Millions of British adults are suffering from serious illness because their bodies are unable to safely digest milk, a husband and wife team of biochemists have claimed.
Dr Stephanie Matthews and Professor Anthony Campbell believe at least seven million Britons suffer from chronic fatigue, arthritis-type joint problems and bowel problems because they are intolerant to lactose, a sugar which naturally occurs in milk.
Their findings - being presented at a national conference on food allergies organised by the Royal Society of Medicine on 1 July - will provoke fierce controversy in the medical world and alarm the farming and food industries.
The couple, who have already had one case study based on their theory published in the The Lancet, claim that millions of people are suffering from these illnesses because their intolerance has gone undetected.
Dr Matthews, who runs a special NHS clinic dealing with lactose intolerant patients at Llandough Hospital in Cardiff, said more than 250 patients showed marked and often complete improvements in their health after cutting milk from their diets.
After being checked with a widely recognised breath test for milk intolerance, the patients were cured of illnesses such as debilitating fatigue, headaches, persistent bowel and stomach upsets, and even asthma and tachycardia - a rapid and irregular heart beat.
"Milk is very good for you - if you can tolerate it - but if you can't, it can do you a lot of harm, and this hasn't been recognised," Dr Matthews said. Her husband, a professor at the University of Wales College of Medicine, added: "We believe we've found a major new syndrome here."
However, two of Britain's leading authorities on lactose intolerance were deeply sceptical about their claims.
Dr Paul Clayton, who will co-chair the Royal Society of Medicine conference, said he believed they had confused lactose intolerance with an allergic reaction to other proteins in milk. "I find this very hard to understand," he said.
Professor Dallas Swallow, a geneticist at the Galton Laboratory at University College London, said their theory was "implausible". Few of these illnesses, except bowel and stomach problems, had been scientifically linked to lactose intolerance. "I'm puzzled about this," she said.
Medical experts agree that about 5 per cent of white Britons become unable to digest lactose when they reach adulthood. They fall ill if they drink milk or eat foods such as breads, ready-meals or sauces which contain milk or its natural sweetener, lactose.
The proportion of lactose-intolerant adults from other ethnic groups rises sharply in the southern hemisphere and the Far East. While nearly all babies can safely drink milk, about 95 per cent of Chinese adults and about 50 per cent of north Indians grow up to be lactose intolerant.
Dr Matthews and Professor Campbell believe that at least four million white Britons suffer from this intolerance - double the accepted figure. A large majority of Britain's three million non-white adults also have that intolerance, they suspect.
Their symptoms go undetected because there can be a 24- to 36-hour gap between someone drinking milk and suffering symptoms, because lactose intolerance is often unrecognised and because their symptoms are so similar to other illnesses.
They also believe the rate of lactose intolerance has grown because far more processed foods, such as bread, beer and even sausages, use lactose as a bulking agent, texturising agent and sweetener. The lactose additive is often not labelled - chiefly because it is not seen as risky.
Professor Campbell, a biochemist who achieved fame last year after inventing a test using genetically modified proteins that change colour, said: "We're not saying milk is bad for everybody - it's a tremendous product. I take a lot of it because I'm okay."
The couple believe that sufferers become ill because they don't have the lactase enzyme that processes lactose in their small intestine. As a result, the lactose passes into the large intestine, and is then "eaten" by unsuitable gut bacteria, which then discharge toxins into the body - a theory disputed by Dr Clayton.
They admit their theory needs to be tested in a peer-reviewed scientific trial, and will apply for research funding. Their local NHS ethics committee has approved their plans to test patients referred by hospital consultants.