All the evidence coming out seems to show that it is not effective to increase leptin levels

Dr John Wilding, University Hospital Aintree

Those taking leptin said their appetite had been reduced.

But the researchers said these subjective assessments were made in a "fasting state" before breakfast - and did not last for the whole day.

Search for a cure

The most well-known treatment of obesity is Xenical, the brand name for orlistat, which helps people shed dramatic amounts of weight quickly.

It works by blocking the digestion of fat, and is the first treatment that does not rely on suppressing the appetite.

Previous experiments on mice had shown leptin was successful at reducing appetite.

In this US research, doctors hoped to take advantage of the message leptin proteins send to the central nervous system to regulate appetite.

Thirty men with an average age of 45 were selected. All had a Body Mass Index (BMI) of 34.2.

Body Mass

Obesity is classified as having a BMI of 30 or more. It is calculated by dividing weight in kilograms by the square of your height in metres.

Once a week over the 12 week study, half the men were injected with a form of leptin chemically modified so the body would not react to it.

The other men were injected with a solution which would have no effect on their appetite.

All were asked to cut their calorie intake to 2,100 calories a day.

Doctors then measured their daily appetite, calorie intake, energy expenditure, BMI and leptin levels were measured at regular intervals.

It was found appetite and hunger levels fell in the group taking leptin from the first day, while in the other group they rose.

The men taking leptin also did not want to eat as much as they had and felt they did not need to. They also reported greater feelings of fullness.

But the low appetite was only recorded before breakfast, and was not reflected in lower food intake at breakfast or dinner.

After 12 weeks, the group taking leptin had lost an average of 4.3kg and the group who had received the inert injections had lost 6.4kg.

Dr John Wilding, of the University Hospital Aintree, Liverpool, a senior member of the Association for the Study of Obesity, said the US findings mirrored what their own findings that leptin did not help weight loss.

He said: "All the evidence coming out seems to show that it is not effective to increase leptin levels."

Dr Wilding said some people have a deficiency of leptin and are seriously obese. For them, extra leptin helps."

He said just that did not mean leptin would be useful as a cure for all people with obesity.