It was easier to stay slim in the 1980s than it is now, researchers have found.
You can eat the same amount of calories as someone did back then, and get the same level of exercise, but you will still weigh more.
The difference amounts to an increase of 2.3 on the body mass index, an assessment of weight, where between 18.5 and 24.9 is regarded as a healthy range for adults.
The reasons are not clear, but scientists believe different lifestyle factors between now and the era when Jane Fonda began selling exercise videos must be the cause.
Researchers looked at data of more than 36,000 people in the US tracked between 1971 and 2008. Professor Jennifer Kuk, of York University in Toronto, said her research suggests a 25-year-old today has to eat less and exercise more just to be same weight as their parents were at the same age. 'Maintaining a healthy body weight is now more challenging than ever,' she said.
