But this is not because compulsory treatment is ineffective.

The patients themselves are more likely to have deeper psychological problems which make them less likely to respond to treatment.

The study, published in the British Journal of Psychiatry, compared 81 compulsory patients at the Eating Disorder Unit at the Maudsley Hospital in London with 81 voluntary patients.

The compulsory patients tended to be admitted under Section 3 of the Mental Health Act, which allowed for detention for up to six months.

This is because these patients are thought to need time to return to a healthy weight.

Psychological problems

The researchers, led by Rosalind Ramsay, say there was no significant difference in body weight between the voluntary and compulsory patients.

The difference between the two groups tended to be that they were less likely to accept they had a problem and were more likely to have deep psychological problems.

Some, for example, had a history of physical or sexual abuse in childhood, many had a history of self-harm and several had been admitted to hospital on many previous occasions.

In the short term, the researchers said the compulsory patients responded to treatment and put on the same amount of weight as voluntary patients.

However, they tended to need a longer time.

Compulsory treatment did not necessarily involve force-feeding. Indeed, this was rarely needed.

However, on follow-up around six years later, the compulsory patients were much more likely to have died than voluntary patients.

Ten of the compulsory patients died, compared with two of the voluntary ones.

High death rate

Anorexia has a very high mortality rate compared to other mental illnesses.

The annual mortality rate is twice that of other female psychiatric patients, running at 0.56% a year.

Most deaths are due to complications arising from the illness, such as heart attacks, but suicide accounts for 27% of deaths.

The researchers say: "The findings of an increased mortality rate [among compulsory patients] should not be misread as reflecting adversely on the compulsory treatment."

They add: "Compulsory treatment was clearly beneficial in the short term."

And they say some anorexics who appear untreatable have gone on to recover.