"We're really designed physiologically to burn fat, it's what our body is designed to do. Shifting the major energy systems from sugar to fat is the transition you go through when you take a gluten-free option," McCraw said.

The first step in treating problems with gluten is to stop eating grain-based foods for at least four weeks, says Everard, but not -- especially for high-performance athletes -- to eliminate carbs altogether.

"We educate the patients about consuming gluten-free ancient whole grains. These include amaranth, buckwheat, corn, millet, uncontaminated organic oats, quinoa, sorghum, teff and rice. This gives the gastrointestinal tract (GIT) a chance to start repairing itself."

It needs help, and probiotics will aid restoration of the stomach's healthy bacteria and digestive enzymes, and also protect against further damage, Everard says.

"The research and my clinical results have found the GIT is weakened when we are exercising, especially in hot conditions and leaves the body susceptible to illness," he adds.

"The lining of the gut is very sensitive to the intense heat that endurance athletes train under. My patients have often complained of cramping in the gut, diarrhea or nausea and an increase in colds and flus after competitions and intense training.

"As the body heats up, small cracks form in the intestinal wall, allowing bacteria into the blood stream. We monitored our patients and found that when they did extensive exercise 82% showed symptoms of gut discomfort, reduced tolerance to the heat or a decrease in immune function."

In the case of Djokovic, who clinched a record third successive Australian Open title on Sunday, he had to give up childhood staples such as pizza, pasta and pancakes while introducing more vegetables and rice, fruit and sushi for easily digestible protein.

"At first, it was difficult for him but he was fed up of being stuck in third place and his determination to be the champion combined with his confidence in my knowledge was all the motivation that he needed to change his diet," says Cetojevic.

"I also suggested that he cut down on his consumption of meat, particularly before a match. Coming from Serbia, that was a radical suggestion -- even more so than cutting out gluten!

"Because Novak was in very good condition from his constant training routine, the benefits were apparent almost immediately, which encouraged him to continue. The breathing problem vanished as did the frequent injuries and strains that had been hampering his progress."

McCraw says he has also seen "life-changing" results in his clients.

"Personalities changed, they were much calmer, had more clarity, better grades at school, their relationships with parents have improved, my relationship with them has matured," he said.

"Less sweating on the court, their endurance levels have increased, ability to maintain a leaner figure has improved, ability to build and hold muscle mass has improved. All of the things that an athletic trainer and coach would strive for, I've seen evidence of that on a daily basis."

However, a gluten-free diet might not suit everyone, says Susie Parker-Simmons, a sports scientist who works with the U.S. Olympic Committee as well as women's tennis players on the WTA Tour.

"There is no benefit in avoiding gluten if you do not have celiac disease or gluten intolerance," she told CNN.

"Therefore I would not recommend an athlete try this diet unless diagnosed by a medical professional."

She said that athletes might find it difficult to adhere to such a diet, especially tennis players who have to travel a lot and might have restricted food options.

"Gluten-free standards and labeling differs in each country. Before going overseas WTA players need to gain advice on the best foods to eat in their country of destination and order gluten-free meals on the airlines they are traveling with," she said.

"A large amount of carbohydrate-rich foods contain gluten in them e.g. bread, pasta, sports bars etc. An athlete with celiac disease needs to be very careful when selecting carbohydrate-based foods to ensure they receive their daily requirements."

Another problem is that many foods containing gluten are also important sources of fiber -- which the body needs to keep the intestines clear of toxins.

"Common gluten-free sources of fiber include: vegetables, fruit, brown rice, legumes, nuts and seeds," Parker-Simmons said.

She believes that while research has shown positive links between health improvement and a gluten-free diet, there is little to suggest it helps in terms of injury prevention and recovery.

McCraw, however, said that his players had shown improved ability to bounce back from intense workouts.

"There's always micro trauma in the muscles and joints, and the body has a natural anti-inflammatory response to that after exercise," he said.