If you had bumped into nursing student Adde Karimi last September, he probably wouldn't have had much time to stop and chat. He was too busy stuffing his face with burgers, cola and milkshakes. It takes a lot of planning to get 6600 calories of junk food down you in a day, he explains. If you are not a born glutton, serious overeating also requires a high level of commitment. Karimi's motivation was commendable. "I did it because I wanted to hate this type of food," he says. He also did it for science.

Even if you have never tried to cure your cravings for fast food by overdosing on it, you may be getting a sense of déjà vu. That's because Karimi was a volunteer in an experiment based on the 2004 documentary Super Size Me. In the movie, film-maker Morgan Spurlock spent 30 days eating exclusively at McDonald's, never turning down an offer to "supersize" to a bigger portion, and avoiding physical exertion. Karimi followed a similar regime, gorging himself on energy-dense food and keeping exercise to a minimum.

That is pretty much where the similarities end, though. By the end of Spurlock's McDonald's binge, the film-maker was a depressed lardball with sagging libido and soaring cholesterol. He had gained 11.1 kilograms, a 13 per cent increase in his body weight, and was on his way to serious liver damage. In contrast, Karimi had no medical problems. In fact, his cholesterol was lower after a month on the fast food than it had been before he started, and while he had gained 4.6 kilos, half of that was muscle.

The brains behind this particular experiment is Fredrik Nyström from Linköping University in Sweden. In the past year he has put 18 volunteers through his supersize regime, and what fascinates him most is the discovery that there was such huge variation in their response to the diet. Some, like Karimi, took it in their stride. Others suffered almost as much as Spurlock, with one volunteer taking barely two weeks to reach the maximum 15 per cent weight gain allowed by the ethics committee that had approved the study. We are used to being told that if we are overweight the problem is simply too much food and too little exercise, but Nyström has been forced to conclude that it isn't so straightforward. "Some people are just more susceptible to obesity than others," he says.

Nyström had been intrigued by Spurlock's experiment ever since seeing the movie when it came out, but was bothered by its unscientific nature. So when one of his PhD students unexpectedly quit, freeing up some research money, he decided to have a go at replicating it under clinical conditions.

Things got off to a good start. Following one of his regular lectures to medical students on the perils of obesity, Nyström casually asked whether anyone would be interested in taking part in an experiment involving as much free food as you can eat. The response was very positive, he recalls. As it happened, most of the volunteers were male. "The boys are very committed," says Nyström, "but it has been really tough to get girls to sign up." He wanted 10 of each, but in the end has had to settle for 12 men and six women.
Eat, drink and be lazy

The first batch of seven healthy, lean volunteers began their month-long challenge in February 2006. First Nyström calculated their normal daily calorie intake and then asked them to double it in the form of junk food, while also avoiding physical activity as much as possible. Nyström allowed them to do just 1 hour of upper body weight training per week. "I thought it would help some of the guys to stick to the diet if they believed that some of the extra weight could be in the form of muscle bulk," he says. Aside from that, though, they were encouraged to be as slothful as possible, and were issued with bus passes and pedometers to help.

In another difference from the movie, Nyström didn't order his volunteers to eat only at McDonald's. They were also allowed to eat pizza, fried chicken, chocolate and other high-fat food whenever they could no longer stomach burgers.

During the experiment Nyström's volunteers had weekly safety check-ups to monitor their health. In addition, they were subjected to a barrage of tests and examinations before starting the diet and again afterwards to find out what it had done to their physiology, metabolism and mental health. "We've done almost every test apart from a muscle biopsy," Nyström says.

His team used a state-of-the-art X-ray technique called DEXA (dual energy X-ray absorptiometry) to measure body composition, including muscle, fat and bone density. They subjected the volunteers to glucose tolerance tests to look for early indicators of metabolic syndrome and diabetes, plus a new spectroscopy test to assess the amount of fat in the liver. They measured their basal metabolic rates before, after and during the experiment, using a standard measure of how much oxygen they inhaled and carbon dioxide they exhaled over a period at rest. They took blood samples and measured levels of hormones such as thyroxine that play a role in setting metabolic rate. And in a bid to work out exactly what metabolic changes occur in fat cells during a fatty diet, the researchers even screened mRNA, the molecule that acts as an intermediate between genes and the proteins they code for. "We looked at all proteins - that's around 30,000," says Nyström.

Overeating has psychological effects too, so the students were also subjected to four questionnaires to assess how they were feeling before and after the experiment. Nyström also kept a paternal eye on their welfare whenever they came to him for money. They were spending up to ¬300 per week on food, he notes, so these meetings were quite regular. He now estimates the final cost for the experiment at around ¬100,000, none of which came from the food or restaurant industries.

Surprisingly, the volunteers' most common niggles were not about food at all, but about the lack of exercise. Although few were fitness freaks in their normal lives, it seems they missed cycling and walking more than they missed being lean. Another regular and surprising complaint was about feeling full all the time - clearly the sensation of hunger is something we don't appreciate until it has gone.

Nyström cannot disclose the full results of his experiment until the study is published later this year. Even then it will take years of analysis to coax the detailed implications from all the data. However, New Scientist can reveal some unexpected early results.

I talked to volunteer Lotta Karlsson while she was in her final week of eating 5000 calories a day. She had already gained 9.1 kilograms, almost the maximum allowable increase over her starting weight of 60 kilos. "I feel [psychologically] very, very good," she said, "but the physical part is bad. I'm so clumsy and I don't like what I see in the mirror. It looks like I am four or five months pregnant."

