THURSDAY, Nov. 16, 2006 (HealthDay News) -- What you choose to eat might determine your risk of developing some common cancers.
That's the conclusion of several new studies presented this week at an American Association for Cancer Research meeting in Boston.
Women who eat high amounts of soy, especially as children, may have a significantly lower risk of breast cancer, according to one study. Other studies show that men who eat a fish-rich diet may have a decreased risk of colorectal cancer and that male smokers who eat foods containing high amounts of vitamin E -- such as nuts, whole grains and green leafy vegetables -- may have a decreased risk of developing tobacco-related cancers.
Together, this research offers some of the strongest evidence to date of a link between diet and cancer, the study authors said.
"This is the first study to look at childhood soy exposure and the later risk of breast cancer. It suggests that there really is a biologic effect for soy, and we're excited about that," said lead researcher Dr. Larissa Korde, of the U.S. National Cancer Institute.
"The more we know about cancer, the more it's clear that diet is related to cancer," added Dr. Alan Kristal, of the Fred Hutchinson Cancer Research Center in Seattle, who moderated the meeting's diet and cancer session but was not connected with the three studies. "But the relationship is complex. Diet and cardiovascular disease is simple compared to diet and cancer because the risk factors differ for different cancers."
In their study, Korde's team examined diet and lifestyle factors in 1,563 Asian-American women, 597 of whom had breast cancer and 966 of whom did not.
The researchers found a 58 percent lower risk of breast cancer in women who ate the most soy as children -- an average of a little over two servings per week -- compared to women who ate the least soy -- an average one-quarter serving per week. They also found a 25 percent lower risk of breast cancer in women who ate the most soy as adolescents and adults.
A recent review of 18 epidemiological studies showed a more modest overall reduction in risk: 14 percent.
However, the new study looked at women of Chinese, Japanese and Filipino ancestry because "it is a population that has a very different diet and lifestyle than most Americans," Korde explained.
Asians consume much more soy than Americans do and breast cancer rates in Asian countries are four to seven times lower than they are in the United States, she said. But after Asians migrate to the United States, it usually takes only three generations for their breast cancer rates to catch up to those of American white women.
Korde's team used food-frequency questionnaires to study the women's diets. They also interviewed the women about other lifestyle factors, such as whether or not they lived in mostly Asian or non-Asian neighborhoods, shopped in Asian or non-Asian grocery stores and read Asian or non-Asian newspapers. They also interviewed 255 of their mothers to get additional information about their daughters' childhood exposures.
"We found that soy was more important than these other measures of acculturation," Korde said.
Although scientists don't know exactly how soy affects breast cancer risk, animal models suggest that a soy component -- isoflavones -- may have protective, estrogen-like effects. "The hypothesis is that exposure to estrogen-like substances early in life can cause changes in developing breast tissue that decreases its sensitivity to carcinogenesis later in life," Korde said.
That may explain why soy seemed to be especially protective in women who ate high amounts of tofu, miso and natto between the ages of and 5 and 11, she said.
The timing of exposure to hormones or hormone-like substances may have a critical effect on breast cancer risk, Korde added. "For example, obesity earlier in life seems to decrease the risk of premenopausal breast cancer (because fat tissue also secretes estrogens). But it's been well-documented that hormone replacement therapy increases the risk of post-menopausal breast cancer."
Although Korde called the study findings "very promising," she cautioned that it's premature to tell American women and girls to start eating more soy. "Before we can make any recommendations, the findings need to be replicated by other studies," she said.
Other studies presented at the meeting also showed that eating certain foods may reduce the risk of certain cancers. In a new analysis of data from the 22,071 participants in the Physicians' Health Study, researchers from the Harvard School of Public Health found that men who ate fish five times a week or more had a 40 percent lower risk of developing colorectal cancer than men who ate fish less than once a week.
"We already know that eating fish can reduce the risk of sudden cardiac death, and this might provide another reason to add fish to your diet," study author Megan Phillips, of the Harvard School of Public Health, said in a statement.
Previous research has shown that high fish consumption may reduce women's risk of renal cell carcinoma -- the most common form of kidney cancer -- by 44 percent.
In a study of 280 smokers, researchers from Columbia University found that higher blood levels of vitamin E from food were associated with lower levels of oxidative damage in white blood cells, which is a marker of increased cancer risk. But the effect was only seen in male smokers, and it appeared to be most significant in men with a beneficial form of a common "detoxifying" gene: GSTM1.
"This suggests that while working toward the goal of quitting smoking, which is the very best way to prevent development of smoking-related cancers, it could be helpful to eat a diet rich in vitamin E," study author Frederica P. Perera said in a statement. "We don't know why this relationship was not found in women, but a good diet is beneficial to health in many ways."
Kristal, who is also professor of epidemiology at the University of Washington, called the new studies "interesting."
"They show that the relationship between diet and cancer is a complicated story," he said. "Diet interacts with many other factors, such as genes and other exposures you might have such as smoking."
"If you look at sum total of studies on diet and cancer, what you come away with is that it's important to prevent obesity, avoid toxins like excess alcohol, and eat a balanced diet that's moderate in fat and high in whole grains, fruits and vegetables," Kristal said.
To learn more about soy and health, head to the U.S. Food and Drug Administration.