Clear evidence that DNA affects whether you are fat or thin was published last night following groundbreaking research involving thousands of Scots.

People with two copies of a particular gene have a 70% higher chance of being obese than people without the variety, according to the study.

The breakthrough may help explain why some people appear to stay slim no matter what they eat while others struggle to shift excess pounds.

The study, however, does not help explain why rates of obesity and poor health are particularly high north of the border.

One researcher said: "There is no difference in the frequency of this gene between Scotland and England. I am afraid Scottish people cannot say the reason we are fatter than England is because of our genetics."

Around one in six Europeans has a double dose of the fat gene, it is claimed.

Professor Andrew Morris, one of the authors of the study and chair of diabetic medicine at Dundee University, said: "This discovery is very exciting. It is a major international breakthrough. It has demonstrated for the first time a very clear genetic contribution to obesity in the general population."

Obesity and related health conditions such as type 2 diabetes have become a serious issue for western nations, including Scotland, and millions of pounds are being spent on trying to tackle the problem.

Identifying a clear link between genes and weight gain could have far-reaching implications for health care for decades to come.

Scientists in England were comparing the DNA of diabetic patients with samples from people without the condition, looking for patterns, when the breakthrough occurred. A report in the journal Science last night revealed they found a variation of the FTO gene was strongly associated with a higher body mass index.

Genetic samples, along with a number of physical measurements from 37,000 people, including 7000 in Fife and Tayside, were then examined to see if they corroborated the discovery. Some people have two copies of the fat' variety of the FTO gene, some have two copies of the alternative thin' FTO gene and some have one of each.

The researchers, funded by the Wellcome Trust, discovered people carrying one fat' FTO had a 30% higher risk of being obese than those with none. People with two copies had a 70% higher risk of being obese and were, on average, 3kg heavier than a similar person with none.

Professor Andrew Hattersley, researcher from the Peninsula Medical School in Exeter, said: "As a nation, 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 than others. 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?' There is clearly a component to obesity that is genetic."

The researchers stressed it was important to eat a healthy diet and exercise regularly, regardless of genetic inheritance.