Scientists have produced evidence that a food supplement can help to reduce the long-term suffering associated with the crippling disease osteoarthritis.
Osteoarthritis, or degenerative joint disease, is the most common form of arthritis, most often affecting middle-aged and older people.
The disease, which can cause severe disability and pain, tends to develop in the joints of the neck, lower back, knees, hips and fingers.
However, it may also occur in joints that have been previously injured, or subjected to prolonged heavy use.
Experts estimate that 60% of those aged 65 have moderate to severe osteoarthritis in at least one joint.
It is caused by the degeneration of the cartilage, the protective material that stops bones rubbing together in the joints.
The dietary supplement glucosamine sulphate has been shown to reduce the severity of symptoms in the short term.
But now researchers have achieved the same effect in the longer term.
The work was carried out by Professor Jean Reginster and colleagues from CHU Centre Ville, Liege, Belgium.
Patients with the disease either received a daily 1,500 mg dose of a form of the compound, or a placebo over a period of three years.
The researchers took radiograph images of the patients' knees while lifting a weight after one and three years.
They found that the space between the joints of the knee narrowed significantly over time in the 106 patients who received the placebo. After three years the average loss was 0.31mm.
However, there was no such narrowing among patients on glucosamine sulphate.
Those patients who completed the full course of treatment showed a 20-25% improvement in their symptoms, compared with slight worsening of symptoms in the placebo group.
Dr Madeleine Devey, scientific secretary of the Arthritis Research Campaign, said many people with osteoarthritis had been taking glucosamine sulphate for many years.
She said a small scale trial of the supplement in the UK found that it had a small beneficial impact on patients who were in mild pain - but not those in severe pain.
A larger scale study is currently being undertaken in the US.
Dr Devey told BBC News Online: "Many people with osteoarthritis take glucosamine sulphate, but until now the evidence that it does any good is very inconclusive.
"These results are encouraging, but we would welcome properly conducted clinical trials to answer the question one way or another.
"Osteoarthritis causes a great deal of social exclusion for elderly people who cannot get out."
Dr Tim McAlindon from Boston University Medical School, USA, said the study was a landmark piece of research.
He said: "It is time for the [medical] profession to accommodate the possibility that many nutritional products may have valuable therapeutic effects and to regain the credibility of the public at large".
The only current treatments for osteoarthritis are painkillers and non-steroidal anti-inflammatories, which are also given to dull pain. Some patients undergo joint replacement surgery.
The best way to minimise risk is to take regular exercise and to lose excess weight. It is also a good idea to wear proper trainers when taking exercise to protect the ankle and knee joints.
The research is published in The Lancet medical journal.