Metabolism is bunk: Hester Lacey meets the scientists who can prove it
"You're fat because YOU EAT TOO MUCH. There isn't any other reason." These bracing (or shattering) words are from the Better Homes and Gardens Diet Book of 1955. Published before slimming became an industry worth £1 billion a year, and before diet gurus had to sell their books by flattering their public or blinding them with science, the editors felt no need to mince their words. Forty years on, however, the overweight can toy with dozens of excuses: slow metabolism or "big bones", food into l erance (as in the Nutron diet), enzyme deficiency (the pineapple-pushing Beverley Hills diet), digestive difficulties (the "don't mix starch and protein" Food Combining diet). So it's really not our own fault if we're plump rather than svelte. Or is it?

The Dunn Clinical Nutrition Centre, a medical research centre at Addenbrooke's Hospital in Cambridge, has been analysing the nation's diet for over 60 years. Nutritionists Dr Andrew Prentice and Dr Susan Jebb, both whippet-slender and bubbling with enthusiastic energy, have been investigating the failed slimmer's favourite cop-out: the notion that their metabolism is out of kilter. Discouraged dieters claim the rate at which they convert fuel into energy is sluggish, so that their body insists on layingfood down as fat, however little they eat.

Dr Prentice is in charge of the Calorimeters; three small neat boxlike rooms, each furnished with a bed, a desk, a phone and a television plus video. "We try to make it as comfortable as possible but it is in fact a scientific instrument," he says; sure enough, a fearsome array of tubing and piping leads from each one to a huge winking bank of computers.

In one of the rooms, a plumpish woman is puffing away on an exercise bike; nearby on a plate are carefully weighed (small) rations of fruit cake, ready for tea-time. Volunteers spend five days in the sealed Calorimeter. Fresh air is circulated through the room from the outside, sucked out again, and analysed for oxygen and carbon dioxide levels; from these, the subject's energy output and metabolic rate can be calculated.

Weight levels are analysed by dividing volunteers' weight in kilograms by height in metres to give a figure called the Body Mass Index; a BMI of below 20 is underweight; 20-25 is healthy, 25-30 is overweight, and above 35 is obese. Allowances are generous; someone 1.68 m tall (approx 5' 6") would be considered healthy at any weight between 55 kg and 70 kg (approx 8st 9lbs and 11st 2lbs).

Surprisingly, a high metabolic rate and a high BMI seem to go together. "Whereas most obese people think they have low metabolisms and can't lose weight," Dr Prentice says, "in fact, people who are overweight have a higher basic metabolic rate. When you

become fat, organs like your heart, digestive tract and liver need to be bigger and they cost more energy. And the effort required to carry fat around with you takes energy, so the rate goes up." He has found people who are overweight can use over a third more energy per day than lean people. "This turns on its head the `doctor, it's my metabolism' excuse," he says.

Dr Jebb has also investigated the effect of intermittent prolonged weight-loss and gain on metabolic rate - so-called yo-yo dieting - and found the long-term effects to be negligible.

But if most healthy metabolisms tick over at much the same rate, why are people so many different shapes? The reply has a stern Fifties feel about it. "Lack of control of food intake. We are eating too much." But every supermarket has shelves crammed with low-cal foods, every bookshop has a whole section devoted to diet and health. And, points out Dr Jebb, calorie consumption per head has actually declined by around 700 per head over the past 40 years. "We are eating less than we used to," says Dr Prentice, "but our rates of energy output have gone down even further." This is a sensitive issue. Asking people whether they think it's possible they over-eat and you risk a very dusty answer. They translate this straightforward question into an insult alongthe lines of "Are you a blubbery lard-butt with no self-control?"

"No! It's simply that my fat-to-muscle ratio is wrong," replied one man, mystifyingly.

"I swear I don't. It's a very cruel thing to ask. I never touch sweets or puddings and it seems so unfair that I weigh ten stone," says Maria, 5' 2".

"I'm sure metabolism plays a part. If I ate what my friend Lucy eats I'd turn into a barrage balloon," snorted Suzanne, 25. (Further questioning elicited that Lucy was four inches taller than Suzanne and never touched alcohol, which is extremely fattening.)

Others are more honest. "Yes, I'm afraid I do," says Annabel, 56, 5' 4" and 12 stone. "I love butter, cream, gravy, sauces and pastry. Those are my downfall."

