Diet, Exercise, and Breast Cancer: A Metabolic Connection?

Breast cancer is one of those diseases where every other week, there’s another study proving that X or Y causes (or prevents) it. Broccoli! Blueberries! Fish! Red meat! Olive oil! It gets to the point where it's almost hard to keep track of what's supposed to be good and what's supposed to be bad.

The hard truth underlying all those headlines is that nothing can completely take away the risk of cancer. Anyone can get breast cancer, even if they do everything "right" (including men - breast cancer is rare in men, but it does occur). There’s no magic diet plan or exercise routine or supplement that will completely eliminate that risk.

But with that said, reducing risk isn't pointless even if you can't completely avoid it. So here’s some context for the endless slew of studies on X or Y food and breast cancer, and which of them actually do reduce the risk of cancer. Taken in context, diet studies are very conflicting and it’s hard to pin down any one particular food as harmful or helpful. But studies also show that big-picture metabolic factors like insulin resistance and blood sugar do matter, and those things are strongly related to dietary patterns. And there's one intervention that shows consistent benefits in most studies: exercise.

Diet Studies are Conflicting.

Doing research into diet and breast cancer is hard. Cancer takes years to develop, and there are so many different factors that it’s hard to control for. Most of the studies that we have are just long-term associations - people who eat this food have a slightly lower rate of developing breast cancer.

A lot of different foods have been associated with breast cancer prevention this way. The positive findings get reported: we all hear about it every time broccoli (or fish, or fruit, or whatever else) might help prevent breast cancer. But studies with negative findings rarely get the same attention: when was the last time you saw a headline like “Study Finds No Association Between Broccoli and Breast Cancer, So Eat Whatever You Want, Maybe.”

So we only get one half of the story, and a lot of people come away thinking that particular foods have a very large or noticeable effect on breast cancer risk when in fact, there are just as many studies showing they don’t do anything. Looking at reviews of the evidence can help get around this, because reviews pull together a bunch of different studies and look at them all in context.

Unfortunately, most reviews show that “miracle cancer-preventing foods” are anything but.

This review covers a bunch of the diet patterns that typically get recommended for people with breast cancer:

• Fruits and vegetables: most studies show at least a small benefit, presumed to be from the antioxidants and fiber, but some

studies found benefit only from fruits and other studies found benefit only from vegetables, which is kind of weird and raises more questions than it answers.

• Soy and soy foods: association studies in women eating an Asian diet haven’t panned out in women eating a Western diet - this may be caused by a difference in gut flora (which can process an antioxidant in soy into a more powerful cancer-protective chemical), or by the influence of eating soy in early life. At any rate, there’s no evidence that an American woman in her 30s or 40s who starts worrying about breast cancer will get any benefit from suddenly adding soy to her diet. And the study also points out that other components of soy may increase breast cancer risk.
• Milk and dairy: some studies show that dairy increases risk; some show that it decreases risk; some show that it has no effect. The authors basically threw up their hands and shrugged.

Reviews focused on other foods are equally dubious. Just for example, take some reviews looking at antioxidants and foods that provide them:

• Green tea: this Cochrane review found very weak evidence for green tea for prevention of breast cancer (or any other kind of cancer). The authors concluded that there’s no risk of harm, but there’s also no strong evidence of a benefit.
• Vitamin D: another Cochrane review found no real benefit for Vitamin D.
• Mediterranean Diet: this review found insufficient evidence to conclude that a Mediterranean Diet pattern (fruits, vegetables, olive oil, fish, whole grains, legumes, little to no refined sugar) has a significant effect on breast cancer risk.

The “bad guys” don’t have much stronger evidence than the “good guys.” This review went over the typical scapegoats - red meat, processed meat, animal fat, total dietary fat - and couldn’t find any conclusive connections between any of them and breast cancer. The only really consistent association that everyone agrees on is alcohol consumption: a high alcohol intake is associated with an increased breast cancer risk.

Is this proof that diet has no real effect on breast cancer? Maybe it actually suggests that we’re looking in the wrong place - instead of worrying about particular antioxidants or individual superfoods, maybe we should be looking at big-picture metabolic health, and what foods contribute to that for each individual person.

Weight, Metabolic Health, and Breast Cancer

Breast cancer is associated with obesity, and with insulin resistance and other metabolic problems. Higher levels of body fat and insulin resistance have some common effects on estrogen levels and the activities of estrogen receptors, which might explain the similar associations with breast cancer.

If the diet affects the risk of breast cancer by improving metabolic health and maintaining a healthy weight, then it’s no surprise that dietary risk factors would be very individual, because the best diet for weight and metabolic health is so individual. For some people, that might be high-fiber/low-fat; for other people, it might be low-fiber/high-fat. Or it could be anything in between.

A recent study just showed that blood sugar responses to different foods have huge variation from person to person - some people have a higher blood sugar spike after eating bananas than they do after eating the same number of calories from cookies. So if a good diet for avoiding breast cancer is a diet that avoids blood sugar spikes, then that diet would look very different from person to person, and it’s not surprising at all that the associations are conflicting and inconsistent.

Overall, the evidence on diet, weight, and metabolic health might suggest that the “best” diet for breast cancer might be the one that reduces inflammation, manages hormone levels, and promotes metabolic health in the person who eats it.

Studies on Exercise Show Consistent Benefits

The diet -> metabolic health -> breast cancer connection makes even more sense when you look at studies on exercise. Unlike the ridiculous tangle of diet advice, studies on exercise are actually pretty encouraging to read. Physical activity helps reduce circulating estrogens independently of weight loss, and this study found that exercise may also help by modifying the expression of genes (specifically FGFR2 and MTHFR) that make people more susceptible to breast cancer. Physical activity may also be protective against breast cancer because it helps reduce inflammation, lower blood sugar, and improve insulin sensitivity - in other words, it supports that all-important metabolic health.

The benefits are pretty impressive:

• This review of 126 association studies found that people with the most leisure-time physical activity had a 10% lower risk of breast cancer than people with the least activity.
• This review found even more of a benefit, a 25% average reduction in risk for women in the highest-activity group compared to the lowest-activity group.
• This review found that the benefits were even stronger for postmenopausal women (20-80% risk reduction) than for premenopausal women (15-20% reduction), but overall, every additional hour of physical activity each week was associated with a 6% risk reduction.

This review cited studies that showed a benefit from exercise as gentle as walking (2-3 hours per week at a brisk pace), although the authors noted that more intense activity was even better.

Summing it Up

In studies of individual foods, cancer-preventing results have been conflicting and frankly pretty disappointing - there’s not a lot of convincing evidence that any one particular food will significantly reduce the risk of breast cancer. The real benefit might be from reducing unhealthy visceral fat and improving metabolic health, which would take a different diet for each individual person. But studies about physical activity are much more promising - maybe because exercise improves insulin sensitivity and metabolic health on top of its direct effects on estrogen levels..

The next time you read a headline about how green tea (or tofu, or olive oil, or raw fruit…) “prevents” breast cancer, take it with a big grain of salt, and maybe a nice long walk.