One expert says it's no surprise teenagers are fatter and less fit than their parents were at the same age.

Research at the University of Otago comparing the physical fitness of 15 year olds in two separate generations, found there has been a decline in adolescent physical fitness.

The decline in fitness between generations is also in line with an increase in body weight.

Otago University researcher Bob Hancox told Mike Hosking the research supports the common perception held by many New Zealanders.

"People are just generally being less active and that might just be less walking. Kids tend not to walk to school anymore likely because their parents are concerned about safety and all sorts of things. So there is a whole lot of physical activity that young people don't do now."

LISTEN ABOVE AS BOB HANCOX SPEAKS WITH MIKE HOSKING