Die-hard fans of the Atkins, South Beach and Cabbage Soup diets may be dismayed to hear that little science supports the claims of celebrity diets. Participants at the Festival of Science were told that cutting calories is all that really matters to the waistline.

Fad diets work because people eat fewer calories on these plans, not because of the specific foods allowed. Sticking to fewer calories over a long time period, which is easier if friends and colleagues are doing the same, is also important for steady weight loss.

Dieters want to lose lots of weight fast. ‘Inappropriate weight aspirations are common among women,’ said Dr. Claire Mac Evilly, of the Medical Research Council’s Human Nutrition Research in Cambridge, as ‘individuals turn to social icons and role models to define an unrealistic weight target.’

People enter diets with the unrealistic expectation of quickly losing 20-30 per cent of their weight, whereas even complete starvation will only result in a weekly loss of about 2.5 kg of fat, and will have serious side effects.

‘Obesity is a serious and threatening condition,’ said Dr Mac Evilly. To lose weight healthily, she recommends eating a variety of foods from all main food groups; thinking about portion size; cutting back on saturated fats, sugars, and salt; increasing wholegrains; increasing fruit and vegetables to at least five portions a day, and exercising more.

Dieting is part of modern life. The National Diet and Nutrition Survey of British Adults found that 24 per cent of women and 10 per cent of men are trying to lose weight at any given time, and a further third are ‘watching their weight’. About 30 per cent of children are overweight and obese, and are being caught up in the dieting culture at an ever earlier age. 49 per cent of teenagers have dieted to lose weight, and 38 per cent are currently on a diet.

But dieting must be a long term lifestyle change, not a quick fix decision. With fad diets, 42 per cent of participants in one study had abandoned their diet after one year. ‘Weight loss depends on cutting calories and sticking to it,’ says Dr. Mac Evilly. ‘Whether you exclude protein, fat or carbohydrates is irrelevant – adherence is the key.’