Reductionist thinking neglects the broader approaches of cancer nutrition research, including eating patterns and the mechanisms of microbiology. The quest now is for answers about nutrition's relationship to the many challenges of cancers, challenges that go beyond any individual study.
When you hear that a certain food helps prevent cancer, ask: Which cancer? "Cancer is multiple diseases," said Marian Neuhouser, a nutritional epidemiologist at the Fred Hutchinson Cancer Research Center in Seattle.
Whereas cardiovascular disease might be broken down into several types, including myocardial infarction, stroke and peripheral vascular disease, she said, "for cancer, it's really over 100 different diseases."
"Cancer is a very complex, very challenging disease to study whether you're looking at it on the cell level or the clinical level or the epidemiologic and preventive level," Willett said.
Cancer occurs when abnormal cells divide uncontrollably. But one cannot assume that all cancers operate in the same way, said Geoffrey Kabat, a cancer epidemiologist at the Albert Einstein College of Medicine in New York. Different cancers can have somewhat different risk factors, which may or may not overlap: The cancers linked to tobacco, for example, might differ from those linked to radiation.
Researchers caution about overreacting to a single study. New findings come out every week, but "we never take any one study to be the answer to anything," said Nancy Potischman, a nutritional epidemiologist at the National Cancer Institute. Only if the same results come up in multiple studies across multiple populations, "then you might think that, yes, this food might be important," she said.
While it's relatively easy to see the effect of a food on a lab animal, it's difficult to study humans, who put variety on their plates. Population studies query participants about what they eat and follow them through a time period. Clinical trials might have a group that eats a certain food or nutrient and a control group that doesn't consume that food, but these studies are very expensive, specific and hard to organize and maintain. Sometimes findings that are promising in early research don't prove to be definitive in follow-up studies.
Still, we yen for super foods - and nutrients. For a while, attention focused on folic acid, which didn't quite live up to all its hopes, at least for cancer. Now there's a great deal of interest in vitamin D.
"There's a process of making hypotheses and testing them," said Jo L. Freudenheim, a professor of epidemiology and environmental health at the State University of New York at Buffalo. "To the extent this goes out to the public, it can lead to unreasonable expectations." And that can set off the next new fad.
Tobacco use remains the leading preventable cause of cancer incidence and death worldwide. After tobacco, the lifestyle trio of diet, weight control and exercise may be linked to one-third to two-thirds of cancers.
"They're inseparable," Neuhouser said. "You can have a great diet and you can have a healthy weight, but if you're extremely sedentary then there's a risk."
And there's a strong link between excess weight and several kinds of cancer, including the esophagus, breast (after menopause), endometrium, colon and rectum, kidney, pancreas, thyroid, gallbladder, according to the NCI. Exercise helps balance calories consumed and calories burned.
Evidence mounts about how lifestyle may affect risk of cancer.
In the largest study of its kind, nearly half a million Americans were evaluated for adherence to American Cancer Society cancer prevention guidelines that include smoking avoidance; a healthful, consistent weight; physical activity; limiting alcohol; and a diet emphasising plants.
Those who followed the guidelines most closely had lowered risk of developing cancer (10 per cent for men, 19 per cent for women) and dying from cancer (25 per cent for men, 24 percent for women) compared with those whose habits were least in line with the guidelines. Most striking was the reduction of overall risk of dying: 26 percent for men, 33 percent for women during the 14-year study period.
Fourteen types of cancer seemed affected by lifestyle behavior, most particularly gallbladder, endometrial, liver and colorectal. For men and women, a healthful weight and physical activity were the top factors in reduced deaths overall. Albert Einstein College of Medicine Researchers published this analysis online in January in the American Journal of Clinical Nutrition, based on data from a National Institutes of Health/AARP study.
Kabat, the study's lead author, said that these results, while encouraging, might be explained by unknown factors: Those who best follow guidelines might be particularly health-conscious and have good access to health care. But overall, the findings affirm decades of other studies showing that "maintaining a healthy weight, physical activity and maybe certain aspects of diet are associated with better health," he said.
Another approach to cancer and nutrition considers dietary patterns. "What we eat on any one day is not going to change our cancer risk, but it's the pattern over the long term." Neuhouser said. Several diets that emphasized fruit, vegetables, whole grains and plants or plant-based proteins were analyzed against information collected over more than 12 years from nearly 64,000 post-menopausal women in the Women's Health Initiative Observational Study. Consuming a high-quality diet was associated with lower death rates from chronic diseases including cancer, as reported last year in the American Journal of Epidemiology.
For researchers in the field of developmental nutrition, the quest is not what you eat, but when you eat it.
"We don't fully understand whether or not there are certain periods of life which are more important to have healthy aspects of the three pillars" of diet, exercise and weight, Neuhouser said.
For breast cancer, for example, researchers are looking at nutrition at birth and time of first period through first pregnancy, Freudenheim said.
Evidence is increasing that eating red meat in high school might have a bearing on the development of some kinds of breast cancer decades later, Willett said. Also, alcohol consumption by young women may raise the risk of breast cancer later in life.
Greater understanding of metabolism might offer clues to how cancer cells develop, according to the National Cancer Institute's Potischman. Metabolomics is an emerging branch of science that focuses on metabolites, the substances produced through digestion and other bodily processes.
The bacteria, viruses and other organisms that live in and on humans seem to play a bigger role in health and disease than was previously understood, Freudenheim said. How the countless microbes in such areas as the gut and the mouth might contribute to or prevent cancers is one of the open questions in the new area of study of the microbiome, which refers to the many organisms in the body, 10 percent of which are human and 90 per cent nonhuman.
Nutrigenetics considers what we eat, the components in our foods and their interactions with genetic processes. Regarding diet, "right now we're making blanket recommendations, but one size doesn't fit all," says Stephen Hursting, a professor of nutrition at the University of North Carolina at Chapel Hill. He said that someday research on our genetic and biochemical differences might lead to personalised dietary recommendations to reduce cancer risk.
When considering links between cancer and nutrition, "one shouldn't lose the big picture by focusing on: Is green tea going to be a magic bullet?" Kabat said. "It's unlikely to be."
But, he said, healthful lifestyle habits can make a difference.
"There's no reason to believe they can do any harm, and there's a lot of either hard evidence or suggestive evidence that they will do good."
