LONDON -- Wading through 30 years of confusing and sometimes contradictory studies on cancer and diet, experts have summarized the state of scientific knowledge: alcohol is bad, obesity is bad and lots of fruits and vegetables are good.
Poor diet is thought to account for about 30 percent of cancer in the developed world and about 20 percent in poor countries, and scientists have long sought to determine what foods cause or ward off cancer. A review of the evidence, published this week in The Lancet medical journal, concludes that studies so far have confirmed little.
"Because the public is so bombarded and confused by stories that broccoli is the answer, or whatever, we wanted to get away from that and report what we know is really important," said the study's lead investigator, Dr. Tim Key, a diet expert at Oxford University's cancer epidemiology unit. "The problem is you keep getting stories, and the more bizarre the connection, the more press coverage it gets."
There have been few studies that have tested the link between cancer and specific foods by randomly giving some people specific foods and comparing their cancer rates with people who got no intervention. A positive result in such a study is considered real proof.
The few such studies show a diet rich in fruits and vegetables reduces the chance of cancer, while alcohol and obesity increase the risk.
"The results ... that have been published have been important in suggesting that some previous observations were misleading," the study said.
Two prominent examples are beta-carotene and vitamin E. Both looked promising as anti-cancer nutrients, but showed no effect on lung cancer rates when tested in rigorous experiments.
Studies have suggested that such dietary components as red meat, broccoli, garlic, fiber, folic acid, vitamin C and soya can either encourage or prevent certain cancers, but the links have not been proven.
The study also identified aspects of nutrition where further research might soon clarify the issues.
For red meat, it looks as though the important thing could be how the meat is prepared. Recent studies have suggested that preserved meats such as cured ham, bacon and sausages could increase cancer risk, but that fresh red meat may not.
The idea that high intake of calcium and vitamin D might reduce the chance of colorectal cancer looks promising, the study said.
However, the evidence does not support the theory that dietary fat increases the risk of breast cancer, and findings on other foods such as dairy products and meat are inconclusive, the study said.
"This is a good update of the situation," said Dr. Elio Riboli, chief of the nutrition and cancer unit at the World Health Organization's cancer research agency, the International Agency for Research on Cancer. Riboli was not involved with the study.
Riboli said emerging evidence suggests many of the factors that contribute to heart disease are also involved in cancer, such as lack of exercise, being even moderately overweight and problems with insulin, the hormone that goes wrong in diabetes.
"I think it's possible that we will realize that some of the benefits which were in the past attributed to the diet in itself should actually be attributed to the global balance between how we eat, how we move and our body shape, where we are actually pointing more to the energy balance," Riboli said.
"This is a major change in the intellectual view of the problem," he said. "Within the World Health Organization there's been a clear understanding that obesity is the real worrying epidemic around the world. There is a strong movement in the direction of 'yes, we have to do something."'
Earlier this year, WHO officials said obesity has reached such epidemic proportions worldwide that a more aggressive approach is needed to try to head off a global explosion of fat-related diseases.
Experts are also starting to advocate a tougher strategy. "The individual awareness approach has been shown repeatedly to have failed," experts said in a report presented Wednesday at a European Union summit on obesity.
In its report, the International Obesity Task Force called for European restrictions on the advertising of junk food.
Other measures mentioned in the report were: redesigning roads to accommodate networks of bicycle tracks, removing junk food vending machines from schools, reintroducing cooking skills into the school curriculum and the establishment of a new medical specialty that takes a comprehensive approach to obesity.
On the Net:
International Obesity Task Force, http://www.iotf.org
International Agency for Research on Cancer, http://www.iarc.fr