THE WEIGHT-LOSS diet is surely a nominee for The 20th Century's Biggest Red Herring award. All it has really taught us is that perfect weight is best achieved not by dieting (98 per cent don't work long-term), but with healthy habits and attitude. Habits we all had anyway before fast food and unprecedented wealth turned us into a nation of lard-arses.

Forget laxatives, vomiting and diet groups, a study published last month in the American Journal of Clinical Nutrition found that one of the most effective diet aids has a far more appealing name: soup.

In the study, 24 normal-weight women were given a lunch of either chicken rice casserole or chicken rice casserole served with a glass of water or chicken rice soup. The soup contained the same ingredients as the casserole. The results showed that the women who ate the soup felt fuller and more satisfied with a reduced calorific intake. Drinking water with the casserole had no effect on calorie intake, contrary to popular myth. Best of all, the soup eaters, despite eating less, did not eat more later in the day.

"A lot of the explanation is psychological," says one of the authors of the paper, Elizabeth Bell, graduate researcher at Pennsylvania State University's department of nutrition. "The women were presented with a huge bowl of food and it felt like a lot. If it could trick normal-weight women, who successfully regulate their food intake, then it could be even more successful with overweight women whose food regulation is less good."

In another recent study at Penn State, nutrition scientist Barbara Rolls found that normal-weight men who aren't hung up on their weight have an internal, subconscious calorie counter. In the test, volunteers were given a yoghurt with either fat or carbohydrate added. The yoghurts tasted the same and had the same volume but the fat yoghurt had more calories. The healthy-attitude men automatically ate less of the higher calorie yoghurt. Overweight volunteers and normal weight volunteers who were nonetheless worried about their weight ate the same amount of yoghurt, regardless of calories.

"Some adults regulate food intake based on their bodies' physiological signals," said Rolls. "These men had internal mechanisms that rapidly detected the energy content of their foods, allowing them to adjust their calorie intake unconsciously. A relative insensitivity to fat could be involved in developing and maintaining obesity."

So the best way to lose weight is to eat normally, substituting fat and high-energy dense food for vegetables and watery things. Don't nibble at half a portion of chocolate cake and expect to be satisfied. And don't use this information as an excuse to give the Cabbage Soup diet another try. This fashionable self-torture requires you to boil a huge vat of cabbage soup - Victorian workhouse style - and exist on that alone. You really can lose 10lbs in a week, but the long-term effect is to leave you turned off cabbage and soup for life.

The soup study (which only shows what starving medieval peasants knew - that rotten turnip soup goes further than rotten turnips) is part of on-going work by scientists to try and put some sense back into eating habits. Dieting is so simple: it only works if the food is satisfying and pleasant enough to be stuck to until the goal is reached. Let's hope in the next century, science encourages healthier diet products. Sadly the chances, unlike increasingly few of us, are slim.

Barbara Rolls new book `Volumetics - eat low energy foods to trick yourself thin' - is published by HarperCollins in the US in December.