Concern about breast cancer is both widespread and wise.
One in every eight American women — even a few men — will eventually face the challenge.
We really need to know whether diet and other lifestyle choices can raise or lower the risk — or improve the odds of a robust, long-lasting recovery.
Two new studies shed light on food choices — one that’s risky and one that may help — women can make to help protect their breast health.
Before delving into these findings, here’s a quick refresher on how lifestyle appears to affect breast cancer.
Breast cancer and lifestyle: The story so far
According to the American Cancer Society, certain factors beyond our control can affect your risk of breast cancer, including:
Fortunately, the risks include some lifestyle factors we can at least partly control.
Weight gain and obesity are both linked with a higher risk of breast cancer after menopause.
Excessive alcohol is a risk factor, and many studies suggest that moderate-to-vigorous exercise can reduce your breast cancer risk.
And diet is believed to play a considerable role in your risk of breast cancer.
The American Cancer Society summarized the available evidence this way:
“A diet that is rich in vegetables, fruit, poultry, fish, and low-fat dairy products has also been linked with a lower risk of breast cancer in some studies. But it is not clear if specific vegetables, fruits, or other foods can lower risk.”
Fiber looks very favorable
Fiber may be a key reason why fruits and vegetables appear protective. That was the conclusion of a large Harvard study published last year … see Can Fiber Really Cut Breast Cancer Risk?.
Of course, fiber may just be a marker for intake of other beneficial plant-source chemicals. But that doesn't matter if eating more fiber-rich foods results in reduced breast cancer risk.
Fishy diets may help in the fight
Omega-3-rich diets — especially omega-3 DHA and EPA from fish and seafood — appear helpful. Most large epidemiological studies link diets high in seafood or omega-3s to lower breast cancer rates and improved outcomes.
For more information, see Omega-3s Restrain Breast Cancer Risk, Breast Cancer Risk Factor Curbed by Fishy Omega-3s, Fish and Omega-3s Linked to Breast Cancer Survival, Omega-3s May Fight Breast Cancer Fatigue, and their links to related reports.
The lab evidence on omega-3s versus cancer is quite compelling, and reveals variations in how omega-3s may affect different kinds of breast cancer.
For example, a 2013 test tube study found omega-3s particularly effective against “triple-negative” breast cancer cells — see Omega-3s May Target Toughest Breast Tumors — and this preliminary evidence should be pursued.
Most of several large diet-health population studies comparing seafood intakes or omega-3 blood levels to breast cancer risk suggest protective effects from fishy diets — though such epidemiological studies can’t prove cause-effect links between foods and health.
And in Breast Cancer Study Questions Omega-3s' Preventive Power, we noted that diet-health population studies often fail to account for the opposing effects of many participants’ highly excessive — hence pro-inflammatory and apparently risk-aggravating — intakes of omega-6 fatty acids from cheap vegetable oils (see our Omega-3/6 Balance page).
Rodent research links sugary diets to higher breast risk
A lab study in mice found that sugary diets raised the animals’ risk of breast cancer, and the risk that it would spread to their lungs.
The study comes from University of Texas MD Anderson Cancer Center, where researchers examined sugar’s effect on breast cancer risk and on a biological pathway — called 12-LOX — linked to tumor growth (Jiang Y et al. 2016).
The Texas-based team injected mice with breast tumor cells and divided them into five groups, each fed mouse chow providing a different proportion of sucrose:
After six months, 30% of the mice on the sucrose-free control diet had measurable tumors, versus from 50% to nearly 60% of those fed sucrose-rich diets.
In addition, the rate of cancer spreading to the lungs was much higher in the mice on sugar-rich diets.
As the researchers wrote, “We found that sucrose intake in mice comparable with [the] levels of [sucrose in] Western diets led to increased tumor growth and metastasis, when compared with a non-sugar starch diet."
Most of the blame assigned to the fructose in sucrose
Sucrose is the only sugar in cane sugar and it predominates in most fruits.
It's a “disaccharide” — which simply means that it consists of two parts: one glucose molecule bound to one fructose molecule, thereby making sucrose 50% glucose and 50% fructose.
(Cane sugar and high fructose corn syrup are chemically comparable, with corn syrup containing about 50% glucose and 50% fructose, give or take five percent.)
