The largest ever study on how obesity affects mortality, carried out by Oxford University, has found that severe obesity can shorten a person's life by 10 years whilst moderate obesity, which is now common, can reduce life expectancy by three years.

Nearly a million people, mainly from North America and Europe, were participants in the trial, which continued for a period of 20 years, over which, 100,000 of them died. Body Mass Index (BMI) was used to calculate obesity, this is determined by dividing an individuals weight in kg by the square of their height in metres. Moderate obesity equates to a BMI of 30-35 and severe obesity falls between the range of 40-50. The researchers found that the best BMI to have for the longest life span is 24 - for somebody that is 5ft 7in (170cm) tall, they would have to be around 11 stone (or 70kg).

So what is it that causes death following obesity? Most obese adults die of heart disease and stroke although cancer figures are also rising. Sir Richard Peto of Oxford's clinical trial service unit, who was the study's main statistician said that it is clear that efforts need to be made to stop people from putting on weight, which happens mostly between the ages of 25 and 50; "It's very difficult to lose weight and keep it off," he said. "But stopping putting on weight is practicable." However, after comparing the effects of obesity of those with smoking, he expressed some worry that people would get the wrong idea - it is true to say that obesity kills, but only in rare cases will it kill as fast as cigarettes.

Peto said 'This study has shown that continuing to smoke is as dangerous as doubling your body weight, and three times as dangerous as moderate obesity. Changing your diet but keeping on smoking is not the way to increase lifespan. For smokers the key thing is that stopping smoking works. "I think smokers are getting the wrong message if they keep on smoking and think what matters is obesity. Smoking matters enormously more."

One strategy put forward for tackling obesity in Scotland - for chocolate to be taxed in the same way cigarettes and alcohol are, has been voted against. Dr David Walker, a GP and trained scientist and nutritionist, from Lanarkshire put forward the motion as he warned that chocolate has lost its status as a 'treat' and had become an unhealthy addiction for some people.

He said that obesity was particularly a mushrooming problem in Scotland and they risk going the same way as the United States; "A little of what you fancy may do you some good, but as nearly one in four people in Scotland are obese, a lack of physical activity, an unhealthy diet and larger portion sizes are clearly taking their toll on the health of Scotland. "There is lots of negative publicity about other fast food and junk food but chocolate is sneaking under the radar.

. "People have been lulled into a false sense of security about chocolate. "I had one patient recently who said to me she thought chocolate was good for you. People are being brainwashed into believing this." He warned that a 225g bag of chocolate contains nearly 1,200 calories - almost half the recommended daily amount for a man and believes that if chocolate was taxed then it would restore its status as a special treat.

Despite the motion being defeated by two votes, Dr Walker said he was glad that his suggestion had provoked debate.