Clare, 28, a successful dieter who has lost three stone and maintained her weight, says, "I'd dieted in the past and thought I couldn't do it because my family are all big. But this time I ignored that excuse, stuck at it, ate less, and it worked. My appetite's a lot less now - I still eat chocolate but just a tiny bit. I must have been eating too much before."

But what about the unfortunates who claim they can't lose a single ounce on a sparrow's diet, and the lucky ones who eat like horses without gaining an ounce? "They are misleading themselves about what they are eating," says the implacable Dr Prentice. "I remember experiments done years ago by Professor Garrow, who is the obesity expert of the medical world - he was at Barts until he retired recently. When people swore they couldn't lose weight they'd be locked in a ward, and when they were forced to stick to a diet the weight piled off in a perfectly predictable way."

Dr Prentice conducted a similar experiment in Cambridge. "We specifically advertised for men who claimed they could eat as much as they wanted without getting fat." The skinny volunteers were tested with a group of overweight men. "We fed them 50 per cent of calories in excess of a normal diet. When force-fed under these conditions they all put on weight. The thin ones gained weight just as fast as the fat ones, and we could detect no special mechanism for burning off calories any faster. And when we underfed them, both groups lost weight at the same rate."

Can it really be true that everyone is completely equal when it comes to losing weight? Not quite. There is one area where some people seem to have a natural advantage - knowing when enough has been eaten. In another Dunn experiment, healthy lean young men ("primordial eating machines," as one researcher calls them) were given yogurt drinks containing varying amounts of fat, then allowed to choose a meal. Afterwards they were all found to have taken in a similar calorie total; those who had been given high-calorie drinks had automatically eaten less. In a similar experiment with overweight volunteers, the meals chosen did not vary according to the calories secretly administered beforehand.

Blaming your well-covered family for your own generous upholstery is not fair, however. "There is a genetic effect, but it's more likely to control fat distribution or control of appetite than anything else," says Dr Prentice. "And obesity has doubled inthe last decade though we all have the same genes as we had before."

And what about all those best-selling diets? In fact, yes, they all work - or none of them do. "They are all packaged differently," says Dr Jebb, "but they all involve calorie reduction, and equally importantly make you think hard about exactly what you are eating. But boredom and deprivation mean they often fail. Simple dietary changes like switching to skimmed milk and low-fat spreads, stopping using fats and oils in cooking, work far better in the long term."

Why are many people so hopeless at gauging and controlling what they consume? "Very many people have a difficult relationship with food," says Dr Prentice. "It completely dominates their lives. The only other comparable question is when you ask people how much alcohol they drink; they underestimate in the same way."

"Anorexia and bulimia are disorders that reach the point where we see them and treat them, but there must be many people who can't be classified in the same way but who do have disorders," adds Dr Jebb.

Dr Prentice, despite his uncompromising stance, believes his message is one of hope - that if you weigh a stone too much, it's simply because you need to change your eating habits, not because there is anything physically wrong with you. "It's essential l y good news," he says. "We knock away these excuses robustly - even brutally - but it's scientists like us that have set up these ideas. Being overweight is not down to metabolism, big bones, hormones or whatever else - the important message is that weig ht is controllable."

WeightWatchers are developing a similar philosophy. "A lot of people believe there is something faulty within their own mechanism," says vice-president Linda Huett, "but people have selective amnesia about what they do eat - and if you're overweight you're very careful about what you eat in public, as others can be critical. If other people then don't think you eat very much it increases your own non-eating fallacy. It's very hard to tell people they eat too much, because often they don't believe t h eydo, and it seems as though you're blaming them for being heavier than they need be. Being `on a diet' is detrimental though - what we aim to do is change people to a healthy eating regime forever. It's true that anyone can lose weight and keep it off.


Tom Sanders, Professor of Nutrition at King's College, London believes that if current trends continue, 25 per cent of the British population could be obese by the year 2000. "People point to metabolic or genetic differences, but it's noticeable that fatpeople tend to have fat dogs." Sanders is an energetic debunker of miracle fast-weight loss diet plans, and the co-author of You Don't Have To Diet, published last year - but recommends reducing calorie and fat intake. "There's no magic about retaining a stable weight. What is needed is small, long-term changes in eating habits. If you go out to lunch, have poultry or fish with vegetables and don't feel you have to go on through the pudding and cheese. Dietary restraint is life-long. You can't get awaywith stuffing your face all the time."