"If people already think 30 minutes is hard to achieve, then if you tell them to do 60 they won't do anything," Sport and Recreation New Zealand (SPARC) general manager of participation Deb Hurdle said.

Thirty minutes exercise was enough to maintain a steady body weight, she said.

National Heart Foundation nutritionist David Roberts said the long-duration message might discourage people from exercising, but more than 30 minutes of exercise a day was necessary for modern lifestyles.

"It makes perfect sense to me that we need to exercise more than we are currently," he said.

"I don't think people realise how much energy is in food, especially the pre-prepared food. Portion sizes also seem to be increasing out of balance with the levels of activity."

Newly revised dietary guidelines released by the US Government recommended 60 minutes of moderate-intensity activity daily to prevent weight gain.

For sustained weight loss, the report recommended at least 60 to 90 minutes of exercise a day, and to reduce the risk of chronic disease a minimum of 30 minutes a day. Pregnant women were advised to exercise moderately for about 30 minutes a day if they did not suffer from medical complications.

Servings were replaced by cups in the new guidelines, which called for four and a half cups of fruit and vegetables a day – equivalent to nine servings.

Community and Public Heath dietitian Vicki Marsh said recommending more than nine servings a day would not be realistic in New Zealand, where, on average, people ate just two servings of fruit and vegetables daily.

"Pushing that to nine would be very difficult," she said. "There's not enough research to say it would improve health."

Marsh said US guidelines were not directly applicable here.

"Over there it (obesity) is much higher. It's a different thing for Americans."

In New Zealand, 31 per cent of children and 46 per cent of adults are classed as overweight. In the US, nearly 33 per cent of the population is overweight.

Courtesy of stuff.co.nz