Why do some people never seem to get fat?

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.

FIND OUT MORE... Horizon's Why Aren't Think People Fat is on BBC Two at 2100 BST on Tuesday, 27 January Or watch it later on the BBC iPlayerlink text here fact here

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-experiement, 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.

Keeping a close watch on the trial was Dr Rudy Leibel of Colombia 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, 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 for 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