Some tennis stars say gluten-free is best for them

German Lisicki was on the verge of upsetting third seed Vera Zvonareva in the second round of the French Open on Wednesday but, with the finishing line in sight, she crumbled on court and had to be carried off on a stretcher sobbing.

On Thursday, the 21-year-old explained why her health had suddenly deteriorated.

"I am sad that my body let me down. Doctors recently discovered that I am intolerant to gluten -- meaning I can't eat e.g. pasta, one of my biggest energy sources," Lisicki, who was seen munching on a couple of bananas on Wednesday, said on her website .

"My body needs to adjust to the big change and needs some time. It is good that we found out and it will only make life better in the long run."

Gluten is found in wheat, barley and rye, and in products that contain these grains or their derivatives.

Pasta and bread were once staple food items for top athletes as they were the most important sources of energy. Athletes on gluten-free diets need to find substitutes for their old standbys.

Serbian Djokovic, who is on a 39-match winning streak in 2011, changed his diet nine months ago after his nutritionist carried out tests and established he was intolerant to gluten.

Like Djokovic, Lisicki's body cannot cope with many carbohydrate products and she will need to find substitute food items so that she can find the energy to last the distance in three-set matches.

The new diet has definitely paid off for Djokovic.

"I have lost some weight but it's only helped me because my movement is much sharper now and I feel great physically," Djokovic, who has beaten Rafa Nadal in four finals this year, said recently.

The gluten-free diet is necessary for people who have celiac disease. It has been gaining in popularity among the general public, but doctors warn that people who eliminate gluten from their diet can also lose out on important nutrients.