EAT LESS, exercise more — the secret of shedding those excess pounds couldn’t be simpler, yet the success of the Atkins, GI and GL diets show that really, we prefer our weight-loss plans to be a little more complicated (or at least, to make us feel that by buying the book, we are halfway there).

But a new diet book seems to offer a more straightforward approach. The Volumetrics Eating Plan by Professor Barbara Rolls, a nutritional scientist at Pennsylvania State University, is a simple diet that allows you to eat more of the kinds of foods that make you feel full yet which help you to lose weight.

Most people naturally consume about the same amount of food every day, says Professor Rolls. The problem is that gram for gram, some foods contain more calories than others. This means that the total calorie intake for the same volume of food can vary greatly.

It is on this energy-density principle of food that her volumetrics eating plan is based. Foods such as fat, which provides 9 calories a gram, are replaced with those that are low in calories, such as fruit, vegetables and soups. Food is categorised into one of three “energy-density” groups. Professor Rolls says that those with an energy density of less than 1 can be eaten in “satisfying portions and should form the bulk of someone’s diet”. Those scoring 1 to 2 should be eaten two or three times a day when included with main meals. Any foods with an energy density of more than 2 should be consumed only sparingly; ideally, no more than once a day.

Working out a food’s energy density is simple — divide its calories per serving by the size in grams of that serving. Calculations can be made using standard calorie books or from the nutrition labels on pre-packaged meals and snacks (the book includes a list of energy-density ratings).

Rolls and her colleagues have found that you can eat the same amount, but save on calories, by following these guidelines. In simple terms the volumetrics plan, like most diets, comprises mainly fruit and vegetables which can be eaten freely. What is surprising is that foods such as yoghurt, cottage cheese and smoothies can also be eaten as often as you like.

The volumetrics eating plan is based on clinical studies which have suggested that hunger can be satisfied without overdosing on calories. In a study presented at the North American Association for the study of Obesity at the end of last year, the Pennsylvanian State University researchers said that making small changes to the type of food eaten, but not the volume, meant that female dieters consumed 800 fewer daily calories. Each of the 24 women, aged between 19 and 35, who took part in the trial were given a total of 2.5kg (5.5lb) of food a day (an amount considered an average intake). Those who were given less energy-dense foods — a turkey sandwich instead of a pizza or a low-fat blueberry muffin instead of a cream cake — not only consumed 23 per cent fewer calories but lost weight more quickly. Yet they reported feeling as satiated as the high energy-dense group.

“In our abundant society we have a huge variety of energy-dense foods that taste good,” says Professor Rolls. “The problem is that we just keep eating them beyond when we should stop. With volumetrics, people get a consistent amount of food but not the same amount of calories.”

In a separate study published in The American Journal of Clinical Nutrition, Professor Rolls showed how starting a meal with a low energy-dense broth or green salad reduced the total amount of calories consumed in a sitting. A group of overweight adults given a soup or salad starter ate 56 per cent fewer calories in the pasta main course that followed and lost more weight more quickly.

Rolls has also found that dieters given two servings of soup a day in place of two dry snacks such as crisps or nuts shed excess pounds more efficiently. She says: “We want to send a message to the food industry that even subtle changes to the composition of a diet can have a huge impact on what people eat overall. You can lose weight this way.”

Claire Williamson, a nutrition scientist at the British Nutrition Foundation, describes the volumetrics plan as “a sensible approach to weight loss”. She says that plenty of studies support the theory that “the higher the energy density of a diet, the higher the total energy intake is likely to be”.

Water content and fibre influence the energy density of a food — which is why fresh fruit and vegetables feature so prominently in the volumetrics diet — as do the amount of carbohydrate, fat, protein and alcohol.

Louise Sutton, head of health and exercise science at Leeds Metropolitan University, says that Professor Rolls has come up with a way inwhich you can physically fill up your stomach nutritiously.

“There is only so much that anyone can eat,” she adds. “And if you are eating that amount of the right foods, it can only be a good thing.”

The Volumetrics Eating Plan by Barbara Rolls is published in America by HarperCollins ($25.95)

ENERGY-DENSITY RATINGS LESS THAN 1

(to be eaten freely) All fresh fruit and vegetables, low-fat yoghurt and cottage cheese, semi-skimmed milk, non-cream based soups and pasta sauces, fruit juices, smoothies

BETWEEN 1 and 2

(to be eaten in moderation) Fish, pulses and beans, brown and white rice, wholemeal pasta and bread, chicken and lean cuts of most meat, eggs, casseroles and stews.

MORE THAN 2

(to be eaten sparingly) Cheese, full fat dairy foods, ice cream, chips, crisps and biscuits, fats (margarine, butter, oils) and most fried foods

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