LONDON (AP) -- Being fat in middle age may slash women's chances of making it to their golden years in good health by almost 80 percent, a new study says. American researchers observed more than 17,000 female nurses with an average age of 50 in the U.S. All of the women were healthy when the study began in 1976. Researchers then monitored the women's weight, along with other health changes, every two years until 2000.

For every one-point increase in their Body Mass Index, women had a 12 percent lower chance of surviving to age 70 in good health when compared to thin women. Researchers defined "healthy survival" as not only being free of chronic disease, but having enough mental and physical ability to perform daily tasks like grocery shopping, vacuuming or walking up a flight of stairs.

Experts consider people with a BMI between 19-25 to be healthy, while those from 25 to 30 are considered overweight and those over 30 are obese.

For every 1 kilogram (2.2 pounds) gained since age 18, women's odds of surviving past 70 dropped 5 percent, researchers found. Women who were already overweight at age 18 and then gained more than 10 kilograms (22 pounds) later in life only had about a 20 percent chance of surviving to age 70 in good health. The most commonly reported diseases were cancer, heart disease, and diabetes.

The study was published online Wednesday in the medical journal, BMJ. It was paid for by the U.S. National Institutes of Health and the Boston Obesity Nutrition Research Center.

"People may think they can safely gain weight through their 20s, 30s and 40s, but there is no evidence that gaining weight is natural," said Aviva Must, professor and chair of the public health and community medicine department at Tufts University School of Medicine. Must was not linked to the study. "These results suggest that small weight gains are not innocuous," she said.

A British study published earlier this year found people with a BMI from 30 to 35 die about three years earlier than normal while those who were morbidly fat, with a BMI above 40, die about a decade earlier.

Other studies have found similar trends in men. Qi Sun, a research associate at Harvard University and one of the study authors, said men were probably equally at risk, since fat acts largely the same way in both genders.

Experts said the findings underlined the importance of preventing obesity in the first place.

"If you are on the obesity track early in life, it could get very dangerous by the time you are middle-aged," said Stephan Rossner, an obesity expert at Karolinska University Hospital in Stockholm. He said it was uncertain if people could regain the health benefits of being thin if they lost weight later in life.

While average life spans have increased in recent years with scientific advances in treating illness, experts warned the obesity epidemic could ultimately undo those gains.

"We know we're extending life span, but we don't know if we're extending healthy survival," Must said. "If one is going to spend the last three decades of one's life with compromised physical and mental function, that may not be the picture of aging we have when we think of living into our 90s."

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