One in eight women in the United States will be diagnosed with breast cancer in her lifetime.
There’s only one cancer—lung—that kills more women than breast cancer. But unlike lung cancer, breast cancer has no obvious cause. Here are six things you may not know.
1. Breast cancer is not a total mystery.
“We know how to prevent almost all breast cancer,” says Walter Willett, professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health. It’s just not a very good solution.
“First, you keep girls in a semi-starved state as they’re growing to prevent them from menstruating until they are about 17 or 18. Then you make sure they get pregnant and have child after child and keep breastfeeding so that they don’t menstruate in between.”
Why would that work? “Breast cancer risk is related in part to the number of menstrual cycles a woman has,” explains Willett. “The cycles make the breast tissue multiply, and the more that cells multiply over a lifetime, the higher the risk.”
That may be why both early menarche and late menopause raise the risk. Until the twentieth century, a life with fewer menstrual cycles was the norm.
“Unfortunately, our lifestyle—which has many benefits that we certainly don’t want to give up—has set us up for breast cancer,” says Willett.
Even if you look beyond menstrual and reproductive cycles, we know how to prevent many breast cancers.
“It’s clear that weight gain during adult life is related to the risk of postmenopausal breast cancer,” says Willett.
Taking estrogen plus progestin after menopause also increases risk, especially in older women who are at least 10 years past menopause.1,2 (Taking those hormones to treat severe hot flashes and night sweats for a few years during early menopause poses a smaller risk.3)
2. Extra weight matters.
If you’re postmenopausal, extra pounds boosts your risk of breast cancer. And it doesn’t have to be many pounds.4 “It’s not just obesity,” notes Regina Ziegler, senior investigator at the National Cancer Institute. “Postmenopausal women who are overweight have a higher risk than those who are normal weight.”
(Premenopausal women who are heavy have a lower risk of breast cancer. But excess weight raises their risk of type 2 diabetes and other health problems.)
How does extra pounds promote postmenopausal breast cancer? After menopause, a woman’s ovaries stop producing estrogen. At that point, most of her estrogen is made by her fat cells. So the more fat cells she has, the higher her blood levels of estrogen—which travels through the blood as estradiol.
“Women who are obese have about three times the circulating levels of estradiol compared to lean women,” says Willett. “That’s a huge difference. And we see some increase in risk even in women who gain 5 to 10 pounds. It’s not just women who gain 50 or 60 pounds.”
Roughly 80 percent of breast tumors are hormone-receptor positive—that is, they are fueled by estrogen and/or progesterone.5 (Those tumors are usually easier to treat—with drugs that block estrogen production—than tumors that are not fueled by those hormones.)
But estrogen alone may not explain why heavier women have a higher breast cancer risk. They’re also more likely to have higher insulin levels, which may also fuel tumors.
When researchers looked at women who were not taking estrogen after menopause, those with the highest insulin levels had 2½ times the risk of breast cancer of those with the lowest insulin levels, after taking estradiol levels and weight into account.6
“Extra weight works only partially by raising estradiol,” explains Ziegler.
But regardless of how extra padding works, the message is the same. “The main point for postmenopausal breast cancer is to weigh as close as you can to what you weighed at age 18,” says Willett.
3. Environmental estrogens may not be a factor.
Could estrogen-like chemicals like BPA and other plasticizers explain why breast cancer strikes so many women?
So far, there isn’t much evidence from human studies that those chemicals matter. That’s what the National Academy of Medicine found in its 2011 report, Breast Cancer and the Environment: A Life Course Approach, which was commissioned by Susan G. Komen for the Cure.7
“I am not a fan of pesticides in food or chemicals in the environment that we now find ubiquitously in human tissues,” says David Hunter, professor in cancer prevention at the Harvard T.H. Chan School of Public Health.
“We definitely can’t give them a pass. But the evidence of an association with breast cancer in humans is very limited, so a lot depends on how confident you are extrapolating from test-tube and animal data to give advice to people.”
And it has gotten tougher to gather data in humans. “Increasingly, there is evidence that childhood and adolescent and young adulthood exposure is important,” says Hunter, who served on the panel of experts that wrote the National Academy of Medicine report.
“We’ve made it even more difficult to gather human evidence by pointing the exposure window back to many decades prior to the cancer diagnosis. Nobody’s got a 50-year prospective study.”
Of course, it’s sensible to avoid potentially harmful chemicals anyway. “If women are worried about their children, and it’s possible to avoid exposure to pesticides and plasticizers without greatly altering one’s lifestyle, I would say, why not?” says Hunter.
“That may have other health benefits that have nothing to do with breast cancer,” he adds. “But we can’t pretend that we know for certain that that’s going to reduce a child’s breast cancer risk later in life. It’s just very, very hard to establish cause and effect, particularly with cancer, when you may be looking at a 40- or 50-year exposure.”
4. Alcohol boosts risk.
“Alcohol is related to both premenopausal and postmenopausal breast cancer,” says Harvard’s Walter Willett. “And the more you drink, the higher your risk.”