Even Karimi, who seems remarkably immune to obesity, noticed some nasty side effects of his unhealthy diet. "I felt disgusting. Even if I washed my hands and face, I still felt dirty - as if there was grease all over me." He had previously been very fit, but describes how during the experiment just walking became a laborious process. "I had difficulty keeping up with the other guys. When I took 15 or 20 steps I started to sweat." Worse still, although his libido wasn't affected, his sexual performance was. "I just didn't have my usual stamina," he says.
Sweating it out

Many of the subjects were shocked at how difficult it was to eat enough calories each day. "They came up with some interesting tricks," Nyström says. One male volunteer would regularly drink a milkshake fortified with a tub of cream. Another, who was particularly dedicated to the cause, found himself 1000 calories short one bedtime, so he just downed some olive oil. Karimi puts his success down to good planning. Karlsson resorted to adding cream to her meals and snacking on chocolate, cheese and high-calorie drinks between times. "I have to eat almost all the time," she said during the experiment.

Nyström also has some initial results from the tests. Take the liver, for starters. When it is under stress it releases large amounts of enzymes into the bloodstream. Nobody is entirely sure why: maybe they are released by dying cells or perhaps produced by overworked cells. Either way elevated liver enzymes are a bad sign - and the main reason why Spurlock's doctor urged him to quit his binge. Yet none of Nyström's first batch of volunteers had this problem, which initially led him to suspect that Spurlock might have had an underlying susceptibility to liver dysfunction.

Now that the Swedish experiment is complete, however, Nyström has revised this opinion. "The liver enzymes did increase a lot in some of the last study subjects," he says. If he can find out what factors affect this variability, the supersize experiment may hold some vital clues for anyone hoping to understand and alleviate the growing problem of obesity-linked liver disease.

Then there is cholesterol. Over recent years it has become clear that it is very difficult to substantially change your cholesterol levels by changing your diet. Nyström's findings push that notion to the limit. Not only did many of his subjects show very little change in the amounts of cholesterol and other fatty biomolecules circulating in their bloodstream, some of them even had less low-density lipoprotein - "bad cholesterol" - and an increase in the "good" form, high-density lipoprotein. It's not clear why a junk-food diet should improve your blood lipids, but it may be down to the precise composition of the diet. With receipts for all the food his volunteers bought, Nyström has the means to find out whether there is any correlation between certain foods and blood fat levels.

The big mystery, though, is weight gain. Why do some people pile on so much more than others while consuming the equivalent amount of food? Nyström's hunch is that it is down to variations in metabolism - some of us are simply better at handling calories than others. If you are lucky, your body can adapt to cope with an extra cream doughnut or even a blow-out dinner by burning off the excess energy in the form of heat. He suspects many of his volunteers fall into this category because they were all slim on their normal diet and because they often commented on feeling warm all the time while overeating. "They certainly looked hot and sweaty when they came to me for more money," he says.
Cold comfort

If Nyström is correct, this is what makes his study so unusual and potentially valuable. Most research into obesity is done on people who are already overweight - in other words, those who are least resistant to calories. The ability to turn excess food into fat has been an adaptive advantage throughout most of human evolutionary history when our ancestors had to deal with alternating feast and famine. But the erratic availability of food has not been the only factor influencing the evolution of human metabolism.

"In cold areas people might have adapted more to cope with temperature and so be more likely to burn off excess calories as heat," says Nyström. People with this type of metabolism seem better able to cope with today's "obesogenic" world, and Nyström hopes that by studying them he will be able to identify new approaches to tackling the obesity epidemic. "Because we have such a huge amount of data we should be able to start teasing apart some of the influences that make some people more susceptible to obesity than others."

By now, Karlsson will be pedalling furiously on the exercise bike she has installed in her living room and Karimi will be pumping iron in the gym. When I spoke to him a month after finishing his blowout he had already lost 1 kilogram. He was still on six meals a day, but eating a much more healthy diet and enjoying being hungry. Best of all, the aversion therapy to fast food seemed to have worked. "I haven't touched a burger since I stopped. I don't think I will for a year," he says.

Downsize me

The study at Linköping University in Sweden is the first to try to replicate Morgan Spurlock's Super Size Me under laboratory conditions. The backlash against his film has, however, prompted several alternative experiments in the past couple of years designed to vindicate the fast food industry. You may question the funding, motivation and scientific credibility of these, but they do seem to confirm the simple truth that you can lose weight no matter what you eat, provided you consume fewer calories than you burn.

In 2004, Dutch journalist Wim Meij spent 30 days eating solely at McDonald's, but choosing mostly salads. He lost 6.5 kilograms.

Also in 2004, US documentary film-maker Scott Caswell lost 8.6 kilograms after a month ofeating only at McDonald's. He charts his progress in a movie Bowling for Morgan, but gives no details about what he ate, except that he avoided supersize meals and drank only water or diet soda.

The 2005 film Me and Mickey D follows New Hampshire resident Soso Whaley as she spends three 30-day periods eating 2000 calories a day at McDonald's. She lost a total of more than 16 kilograms.

After 90 days eating only at McDonald's but limiting herself to a daily maximum of 1400 calories, North Carolina construction worker Merab Morgan claims she lost more than 16 kilograms. Following a similar regime but eating 2500 calories a day and sticking to diet drinks, Texan air force sergeant Deshan Woods claims to have shed around 6 kilos over 90 days.

To make the point that obesity is not so much a matter of what you eat but how much, James Painter from Eastern Illinois University got two students to eat fast food for a month in portions calculated as appropriate for their size. Both lost a little weight and their taste for fast food. Painter made a film of his experiment, Portion Size Me, which he hopes will be used by nutritionists to educate students' fast-food choices.

From issue 2588 of New Scientist magazine, 27 January 2007, page 28-31