In line with other research, the study’s results suggested that the fructose half of sucrose was mostly responsible for two of the negative effects of dietary sucrose:
As they put it, “We determined that fructose derived from the sucrose was responsible for facilitating lung metastasis and 12-HETE production in breast tumors.”
Earlier studies indicated that sugar can promote breast cancer, and the Texas researchers cited considerable evidence that inflammation plays a role in that process.
And this isn’t the first time that fructose has been the subject of cancer-related concern.
For example, see… see Does Fructose Fuel Cancer? and Sugary Diets Raise Risk of Pancreatic Cancer Sharply.
According to the Texas team, limiting sugar consumption is critical to reducing the risk for cancers.
Average sugar consumption in the U.S. has surged to more than 100 lb. per year, and copious consumption of sugar-sweetened beverages appears to fuel the worldwide rise in rates of obesity, heart disease, and cancer.
Soy may boost survival for some breast cancer patients
Soy isoflavones are estrogen-like compounds that bind to estrogen receptors on breast cells, thus offering the possibility of curbing the growth of estrogen-dependent breast tumors.
But soy isoflavones exert mild estrogen-like effects, so concerns about the safety of soy for women at risk for, or diagnosed with, breast cancer have continued to linger.
The worry to date has been that, with a hormone receptor–positive cancer — the most common type — stimulation of those receptors by soy foods’ isoflavones could encourage cancer development, growth, and spread.
Five years ago, the authors of a Vanderbilt University study concluded that women who consumed 10mg or more of soy isoflavones daily were slightly but significantly less likely to die from breast cancer, and enjoyed a substantially reduced risk of cancer recurrence (Nechuta SJ et al. 2012).
Those conclusions echo the findings of a diet-health study we reported six years ago, which was conducted among 5,042 female breast cancer survivors aged 20 to 75 years: see Soy Link to Breast Health Gets a Boost.
Now, a U.S.-Canadian team — which included scientists from Tufts University and Stanford University — say the results of their population study suggest that dietary soy is safe for women with breast cancer, and may benefit people diagnosed with especially dangerous forms.
Importantly, the new findings help clarify some conflicting guidance about soy and breast cancer (Zhang FF et al. 2017).
The international team looked for statistical links between dietary intake of soy isoflavones from food — not soy powders or supplements — and death from any cause among 6,235 American and Canadian women with breast cancer.
The researchers analyzed diet survey data collected from 5,178 women who’d reported their pre-diagnosis diets and 1,664 women who’d reported their post-diagnosis diets.
Next, they compared the women’s reported diets to their health outcomes nine years after their initial breast cancer diagnosis.
Encouragingly, the women with breast cancer who reported consuming higher levels of isoflavones had a 21% lower risk of dying compared with women who reported eating lower levels.
The study’s lead author — Fang Fang Zhang, M.D., Ph.D., of Tufts University — said the results also should allay a lingering fear: “Based on our results, we do not see a detrimental effect of soy food intake among women who were treated with endocrine [hormone] therapy.”
Dr. Zhang added that her team’s findings suggested a benefit to women diagnosed with particularly dangerous “estrogen-negative” forms of breast cancer:
“For women with hormone receptor–negative breast cancer, soy food products may potentially have a protective effect. Women who did not receive endocrine [hormone] therapy as a treatment for their breast cancer had a weaker, but still statistically significant, association [between higher soy consumption and lower risk of recurrence].”
Scientists remain uncertain how soy isoflavones interact with breast cancer cells.
Some propose that the link between soy consumption and reduced risk of recurrence may be linked to anti-inflammatory, antioxidant and anti-angiogenic effects of soy isoflavones.
(Anti-angiogenesis simply means slowing or stopping the creation of new blood vessels necessary for tumor growth.)
Are soy foods generally healthful?
The available evidence suggests that whole, natural soy foods like tofu, miso, and tempeh are healthful for most people.
And, combined with prior findings, the new evidence suggests that whole soy foods may help prevent or slow the growth of certain breast cancers, without raising the risk of other kinds.
However, Dr. Andrew Weil and others suspect that in excess, processed soy foods — such as textured soy protein and soy protein isolates — may not be so healthful.
And we should note that much of the soy grown for human and animal consumption has been genetically modified to resist the herbicide glyphosate (sold as Roundup).
People concerned about the health effects of genetically modified soy — or the environmental and health effects of glyphosate — can choose certified organic soy, which, by definition, has not been genetically modified.