New research from Great Brittan and the United States is showing that being severely obese—80 pounds or more above a normal weight—can reduce your lifespan by up to 12 years, similar to the effect of a lifetime of smoking. And even moderate obesity can cut life expectancy by about three years, according to the British study, which analyzed data from nearly one million people from around the world. The study was published in the medical journal Lancet.

The study used Body Mass Index (BMI), a measurement of body fat based on height and weight, to determine obesity levels and found that death rates were highest among people with a BMI of 40 and above and lowest in people with a BMI of 23 to 24, the high side of the normal range. Those with a BMI in the 30 to 39 range are considered moderately obese and have a reduced life expectancy of about three years.

Study researchers said that moderately obese people had a 50 percent greater chance of dying prematurely than normal-weight people and were also two-thirds more likely to die of a heart attack or stroke. They were also up to four times more likely to die from diabetes, kidney disease or liver problems and one-sixth more likely to die from cancer. And the researchers emphasized that even small increases in a person's BMI was enough to raise their risks for cardiovascular disease and cancer.

However, the U.S. study, which analyzed data on 366,000 people nationwide, found that, unlike the British study, just being overweight or moderately obese, had little or no effect on life span. The U.S. study, published in Obesity, did find that extremely obese people who smoked had a shortened life expectancy of 21 years—60 versus 81—over that of a nonsmoker of normal weight.

Moderately overweight people may be spared a shortened lifespan, speculated researchers, because there are so many effective medications to treat health problems like high cholesterol, high blood pressure and diabetes that are often the result of extra pounds. However, the consequences of being overweight don't affect just your health; they can affect your wallet as well. Research by lead author of the U.S. study, Eric Finkelstein, and obesity experts with the Centers for Disease Control and Prevention, found that obese patients spend about $4,871 in medical bills each year compared with $3,442 for a patient at a healthy weight.