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Page last updated at 12:51 GMT, Thursday, 22 January 2009

Why do some people never seem to get fat?

Thin woman, overweight woman, thin man, overweight man

By Katie Fraser
BBC News Magazine

The UK is a country obsessed by the threat of obesity. As the average person's weight has grown, so has coverage of the subject.

The chief medical officer for England, Sir Liam Donaldson, has said we are facing an "obesity timebomb". Culinary celebrities like Jamie Oliver have launched campaigns, in homes and school kitchens, to fight the fat war.

Horizon's Why Aren't Thin People Fat is on BBC Two at 2100 GMT on Monday, 26 January
Or watch it later on the BBC iPlayer

Yet the science of weight gain is less straightforward than the headlines sometimes suggest. Why, for example, do some people seem to eat what they like and not put on weight, while others limit their diet yet struggle to shed their bulk?

In 1967, a medical researcher, Ethan Sims, carried out an experiment at Vermont state prison in the US. He recruited inmates to eat as much as they could to gain 25% of their body weight, in return for early release from prison.

Some of the volunteers could not reach the target however hard they tried, even though they were eating 10,000 calories a day. Sims's conclusion was that for some, obesity is nearly impossible.

It was with this in mind that 10 slim volunteers - who were not dieters - convened in more hospitable circumstances, for a recent experiment devised by the BBC's Horizon documentary. The 10 spent four weeks gorging on as much pizza, chips, ice cream and chocolate as they could, while doing no exercise, and severely limiting the amount they walked.

'Friends hate me'

Medical student Katherine Hanan, 21, says she had never dieted or done very much exercise before the experiment.

Katherine Hanan
Pre-experiment, Katherine Hanan: 'I've always eaten whatever I want to'

"I've always eaten whatever I want to eat and I've always been quite slim. I'm really lucky and my friends hate me," she says.

During the study, Katherine and the other volunteers had to eat double their usual amount of daily calories, which varied from 3,500 for the women to 5,000 for the men.

The outcome of the trial could bolster the theories of Dr Rudy Leibel of Columbia University, New York, who believes we all have a biologically determined natural weight which our bodies make an effort to stick to, whether it is fat or thin.

"The body will constantly tend to try to bring you back to whatever your normal body weight is," he says.

But he does not think this is the full story. There are other issues that influence a person's weight.

"Fifty per cent is down to genes and the rest is probably down to environment. If you get the gene for Huntington's you have the disease 100% of the time. That's certainly not the case with obesity."

The four-week eat-a-thon was easier for some than for others.

Volunteer Thomas Patel-Campbell, a keen sportsman and runner, struggled with the cap on physical exertion that was one of the terms of the experiment.

Snacks and puddings

"Eating that much was pretty easy as I'd been eating more than usual in preparation for my run," he says. "I was one of the two who weren't sick at all. What was difficult was limiting myself to 5,000 steps a day.

Katerine Hanan
'I'd eat half a tub of ice cream... a couple of puddings... a pint-and-a-half of chocolate milk'

"The least I did was when I spent a day at home, only leaving the house to go to McDonald's and the shops. Even that was 8,000 steps."

Katherine described a typical day's menu for her while taking part in the study. She made up most of her calorie intake by eating sugary snacks and puddings.

"I'd wake up and have two pain au chocolats plus a large hot chocolate with cream. Mid-morning I'd have a packet of high-fat crisps or a chocolate mousse, sometimes it might even be a small meal. Lunch would be substantial - shepherds'' pie or something.

"In the afternoon I'd eat half a tub of ice cream. At night it would be almost the same evening meal as before except I'd have a couple of puddings. I'd also drink a pint-and-a-half of chocolate milk with… ice cream every day."

Unlike Thomas, Katherine found her body rejected this enforced gluttony - leading her to vomit each week.

And two other volunteers couldn't even get that far - finding they couldn't consume the full allocation of food each day, failing to hit their calorie targets.

Persistent hunger

After four weeks Katherine had gained 3.5kg - almost a 7% gain in body weight. Thomas, meanwhile, put on 5.5kg - a 9% gain in body weight.

Think of it like a thermostat and that each person has a set point
Dr Rudy Leibel

Of the two who struggled to reach their targets, one put on just 0.5kg - a mere 1% gain in body weight, while the other actually saw their body fat percentage go down slightly, despite putting on 5.7kg.

The results highlight the different ways our bodies behave when faced with excess calories.

One expert, Professor Jane Wardle, believes there could be a genetic answer, through what's known as the FTO gene. Adults who have one variant of this gene weigh on average more than everybody else.

Prof Wardle believes the gene can influence appetite, leading some people to not know when they are full. Those without the gene, she thinks, find it easier to say no to food.

"It's kind of effortless because they don't even want to eat. They're not having to exert willpower and self-control whereas for other people, their brain responses to foods that they're exposed to aren't being switched off effectively as a consequence of them already having had enough."

Dr Leibel observes that for some people, such as those who couldn't reach their calorie targets, the appetite hardly fluctuates regardless of how much they want, or are told, to eat.

