Why do some people never seem to get fat?

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.

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.

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.

'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."