Exercise is not enough to offset the increased death risk associated with being obese, research suggests.
Keeping weight down and regular exercise is key to a long life
A study of more than 116,000 women nurses found physical activity did not totally compensate for the higher death risk associated with being obese.
The Harvard School of Public Health researchers said the key was both to exercise and lose weight.
Nurses who were lean but inactive also had an increased death risk, they told the New England Journal of Medicine.
Excess weight and physical inactivity together could account for about a third of all premature deaths, two-thirds of deaths from cardiovascular disease, and a fifth of deaths from cancer among non-smoking women, they estimate.
They defined excess weight as a body-mass index (weight in kg divided by the square of the height in meters) of 25 or more.
For example, a 5ft 2ins woman was considered obese if she weighed more than 160 pounds and lean if she weighed less than 135 pounds.
Women who did more than 3.5 hours per week of exercise were considered "active".
Compared with the lean, active women, varying degrees of obesity and inactivity increased the risk of an early death.
Lean women who exercised less than 3.5 hours per week increased their risk of early death by 55%.
Obese women who worked out for at least 3.5 hours a week increased their risk by 91% and those who were obese and inactive increased their risk of a premature death by 142%.
The researchers said the key to a long life, for both men and women, is to keep weight down and take regular exercise.
"Public health campaigns should emphasise both the maintenance of a healthy weight and regular physical activity," they said.
Lead author Dr Frank Hu said: "If you are overweight or obese, exercise is good for you even if you don't lose weight.
"For people who are lean and sedentary, it's really important for them to get out of the couch and exercise, even if they don't have to lose weight."
Professor Neil Armstrong, from the Children's Health and Exercise Research Centre in Exeter, said: "If you really want to do something about obesity, it really needs to be a two-fold process, which includes aerobic exercise and a reduction in energy intake.
"Obesity is related to many diseases, such as heart disease, high blood pressure and diabetes, so it's a very important issue.
"And of course the advantage of exercise is not just related to obesity.
"It reduces the risk of heart disease and in postmenopausal women the risk of osteoporosis.
"Plus it generally raises your quality of life."
Dr David Haslam of the National Obesity Forum said: "An obese person who is exercising and maybe getting a bit despondent because the weight is not falling off should take great comfort from the fact that they are at much less risk of heart disease and stroke than if they hadn't been exercising.
"Inactivity, like smoking, is a massive risk factor for heart disease in it's own right."