Muscle not fat

This can work both ways, says Dr Leibel. If someone loses a lot of weight, they often have persistent hunger, even if they are eating enough to sustain themselves.

"Think of it like a thermostat and that each person has a set point," says Dr Leibel. "When it is reduced below that point the body begins to do things that will force it to recover its lost body weight."

And while excess calories can lead many people to put on body fat, one volunteer in the study defied convention by putting on a lot of weight (4.5kg) while his appearance didn't seem to alter. Instead of fat, the weight had gone on as muscle as the volunteer's metabolic rate had risen 30%.

This is another reason, says Dr Carel le Roux, an obesity specialist who oversaw the experiment, why some people appear not to get fat despite eating at lot.

"Studies have shown that this tendency to lay down muscle rather than fat when we over-eat is genetically determined," she says.

For those who did show any signs of having overindulged after the experiment was over, they soon got back to normal, and not through a rigorous diet and exercise regime. Thomas found it happened easily.

"After the first week," he says, "my trousers fitted almost as well as before, and it didn't take long for my belts to be back to the right button hole."

Below is a selection of your comments.

I've always wondered why my sister struggles to lose weight, whilst I never gain weight, no matter how much I eat or what I eat. Over the Christmas period I went through a similar situation to the results of your test.
Rupert Adams, Bedford, England

I'm glad you've published this story. Since I was young I've had friends ask me straight-forwardly whether I was anorexic or bulimic. But the fact is in a house of four girls I probably eat the most! I consider myself pretty lucky; Thinking how hard I find it to increase my weight, it must be a hundred times harder for those struggling with being overweight to do the opposite.
Sarah, Manchester, UK

This is a fascinating article. I, too, have struggled with weight gain throughout the years and have visited hospital dieticians several times all in the hopes of putting on just ten more pounds. One dietician even suggested eating a spoonful of mayonnaise before bed each night! Nothing has worked and my university friends constantly joke how much they "hate" me. After examining family photos, I'm learning to accept that I'll probably be very slim my whole life. I'm glad research is being done in this field, because for some (heavy or slim), calorie intake is not the simple answer.
Ali Pezeshkpour, California, USA

Two years ago I had a big accident. Beforehand I could eat whatever I wanted but never put any weight on. After my accident I have put on two stone. I am eating less but I do not bounce about as much as I used to and am not as active.
Dom Finn, Nottingham

I work in health promotion in the NHS and I have felt uncomfortable for a long time about people constantly being told to eat more healthily and exercise in order to avoid obesity. I believe that many people will need a genetic or chemical treatment to overcome the widespread urge to eat when food is available, and to know when they have had enough.
Mary Anne, Scotland

A very interesting article. A friend of mine once asked me if I would like to be thin and unhealthy or fat and healthy. I opted for the second [obviously]. Health and weight don't go hand in hand - look at rugby players. Most of them are technically clinically obese but are fitter than the average person.
Jacquie, Yorkshire

Don't eat, don't smoke don't drink… what can the working population do? We are becoming self-obsessed ninnies who do nothing but think constantly about our health etc and worry that this government in particular, will not be able to fund our NHS with our money for treatment. I do believe in a healthy lifestyle but have people not got other things to worry about today?
Janet Ostins, Birmingham

Extremely depressing, as I am not one of these very lucky people. We do not know what the ever decreasing quality of food, air, water due to pollution etc is doing to our bodies and how it is affecting our genes. There needs to be a lot more research on this before wide spread condemnation is applied. Granted, some do not help themselves but there are a lot of people out there who eat healthily, exercise, and yet are overweight, and there are lot who eat rubbish, do not exercise, and are stick thin.
Katharine, Liverpool

I'd be interested if this experiment did shed some light on why some people's bodies are more inclined to put on weight than others.

As a tall, exceptionally lean 29 year-old male, who has been trying to gain weight on and off for 13 years, I can't wait until I hit the big 30 as this is when everyone promises me it will happen, whether I want it to or not!
Richard Gaskell, Cumbria

I have always been medically "severely underweight" despite eating crazy amounts of food including two and a half years working in McDonalds where I took full advantage of my staff discount. I have never put any weight on, no matter how hard I try!
Rich, Wales

I would also love to put on some weight. I eat a very healthy and varied diet but most people are very surprised I remain a size 10. Even after my first pregnancy, my size came right back down.

I enjoy food and long for some curves.
Wendi, London

When I was growing up, there was the myth that having started a baby on fruit rather than cereal determined whether one was slim or not.
Candace, New Jersey, US

I think changes in routine, environment as well as diet have a lot to do with changes in weight. Whenever I am on a two-week holiday to my extended family in Malaysia, as well as not going to work, the hot weather (lethargy) and fabulous, nostalgic food in constant supply - my siblings and I pile on the weight. But as soon as we get back to the UK, we seem to fall back into our usual routine and weights, albeit, a little bit heavier each time!
Yong Lee, Maidstone, Kent

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