But I eat like a horse!

Come off it, love. No one believes (or likes) a skinny person who says they enjoy cake on a daily basis. Yet many factors can affect our ability to gain weight, says Catherine Nixey

Jane Higgins is a girl with a healthy appetite. "Probably I'll have a couple of chocolate bars for elevenses, with a mug of hot chocolate. Then, if I'm hungry in the evening, I'll have some custard slices for tea." If she eats this, Jane will have consumed 1,200 of her recommended 2,000 daily allowance of calories in snacks. But, unlike her diet, Jane has a very low percentage of fat. She is 5ft 11in tall yet weighs only nine stone.

If, as the 20th-century American humorist Helen Rowland said, "the chief excitement in a woman's life is spotting women who are fatter than she is", then the biggest pain is undoubtedly meeting women who are thinner but appear to eat more. "I do sometimes get women giving me beady looks at the pudding counter," says Jane. "It isn't a nice experience." But it's a pain that most women would gladly put up with in return for more cake. How do these women get away with it?

It used to be that metabolic rate was held responsible. Everyone was assumed to have a different metabolism, which explained the varying ability among humans to lose or gain weight. Books and magazine quizzes would help you to discover your own metabolic type, based on factors such as the quality of your hair: dry or silky? Fine or thick? Once you knew your type you would then adopt a seemingly arbitrary diet to complement it, and hence - theoretically - lose weight.

Dr Toni Steer of the MRC Human Nutrition Research Unit at Cambridge dismisses this as hokum: "People's metabolisms don't vary. The first law of thermodynamics states that energy can neither be created nor destroyed, but only transferred." And humans, even thin ones, are no exception to the laws of physics. "A healthy, lean person who says they can't put on weight is simply not right." Or perhaps not trying hard enough. So if you aren't going to store energy as fat, then what goes in must, in one way or another, come out.

If a person with a normal lifestyle really is eating large amounts of food yet not putting on weight then there is clearly something wrong. Many diseases can cause weight loss because the body is not absorbing food properly and calories are lost in urine or faeces. Coeliac disease, diabetes and Crohn's disease are among them.

Recent research puts the figure of coeliac sufferers as high as 1 in 80, though in the UK only 1 in 1,000 are diagnosed. Individuals who suffer from coeliac disease are intolerant to gluten, a protein which is found in wheat, and to other, similar proteins found in cereals such as rye, barley and oats. In coeliacs, intolerance of this protein results in damage to the lining of their small intestine. This in turn leads to malabsorption of certain nutrients, especially fat.

"Some people may have a bad bout of diarrhoea and lose half a stone quite rapidly, and presume that it is because of a stomach bug," says Norma McGough, a dietitian for Coeliac UK, the charity set up to support people suffering from the disease. "Others, in the same way that people can gradually put on weight over decades, lose weight over the years, and are only diagnosed as sufferers via secondary symptoms, such as osteoporosis and anaemia, which can occur due to prolonged malabsorption of nutrients."

Treatment of coeliac disease is simple in prescription but complex in practice, involving as it does the elimination of all foods containing gluten and gluten-like proteins from the sufferer's diet for life. That means no bread, pasta, cakes or biscuits. However, the disease is gaining recognition, and gluten-free foods are becoming more widely available.

Other diseases that can cause weight loss are those involving the thyroid gland, such as Graves' disease. The thyroid produces the hormone thyroxine, which controls our basal metabolic rate (BMR). The BMR is how many calories a body at complete rest needs to carry out just the processes essential for life, such as keeping the heart beating. Produce too much thyroxine ("hyperthyroidism") and your heart will beat too fast, you will feel hot and sweaty, and you will lose weight.

In healthy individuals, BMR should be fairly standard. What variations there are tend to be rather the opposite of what you might expect. "A fat person actually has a higher metabolic rate than a thin person of the same height, because they have a higher energy requirement," says Dr Steer. "When you put on weight you don't just put on fat, you also put on lean tissue, such as muscle, and this all requires energy to maintain," Dr Steer continues. "A larger person simply requires more energy to move their extra weight about."

So how is it that some people eat such a lot and stay slim? The answer is that they don't. "If somebody is not putting on weight, it is because they are only eating the number of calories that they require," says Dr Steer. "There really is no magic metabolic advantage. Although people often think they are eating a lot, if they wrote down what they consume every day, they would probably find that they aren't." A friend of Jane's agrees: "I really don't think she eats all that much. She just eats erratically. It is true that when she does eat she'll eat big meals, or three chocolate bars at a time. But then she'll skip meals. She stockpiles food when she can get it."

But this is not to say that these people are being deliberately misleading. "If a person is very thin but says 'I eat everything I want to', then they probably do," says Dr Steer. "It is just that they want less." So there's no great conspiracy; it's just that some people are better at managing their appetite subconsciously.

It is also the case that such people may simply be using more energy, either through planned exercise or through small but incremental bursts, like walking an extra stop to work.

A recent study published in the American Journal of Clinical Nutrition looked at lean people and their movements throughout the day, how many times they changed position in their chair, whether they were crossing and uncrossing their legs and so on. It put forward the suggestion that the few extra calories expended in these actions were actually helping to keep them thin.

So it seems there is no great weight mystery: nobody has a talent for making calories vanish like magic. Film star Renée Zellweger, who temporarily bulked up to play Bridget Jones, took some persuading to repeat the exercise for the sequel, which she is currently filming. The real key to weight loss was rather nicely summed up by the actress Minnie Driver. When asked how she had managed to shed the pounds that she gained for her role in the film Circle of Friends, the svelte Ms Driver replied: "Eat less. Move more."