"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.
"The gluten inhibits that process and therefore the onset of muscle soreness is worse. The general ability to recover and retain that homeostatic state is compromised. Your threshold of work levels is lower and your recovery rate is higher and longer."
Everard would like to see more studies into the effects of gluten, which he believes could be at the heart of many modern illnesses.
"This is a very complex area and not fully understood in mainstream medicine, however there is a huge amount of research and evidence in this field," he said.
"I see the benefits of a gluten-free diet in my patients every day through clinical experience, but there still needs to be more research done so that this can become more mainstream -- especially in the world of sport."
Many studies focus solely on the benefits or otherwise of eliminating gluten, rather than taking into account other dietary elements.
"The question is -- is it actually gluten that is the problem or is it refined grains or grains in general?" says British nutritional therapist Kate Delmar-Morgan.
"Individuals wishing to go down this route should seek proper nutrition advice and be tested if possible."
There is also the issue that many gluten-free products are processed and contain high levels of added sugars and artificial additives.
"People will look at these products and think they are 'healthy' just because they are advertised as gluten-free," Delmar-Morgan says.
"However if people stick to a wholefoods diet -- i.e. fresh fish, meats, vegetables, fruit, nuts and seeds, wholegrains which do not contain gluten, and they try to cook from scratch as much as possible and have limited pre-prepared foods -- they can then avoid this problem."
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