tried this diet.

"It will melt the fat and build muscle," Saunders said. "It's hard to do for an extended period of time, but for 21 days it will drop a drastic amount of body fat."

A sample breakfast includes six whole eggs, six egg whites or six ounces of red meat (leanest cut filet) or ground turkey breast with steamed asparagus.

Dunn has trained Joe Flacco, Patrick Peterson, Demario Davis and other NFL stars. He uses high-tech equipment to monitor how many calories his players burn during their workouts.

"We found our O-Line players burn an average of 22,000 calories per week above and beyond their basal metabolic rate, which would require between 4500-5000 calories per day to maintain their weight," he said. "Therefore, it becomes critical to replenish that fuel with some of the most nutrient dense foods you can get your hands on."

Before Kelly brought sports science to the Eagles, former coach Andy Reid let them eat whatever they wanted. The team even had junk-food themed days such as "Taco Tuesdays" and "Fast-food Fridays" during the season.

But Kelly banned pizza, burgers, fried chicken and other unhealthy treats. Players have customized protein shakes waiting for them after practice and other healthy snacks available outside the locker room. The team's cafeteria only offers a health-conscious menu now. After Kelly arrived, he had signs hung in the cafeteria explaining each of the four major food groups.

"You use proper nutrition to make them bigger, faster and stronger," Kelly said.

Many players immediately jumped on board.

"Once you're done working out, you need to replenish your body, otherwise it's just going to keep breaking down," said tight end Brent Celek, who also has trained with Saunders.

San Francisco 49ers safety Eric Reid is such an advocate of healthy eating that he partnered with My Sports Dietitian, a new company that's devoted to sports nutrition and education services.

"I would recommend everyone else check them out," Reid said.

Tavis Piattoly, co-founder of the company, said Reid was a case study who inspired the concept.

"When he was a sophomore in high school, he weighed about 160-170 pounds and he was able to put on about 40 pounds of lean muscle in about two years through a good diet and increasing his calories in a good way," Piattoly said. "That made him more attractive to college scouts and it opened him up to more schools recruiting him. I've seen that in a lot of kids."

Piattoly's mission is to inform and educate athletes, parents and coaches on the importance of nutrition versus overuse of supplements.

"Too many athletes are turning to supplements to try and increase their performance," he said. "We believe in a food-first approach to help an athlete reach their peak performance."