Drinking over more of your life also matters. “Women who started drinking earlier in life and then stopped, their risk goes down,” Willett explains. “The highest risk is in women who started consuming alcohol early and continued.”
And it’s not just women who overdo it.8 “We now see a 17 percent increased risk with only one drink every other day,” notes Willett. “What’s remarkable is how modest that amount is. With colorectal cancer, you don’t see much increase in risk until you get to over two drinks a day.”
Alcohol’s ability to raise blood estrogen levels appears to explain at least part of the increased risk.9 “But we’re still not entirely sure whether it’s limited to the increase in estrogen or whether there’s more to it than that,” adds Willett.
Could teenage drinking pose a particularly potent threat? “That’s been a worry from the beginning, because the breast is more sensitive then,” says Willett. When he and others tracked nearly 6,900 teens aged 13 to 20 for five years, each daily serving of alcohol they consumed was linked to a 50 percent higher risk of benign breast disease.10 (Some types of benign breast disease are risk factors for cancer.) “So far we haven’t seen a massive time bomb due to teenage drinking,” says Willett. “But it deserves some more looking.”
5. Exercise helps.
“More than 100 epidemiological studies have looked at the risk of breast cancer and physical activity,” says Heather Neilson, a Canadian exercise researcher at Alberta Health Services in Calgary. “The majority have found that women who are the most physically active have a 10 to 25 percent lower risk than women who are the least physically active.”
Of course, something else about women who exercise might explain their lower risk of getting or dying of the disease. Only trials that randomly assign women to exercise or not can find out.
A year-long trial—the Nutrition and Exercise for Women (NEW) study—was a surprise. The trial assigned 439 overweight or obese postmenopausal women to one of four groups:
■ a weight-loss diet,
■ aerobic exercise (45 minutes a day for five days a week).
■ diet plus exercise, or
■ a control group (which was told not to make diet or exercise changes).
The results: estradiol, insulin, inflammation, and other markers of risk improved (just about equally) in both the diet and diet-plus-exercise groups (which lost an average of 20 pounds). However, the exercise-only group’s markers were no different from the control group’s.11
“The NEW trial suggests that for overweight or obese postmenopausal women, some of the benefit of exercise comes from weight loss,” says Neilson.
Still, exercise could lower breast cancer risk by some mechanism that wasn’t examined in the NEW study. “The epidemiological evidence that exercise lowers the risk of breast cancer is quite strong, so there’s probably something about physical activity that we don’t understand, or different pathways that we’re not measuring,” says Kristin Campbell, an associate professor of physical therapy at the University of British Columbia who co-authored the NEW trial.
For example, says the National Cancer Institute’s Regina Ziegler, “we’re looking at whether physical activity changes how the body metabolizes estrogen.”
In the meantime, they all recommend exercise, whether you lose weight or not. “Physical activity helps prevent weight gain in the future and helps maintain weight loss,” says Campbell. “And it improves cardiovascular health, type 2 diabetes, mental health, and quality of life, so it’s obviously a great benefit.”
6. CT scans are far more than ordinary x-rays.
Studies in humans find little evidence that non-ionizing radiation—like microwaves or radio waves—causes breast cancer, said the 2011 National Academy of Medicine report.7 But it did point a finger at another kind of radiation: CT (also called CAT) scans.
“There’s been a large increase in medical imaging that involves ionizing radiation, particularly with CT scans,” says Harvard’s David Hunter. That’s partly because the imaging is so much better than it was 20 or 30 years ago.12
“Nobody thinks that CT scans are a major driver of breast cancer rates in the United States,” adds Hunter. “It’s just that here is a known carcinogen where the exposures have actually increased over time, even as the machines have in theory been made safer and become better regulated.”
Most people—including some doctors—don’t realize that in order to produce a 3-D image, a CT scan of the abdomen exposes the body to 400 times more radiation than an ordinary chest x-ray.13 “The doses from mammograms are much lower,” says Hunter. (A virtual colonoscopy is a CT scan. An MRI causes no radiation exposure.)
And then there’s the possibility of errors. “There have been documented incidences of overdoses,” notes Hunter. “Regulations probably need to be tighter, given the proliferation of these machines.”
Hunter’s bottom line: “Nobody should be subjected to medical imaging using ionizing radiation without a good rationale for it. If the situation is not urgent, women should ask about the risks and benefits of having the procedure.”
1JAMA 288: 321, 2002.
2N. Engl. J. Med. 356: 1670, 2007.
3N. Eng. J. Med. 374: 803, 2016.
4BMJ 335: 1134, 2007.
5J. Natl. Cancer Inst. 106: dju055, 2014.
6J. Natl. Cancer Inst. 101: 483, 2009.
8JAMA 306: 1884, 2011.
9J. Natl. Cancer Inst. 93: 710, 2001.
10Pediatrics 125: e1081, 2010.
11J. Clin. Oncol. 30: 2314, 2012.
12N. Engl. J. Med. 357: 2277, 2007.
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