The fat in a typical British bird now outweighs its protein, according to a study by a London university, and a single serving has up to 50% more calories than it did in the 1970s.

The idleness imposed by factory farming methods is being blamed for soaring obesity levels among chickens, a problem that affects conventionally and organically produced meat.

Despite chicken’s image as a healthier option than red meat and pork, the researchers warn that it may now be contributing to obesity among consumers.

“Chickens used to roam free and eat herbs and seeds. They are now fed with high energy foods and even most organic chickens don’t have to walk any distance to eat,” said Professor Michael Crawford, one of the authors of the report. “Obesity in chickens is going through the roof. It means that if you eat a 100 gram portion of chicken today, you are eating 100 more calories than you would have done in 1970.

“Our physiology is adapted for eating wild food. You just wouldn’t find anything like these chickens in the wild. People don’t realise how much fat they consume from chicken and we need a new definition of what is a healthy food.”

The study by the Institute of Brain Chemistry and Human Nutrition at London Metropolitan University compared samples of chicken analysed over the past 35 years. They found that a chicken contained 8.6 grams of fat per 100 grams in 1970, compared with 22.8 grams in a supermarket bird in 2004. Over the same period, the amount of protein fell by more than 30% from 24.3 grams per 100 grams to 16.5. For the first time since records began in 1870, a typical chicken now has more fat than protein.

It is not just conventionally produced supermarket chickens that are larded with fat. Researchers found a typical organic chicken had 17.1 grams of fat per 100 grams, although it contained more protein than fat.

Although organic birds have more freedom to roam than factory-farmed birds, most are given artificial food rather than having to forage for their own. This means they often take little exercise beyond waddling out of the door of their huts to peck at food troughs. However, they are not given chemicals to enhance growth.

Much of the fat of a chicken is in the skin, and roasting allows it to permeate the meat. Roast chicken leg with skin now has more fat than a Big Mac. Grilled white chicken meat without the skin, however, remains a relatively healthy, low-fat option.

The research found that, because of changes in a chicken’s diet, the amount of a substance called omega-3 fatty acid DHA, which helps to nourish the brain, promote mental health and prevent heart disease, has fallen dramatically over the past two decades.

In 1980 a typical chicken contained 170 milligrams of DHA per 100 grams, compared with 69 in 1998 and 25 in 2004. “There are miserable amounts of DHA in modern birds,” said Crawford.

During the 1950s, chicken was seen as a luxury food to be eaten on special occasions. But when experts recommended white meat because of its low fat content, it rapidly become the most widely eaten meat in the UK. British consumers now spend £2.5 billion each year on chicken, about 820m birds are slaughtered and most of these are reared on factory farms.

The average life span of a chicken is about six weeks and during that time it is given growth-promoting drugs and large amounts of food to ensure that it reaches the required size.

Justin Kerswell, of the animal welfare group Viva, said: “It’s not surprising they’re getting fatter. They’re selectively bred to put on weight quickly. Some become so heavy so quickly that their hearts can’t cope and they drop dead.”

Phil Stocker, head of agriculture for the Soil Association, said he was concerned at the increased fat levels in organic birds. He claimed, however, that the problem was less likely to occur in birds bred according to the association’s standards, which he said were higher than those of other organic associations.

The British Nutrition Foundation said last week the results of the research should not stop people eating chicken. Brigid McKevith, a nutrition scientist at the foundation, said: “You can cut out a lot of fat intake by removing the skin and grilling the meat. A roast chicken is fine once in a while, but it’s a good idea to pour off excess fat.”

Howard Hellig, a vet and adviser to the British Poultry Council, was surprised by the findings, but said some fat was needed in a chicken for the meat to be moist. He said he was unconvinced by the research because in the past chickens lived longer, allowing them to put on more weight.

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