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Middle-aged people who put on weight live longer than those who remain in healthy shape throughout their lives, according to a new study.
Scientists say that while people who remain obese from childhood into adulthood were most at risk of dying, modest weight gains throughout a lifespan can increase the "probability of survival”.
Experts said individuals who put on weight in later life often lived longer than those who remained trim. The findings were made following a study based on two generations of Americans followed over nearly seven decades.
Obesity campaigners cautioned the results should not be seen as a green light to "let yourself go" when reaching middle-age but added there was evidence gaining weight can be useful in protecting against fatal diseases.
Prof Hui Zheng, a sociologist at The Ohio State University, said: "The main message is for those who start at a normal weight in early adulthood, gaining a modest amount of weight throughout life and entering the overweight category in later adulthood can actually increase the probability of survival.”
Prof Zheng and colleagues analysed 8,329 participants in the Framingham Heart Study - 4,576 parents and 3,753 of their children.
Residents of the Massachusetts town have been tracked since 1948. The parents were followed until 2010 and the children from 1971 until 2014.
Weight gain trends were fairly similar across both groups. But, worryingly, the offspring were more likely to become obese at an earlier age.
What is more, they were more prone to die of it than those from their parents' generation. Almost all the older adults had died by the end of the review, said Prof Zheng.
This provided a window into the evolution of obesity over time and how it affects the human lifespan. The health records of each generation were examined from the age of 31 to 80 - focusing mainly on BMI.
The measurement is based on a person's height and weight which helps categorise patients as either underweight, normal, overweight or obese.
It showed the right weight path can add years to your life, said Prof Zheng.
Tam Fry, spokesperson for the National Obesity Forum and chairman of the Child Growth Foundation, said it is vital people keep the findings in perspective.
He said: "It is quite normal that in middle age you will put on a little weight and true that such weight has been found to be protective against a number of serious diseases such as diabetes and heart disease.
"But don't get the idea you can let yourself go at fifty five and dream about longevity."