What makes us fat?

Lack of sleep

People who sleep for four hours or less per night are 73 per cent more likely to be obese. A team from the Mailman School of Public Health at Columbia University in New York found that, even after factors such as depression, physical activity, alcohol consumption, ethnicity, education, age and gender had been taken into account, people were more likely to be obese the less sleep they had.

Those who got only five hours' sleep were 50 per cent more likely to be obese than those who were getting a full night's rest, and those who slept for just six hours were 23 per cent more likely to be substantially overweight. Dr Stephen Heymsfield, who worked on the study, said it was not as simple as saying that if people were awake for longer, they were likely to eat more. "There's growing scientific evidence that there's a link between sleep and the various neural pathways that regulate food intake." Previous research has shown that sleep deprivation is linked to a decrease in levels of leptin (see Hormones). Levels of the hormone grehlin, which makes people want to eat, have also been seen to increase in people who are sleep-deprived.

Too much choice

Although a varied diet is likely to be rich in nutrients, US scientists found that the availability of lots of different foods can also encourage overeating. Hollie Raynor and Dr Leonard Epstein from the University of Buffalo said that variety decreased the feeling of satisfaction, making people more vulnerable to obesity. "Both people and animals will eat more food when a meal or diet contains a greater variety of food, which can eventually cause weight gain," they said. The research showed that meals composed of foods of a similar shape, taste and colour may curb overeating.

Friends

If your friends are fat, your chances of being fat increase by 57 per cent. Researchers monitored the weight of a network of 12,067 friends and relatives between 1971 and 2003 and found that weight gain in one person apparently had a similar effect in their close friends or partners. Same-sex friends and siblings had a greater influence than did those of the opposite sex. The scientists who instigated the study, Professor Nicholas Christakis of the University of California, San Diego, and James Fowler of Harvard, concluded that obesity was "socially contagious".

Having relatives and friends who become obese changes one's idea of what is an acceptable weight. Prof Christakis said: "It's not that obese or non-obese people find other similar people to hang out with. Rather, there is a direct causal relationship." And the connection was not as simple as saying that friends adopt each other's lifestyles. "It's more subtle than that," he said. "A person becoming obese most likely causes a change of norms about what counts as an appropriate body size.

"We were stunned to find that friends who are hundreds of miles away have just as much impact on a person's weight status as friends who are next door," said Dr Fowler. The study found that a person's chances of becoming obese rose by 40 per cent if a sibling was obese and 37 per cent if a spouse was. In the closest friendships, the risk almost tripled.

Genes

British scientists have discovered a gene that contributes to obesity. The research team, led by Professor Andrew Hattersley of the Peninsula Medical School in Exeter, found a gene variant that regulates the amount of fat in the body. Those with the variant – around half of all Europeans – are on average 2.6lb heavier than those without it, while one in six Europeans carries two copies of the variant and is on average 7lb heavier. "We are eating more but doing less exercise, and so the average weight is increasing, but within the population some people seem to put on more weight, " said Prof Hattersley. "Our findings suggest a possible answer to someone who might ask, 'I eat the same and do as much exercise as my friend next door, so why am I fatter?'"

Hormones

Scientists have discovered that leptin, one of the key hormones responsible for reducing hunger and increasing the feeling of fullness, also controls our fondness for food. A Cambridge University team, headed by Dr Sadaf Farooqi and Dr Paul Fletcher, studied patients with a rare genetic disorder resulting in a complete lack of leptin. These patients ate excessively, liked all types of food and developed severe obesity. After treatment with leptin their hunger reduced, they became more choosy about food and lost weight.

Dr Farooqi said: "While bodyweight remains stable for many people over a long period of time, other people gain weight very easily. More studies are needed to find out how these brain responses vary in people with weight problems in general."

Air conditioning

Air conditioning keeps us in a temperature range in which we do not have to regulate our body heat. Scientists suggest that when people are out of this zone we lose weight. If it's too cold we burn fat to stay warm, and if it's too hot our appetites decrease. Studies show that people are keeping their houses warmer in the winter and cooler in the summer than they did in years past, so these natural influences on weight are negated.

Poor diet during gestation

If a mother has a poor diet during pregnancy, her foetus might predict that future food supplies will be scarce and set its metabolism to store and conserve fat, according to scientists from the universities of Auckland and Southampton. However, if this early prediction proves false, and food – particularly food high in fat – is readily available, the infant may find its metabolism programmed for adult obesity.

Researchers led by Dr Stephanie Bayol of the Royal Veterinary College, London, looked at the effect of the maternal diet on more than 300 baby rats. The young exposed to junk foods in the womb or after birth were more likely to be overweight at 10 weeks, and ate more junk food themselves.

Catching a cold

A virus that causes colds and sore throats could also lead to obesity. In tests, chicken and mice infected with a particular adenovirus, the group of bugs behind up to 10 per cent of colds and sore throats, put on weight more quickly than uninfected animals – even when they didn't eat any more food. Scientists at Louisiana State University found that the virus could trigger the development of fat cells in the body. "We're not saying that a virus is the only cause of obesity, but this study provides strong evidence that some obesity cases may involve viral infections," said LSU's Dr Magdalena Pasarica.

You are what you eat

Despite the factors above that have an effect on obesity, diet is still a huge factor:

* The majority of people put on weight simply because they consume more calories than their bodies require.

* To shift 2lbs a week, you need to eat 500 calories fewer each day, or burn off 500 calories more. This can be achieved by eating less, taking more exercise or, best of all, combining the two. A brisk 30-minute walk burns off approximately 100 calories.

* The idea that people who eat late at night will put on more weight is a myth. Scientists at Oregon Health and Science University in the United States carried out tests on 47 female monkeys and fouthe time that the animals ate and whether or not they gained weight.

* It is also a myth that eating carbohydrates will make you fat. A healthy diet depends on the right balance of protein, carbohydrates and fats. Experts recommend that meals include starchy foods such as rice, pasta, bread, potatoes and cereal.