A new diet pill which aims to “train” people to want less food goes on sale on the high street today.

Appesat, which is made from extracts of seaweed, works by expanding in the stomach and stimulating hunger sensors in the stomach wall. These then send a message to the brain saying the stomach is full.

Appesat sits in the stomach for three or four hours, continuing to suppress appetite, before being fully digested by the body.

Over time, this stimulation coupled with a lower calorie intake is said to help “train” people to want less food.

One clinical trial on 139 overweight and obese people found those taking three Appesat capsules three times a day lost an average of 9.4kg (20.7lb) in 12 weeks compared with 5.6kg (12.4lb) among people not taking the drug.

All those in the trial were on a low-calorie and low-fat diet of 1,200 calories a day for women and 1,400 for men.

Appesat, which costs £29.95 for 50 capsules and is taken with a glass of water before a meal, is said to have no worse side-effects than an upset stomach.

It will go on sale in 1,000 Boots stores in the UK from today and in Tesco, Sainsbury’s and Holland and Barrett from May.

The makers, Goldshield, said Appesat works in a different way to other appetite suppressants on the market by staying longer in the stomach, helping people feel “full up”.

It comes after another diet drug, Alli, went on sale over the counter last week for people with a BMI of 28 or over. Appesat, a natural-based product, will be available on supermarket shelves.

David Towse, marketing manager of strategic brands at Goldshield, said: “An intervention product that can help people modify eating behaviour over time will offer serial dieters a real opportunity to break the cycle of overeating and yo-yo dieting.” And nutritionist Mary Strugar said: “People gain weight and find it hard to lose because their calorie intake is higher than they need.

“The role of appetite is key; a detrimental cycle of food restriction that causes hunger pangs may lead directly to overeating.

“This is why many people simply give up and break their diets early.

“Appesat effectively gives people a tool to help control their appetite allowing them a window of time to train their body to want less food.”