A new study has shown that diet pills are effective partly because they encourage users to eat more healthily. Dieters who used the drugs and also cut out fatty food from their diets lost the greatest amount of weight, researchers said.

The study analysed data on 572 people who were prescribed the slimming pill orlistat, sold under the brand name Xenical, by their GP. It is one of two medications available on the NHS, the other being Reductil, and recently it also became available in a lower dose under the counter at pharmacies, under the brand name Alli.

While Reductil is an appetite suppressant, Xenical works by limiting the amount of dietary fat absorbed by the body.

The participants were questioned about their weight, the side effects they experienced with their medication and how well they stuck to the treatment program, at the start, during and towards the end of the course of slimming pills. The questionnaire also had sections about patient’s motivation for trying to lose weight, and what they thought the cause and solution was to obesity.

The research into the diet pills was carried out at the University of Surrey. One of the researchers, Amelia Hollywood, said, “Our findings support the idea that orlistat works not only on a physical level, but also psychologically – as it encourages people to see their diet as a cause of their weight problem.”

She added that as the Xenical slimming pills could have unpleasant side effects – including oily stools, flatulence and even uncontrollable bowel movements – when taken without following a calorie controlled diet, people avoided eating a fatty diet and so boosted their efforts to lose weight.

When Alli first went over the counter, some experts expressed fears that dieters might be encouraged to see slimming pills like Xenical and Reductil as magic solutions to their weight loss problems.

However it seems that the drug actually trains people to identify the foods they should avoid. Participants on the study indicated that over the course of treatment, they grew to understand which foods are fatty and so which would cause unpleasant side-effects.

Despite the fact that the majority of patients on average lost 4.52 kg over six months and saw their body mass index decrease, there was another group of patients who did try to treat the medication like a lifestyle drug. Instead of following the whole course of treatment, they chose when to take the diet pills, taking it when they were eating fatty foods to reduce weight gain or not taking it when out for a meal or on holiday.

The findings from the study were presented at the British Psychological Society’s Division of Health Psychology annual conference.