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Plant power ... some athletes find a vegetarian diet improves performance.

It's hard to imagine Rocky Balboa shaping up for a fight on veggie burgers but, hey, he's only a movie character. In real life, US boxer Timothy Bradley is in training for the world welterweight title in Las Vegas in June on a diet that includes tempeh and vegetable smoothies. More interesting still, Bradley is a self-confessed meat lover, but when it comes to training for an event he says it's an all-plant diet with no animal foods that works for him.

Meanwhile in his new book Eat and Run US ultra-marathoner Scott Jurek details his astonishing runs of 160 kilometres or more fuelled by foods like quinoa porridge with almond milk. Interspersed with accounts of brutal races up mountains and across deserts in blistering heat are recipes for Minnesota winter chilli, apple cinnamon granola and other vegan dishes that he credits with his endurance.

"If you eat a plant based diet you're getting a lot of extra antioxidants from vegetables and fruit so perhaps you're less likely to get sick so often and you can front up for training every day." 

Jurek is convinced that making the switch from a diet big on barbecues, burgers and fries improved his performance - although he's not sure whether it's because of what he's eating or what he's leaving out, like trans fats and refined carbohydrates.

Ultra-marathoner Scott Jurek.

Meat-free ... ultra-marathoner Scott Jurek espouses a vegan diet.

So can a diet based only on plant food and no meat really let athletes excel at their sport?

Exercise physiologist Dr Darren Morton wonders why I bother asking.

"It's a question that's already been answered because so many athletes have shown that it can," he says, ticking off a list of famous names to prove his point.

There's US basketball player Robert Parish, marathon runner Robert de Castella, tennis player Martina Navratilova, Bill Pearl who won the 1971 Mr Universe title as a vegetarian, and track and field Olympian Carl Lewis who said he performed at his best on a vegan diet.

"When it comes to high-end performance, a high carbohydrate intake is important for endurance events as well as for power and strength and this is what plants can provide," explains Morton, a senior lecturer in health and exercise science at Avondale College of further Education in Lake Macquarie. As for the extra protein required for serious bodybuilding or strength most people use protein shakes based on whey or soy that are suitable for vegetarians anyway, he adds.

Does a vegetarian diet have any added benefits for athletic performance? That's still not clear, according to the Australian Institute for Sport, but Morton wonders if the extra plant foods might reduce the number of sick days that keep athletes away from training. Although regular moderate exercise is considered an immune system booster, some research suggests that the stress of strenuous training at an elite level can make athletes more susceptible to colds and flu.

"It may be that a vegetarian diet is protective. If you eat a plant based diet you're getting a lot of extra antioxidants from vegetables and fruit so perhaps you're less likely to get sick so often and you can front up for training every day," suggests Morton, a recreational triathlete who's followed a diet that's vegan – if you don't count chocolate - for about four years. "What I've noticed is that I don't get sick now, whereas before when I was doing high volume training I got a lot of colds."

But if there's a downside to vegetarian eating for athletes, especially vegan diets that skip dairy products and eggs too, it's that plant foods tend to be low in kilojoules.

"Athletes who train at a high level need to work at eating a lot of food in order to get enough kilojoules," he says."But for non-athletes that can be an advantage of a vegan diet – you can still eat more food and weigh less. "

Eat & Run by Scott Jurek is published by Bloomsbury, RRP $29.95 in July.

Does a vegetarian diet give you the fuel you need  to work out or play sport?