March 19, 2009 -- If you are having trouble sticking to your diet, here’s more motivation: Being overweight may take years off your life.

A new study shows that weighing a third more than your ideal weight could reduce your life by three years on average.

''Excess weight shortens human life span,” study researcher Gary Whitlock of the Clinical Trial Service Unit at the University of Oxford says in a written statement. “In countries like Britain and America, weighing a third more than the optimum shortens life span by about three years. For most people, a third more than the optimum means carrying 20 to 30 kilograms (or 50 to 60 pounds) of excess weight. If you are becoming overweight or obese, avoiding further weight gain could well add years to your life.''

Researchers looked at data from 57 existing studies, examining in total the mortality rates for nearly 900,000 adults. They looked at mortality rates for people five or more years after they began participating in a study. The lowest mortality rate was for people with a body mass index (BMI) of 22.5-25.

BMI is a measure of weight in relation to height. A BMI of 18.5-24.9 is considered normal weight. A BMI of 25-29.9 is considered overweight. Obesity is considered a BMI of 30 or more.

Researchers found that each additional increase of 5 in the BMI translated into a 30% increase in any cause of death. It also translated into a 40% increase in death from heart disease, stroke, and other vascular diseases; a 60% to 120% increase in death from diabetes, kidney disease, and liver disease; a 10% death from cancer; and a 20% increase in death from lung disease. Findings took into account age, sex, and smoking history.

Severe obesity has a similar effect to smoking on mortality rates, according to researchers. However, researcher Richard Peto, also of Clinical Trial Service Unit at the University of Oxford, cautioned that smoking to stay thin is not the answer.

"This study has shown that continuing to smoke is as dangerous as doubling your body weight, and three times as dangerous as moderate obesity,” Peto says in a written statement. “Changing your diet but keeping on smoking is not the way to increase life span. For smokers, the key thing is that stopping smoking works."