No one can match Novak Djokovic on the tennis court. After smashing his way past Britain's Andy Murray in the semi-finals, the Serbian outfought Spain's muscular Rafael Nadal 6-4, 6-4 to win the Rome Masters.

For Djokovic, the most in-form player in the world, Sunday's match was his 39th straight victory in succession. This year the 6ft 2in Belgradian has won seven titles and £3.3m. His success is not just down to his agility, baseline aggression and strong serve, however, but his diet.

Like an estimated 600,000 Britons, the muscular 23-year-old Serbian has coeliac disease, which means he is intolerant to gluten found in many grains. Last month, Djokovic subscribed his new-found success on the court to his nutritionist, Igor Cetojevic, who has steered him away from wheat, barley and rye.

"He's done a great job in changing my diet after we established I am allergic to some food ingredients, like gluten," Djokovic explained. "It means I can't eat stuff like pizza, pasta and bread. I have lost some weight but it's only helped me because my movement is much sharper now and I feel great physically."

Unlike most coeliacs, Djokovic has the benefit of knowing why he was underperforming. Around 1 in 100 people in the UK have the disease, but only an estimated 10 to 15 per cent have been diagnosed. There is no cure or treatment for the disease. A change in diet is the only help.

In speaking publicly about the condition, Djokovic has become the world's most famous coeliac, and living proof that sufferers can not only lead relatively normal lives but can excel. Sufferers of coeliac disease experience an auto-immune response to gluten, a protein in many grains, which irritates the small intestine, sometimes causing symptoms throughout the body. The disease runs in families.

Diagnosis within Britain is slowly improving. Two years ago the NHS watchdog, the National Institute for Health and Clinical Excellence (Nice), advised doctors to carry out blood tests on patients presenting potential symptoms. The list is long and various: headaches, mouth ulcers, irritable bowel synrome, bowel cancer, weight loss, weight gain, infertility, early onset oesteoporosis, and stunted growth in children.

An estimated 500,000 sufferers in Britain are undiagnosed, either because the illness is dormant or because they do not know why they feel unwell. The 100,000 or so in whom the disease has been diagnosed face difficulty shopping and eating out. Excluding wheat rules out ordinary pasta, bread, cereals, biscuits, cakes, pizza, and a huge range of other foods, including fish fingers, sausages, gravies, sauces and soy sauce.

Increased diagnosis of sufferers in recent years has driven a boom in "free-from" foods, which now command an aisle of their own in large supermarkets. Gluten-free food grew by more than 20 per cent in 2009 to reach £100m in sales, according to the market research company Mintel.

Coeliac UK, with 60,000 members, is appealing for greater awareness of the disease in the hope that more people will be tested. This week it is urging people without coeliac disease to take the Gluten Free Challenge – to buy a gluten-free dish in a supermarket or restaurant.

The aim is to spread the word about the condition, as well as to stoke demand for gluten-free products and their greater availability. Coeliac UK's Kate Newman said: "Gluten-free products are a huge growth area in supermarkets and restaurants, but there is still a long way to go."

She could not promise that a diagnosis would turn individuals into world-beating tennis players – as Djokovic explained last month when he disclosed his "secret", that includes years of hard work and practice – but it could make half a million people healthier and happier.

Forbidden foods: what's not on the coeliac menu

* Food manufacturers, industry experts and coeliacs are among the few who appreciate just how widely grains are used in modern processed food.

* In addition to obvious wheat products such as bread, pasta, pies, breakfast cereals, biscuits and cakes – which account for a vast proportion of everyday foods – wheat is also widely used as a cheap filler, coating or thickener in a huge array of other products.

* Commonly, wheat can be found in sausages and the breadcrumb coating of fish fingers, as well as being used as a thickener in gravies and sauces, including oriental ones such as soy sauce. Beer – typically made from barley – and grain-based spirits are also out of bounds for coeliacs.