KEY POINTS:
Obesity and inactivity are leading triggers for cancer, says an international report to be made public this afternoon.
The report, by the World Cancer Research Fund, will re-shape cancer messages about food and underpin official advice for years to come.
A five-year analysis of thousands of studies, the report will say that premature development of many types of cancer should be preventable by lifestyle choices such as not smoking, having a good diet and exercising.
Eating too much red and processed meat and not enough fruit and vegetables will be re-stated as cancer risk factors in the report - challenging New Zealand's meat-loving diet.
But this will be overshadowed by the primary finding of a convincing association between obesity and various types of cancer.
The report is expected to lay down rules on how to avoid cancer, specifying amounts of red meat that can be eaten and amounts of exercise required.
The meat limit is likely to pose problems for meat-lovers, and the required exercise level may exceed the New Zealand prescription of at least 30 minutes of moderate exercise on most days.
Obesity and inactivity are already established cancer risk factors, but the report, by a panel of international and New Zealand experts, greatly elevates their significance. Obesity will be ranked with smoking.
Fighting obesity will become an even higher priority for health agencies than it is now, because of the strong links found between it and cancers, including post-menopausal breast cancer and bowel cancer.
The rate of adult obesity in New Zealand more than doubled between 1977 and 2003.
In 2003, 21 per cent of Kiwis were obese, and a further 35 per cent were overweight.
The first national survey of children, in 2002, found 10 per cent were obese, and other studies suggest the rate is increasing.
Obesity is also associated with cardiovascular disease - mainly heart disease and stroke - and type 2 diabetes.
In a bid to avoid having the health system swamped by obesity-related diseases, the Government is investing heavily in research and programmes to help people eat and exercise healthily.
The long-standing public health advice to eat plenty of fruit and vegetables - "5-plus a day" - will remain because of its general health benefits.
But the report shows that in cancer prevention, the evidence in favour of fruit and vegetables is weaker than that against obesity, so the role of 5-plus a day may be absorbed into the overall effort to maintain a healthy weight.
The report draws on international studies checking the health and eating habits of hundreds of thousands of people.
The studies include the European Prospective Investigation of Cancer, which has more than 500,000 participants.
Such studies do not prove cause and effect, but can find what scientists call "convincing associations" - the tag assigned to obesity in the report.
The European study's findings include that being overweight or obese increases the risk of post-menopausal breast cancer and kidney and oesophageal cancers, high fibre intake reduces bowel cancer risk, and high intakes of red or processed meat increase the risk of bowel cancer and stomach cancer.
New Zealand's Cancer Society and other health and physical activity organisations will meet in Wellington on November 21 to discuss the the new report.
