New Zealanders' expanding waistlines have far more to do with overeating than with lack of exercise, a controversial new study contends.
The international research has highlighted the disagreement over the causes of the obesity epidemic. Two-thirds of adults are overweight or obese.
There are debates over the relative effects of overeating and reduced physical activity; and whether individuals' bad choices or an unhealthy environment - such as super-sized fastfood and lack of safe cycleways - are most at fault.
In February, the Government scrapped the healthy-only requirement for food and drink sales at schools, triggering widespread complaints from the public health community.
The study, based partly on United States food supply statistics, calculated that the average weight of adults today should be 10.8kg more than 30 years ago.
In fact it was 8.6kg more in the US and the increase in New Zealand would be similar, said a study author, Professor Elaine Rush, of the Auckland University of Technology.
The authors believe increases in physical activity may have prevented people gaining the full 10.8kg.
The study used medical sample results from more than 2000 people, including 111 New Zealanders, to find the energy-burning rates needed to maintain stable adult weight and normal child growth.
Professor Rush said the study showed how much the population - especially in the most-deprived areas, where obesity was more prevalent - needed to cut back to return to the average weights of the 1970s.
"Children would have to cut their intake by about 350 calories a day - equal to one can of fizzy drink and a small chocolate bar, and adults 500 calories - the equivalent of a large burger."
Similar results could be achieved by children walking for an extra 150 minutes a day and adults an extra 110 minutes, but this was unrealistic, she said.
Associate Professor Robert Scragg, of Auckland University, agreed diet was the main cause of rising obesity.
His group's study of adolescents in New Zealand, Australia, Fiji and Tonga found much higher obesity among Pacific Islanders in Auckland than in Tonga.
"The big differences across the four countries are not related to physical activity, they are due to food intake differences. That, to me, is compelling evidence that diet is the more important problem than physical activity."
But AUT professor of public health Grant Schofield said reduced physical activity clearly played a big part, because New Zealanders now spent more time than in the past sitting at work, in cars and watching TV.
He also challenged the study's method. Its way of measuring food intake - based on the total food available in the US, less the amounts disposed and not used by humans - were known to be unreliable.
Food Industry Group executive director Vicky Hamilton said increased intake and decreased activity both played a part.
Average weight of NZ adults:
1997 - 74.8 kg
2006 - 77.8 kg
Source: Calculated from national body mass index surveysBy Martin Johnston Email Martin