Carrying a common variant of a particular gene can dramatically boost the risk of diabetes, a study shows.
Diabetes can cause long-term damage to the body
A UK team found people with two copies of the mutant TCF7L2 gene were twice as likely to develop type 2 diabetes compared with those with no copies.
The researchers said their findings suggest a particular genetic make-up can put people as much at risk of the disease as being clinically obese.
The research is published in the Journal of Molecular Medicine.
Currently, more than two million in the UK people have diabetes.
The most common form of the disease, type 2 diabetes, is linked to being overweight. It can cause long-term damage to the eyes, kidneys, nerves, heart and major arteries.
To investigate the association between genes and diabetes, the researchers looked at 2,676 European middle-aged men whose health had been followed over a period of 15 years. Of the group, 158 developed type 2 diabetes.
The researchers discovered those carrying one copy of a variant of a gene called TCF7L2 were 50% more likely to develop type 2 diabetes, while those men who carried two copies of the variant were about 100% more likely to get the disease compared with those who had no copies.
Further research into Indian-Asian and Afro-Caribbean patients found similar results.
Professor Steve Humphries, lead researcher on the study from the University College London Centre for Cardiovascular Genetics, said: "Although being overweight is the major risk factor for developing diabetes, it is now becoming clear that an individual's genetic makeup has a big impact on whether or not they are going to develop diabetes.
"This is the first study that has followed healthy men and shown that carrying this risk gene has such a big effect.
"Because it is so common, and because the risk is so high, this gene seems to be causing as many cases of diabetes in the UK as obesity, which we know is the biggest risk factor."
The TCF7L2 gene's role in the body is not yet fully known, but it is thought to be important in the pancreas, where insulin is made.
Variants of the gene are common. Professor Humphries said about 40% of the population carried one mutated version of the gene, while 10% carried two mutant copies.
The researchers said more work would need to be done to establish why the gene was making people more prone to the disease.
They added in the future genetic screening could help identify those at risk from the disease. They also said their findings could provide a new avenue for developing new treatments for the disease.
Dr Angela Wilson, director of research at Diabetes UK, which partly funded the study, said: "If we can improve our understanding as to why people with certain genes are more likely to develop diabetes, it will help us to find ways to identify those at risk with a view to enabling them to take preventative action by adopting a healthy diet, becoming active and ensuring they do not become overweight - which is a major risk factor for diabetes."
Professor Simon Howell, research dean at Kings College London, said: "It has always been known that type 2 is caused by mixture of genes and environment, but it has been really hard to tie down the genes involved.
"This is the first gene that has really been shown to be generally applicable to the disease. It was initially found in an Icelandic population, but this paper shows that it is prevalent all over the world."