Can dietary fiber reduce breast cancer risk? A new analysis adds fresh insights to a Smart Bytes discussion of just a few months ago about a previously published analysis on this subject.  This new analysis includes additional studies and drills down to see if fiber’s effects differ by amount and type of fiber.

The fiber-breast cancer link
Almost all the 16 studies in this latest analysis found the association of higher dietary fiber with lower breast cancer risk just as strong after statistical methods adjusted for weight.  That suggests boosting fiber consumption may help lower your breast cancer risk even outside of any weight control help it provides.  As noted in my previous post, lower levels of estrogen, insulin and inflammation are three likely ways fiber could reduce breast cancer risk.

Four Key Take-Home Messages
1) Get fiber from a variety of foods.
Each 10-gram increase in dietary fiber was linked with 5% lower breast cancer risk.  You get 10 grams of fiber in:
                1 bowl of super-high fiber cereal
                OR 4 to 5 half-cup servings of fruit and vegetables
                OR ½ cup of dried beans and one slice of whole wheat bread.

Whether that extra 10 grams of fiber came from fruit, vegetables or grain products did not show any clear advantage.  The one type of fiber that stood out as statistically significant in its link to lower risk was soluble fiber.  Every 10-gram increase in soluble fiber showed a 26% lower breast cancer risk. You might be most familiar with soluble fiber as the fiber found in oatmeal that helps lower blood cholesterol, but it’s much more than that.

Actually, “soluble fiber” is not all the same, and differences in viscosity (tendency to form a gel when mixed with water) and fermentability (ability for healthy bacteria in our gut to use it to produce health-promoting substances) likely result in different effects.
 Foods contain a mix of soluble and insoluble fiber. Those especially high in soluble fiber include:

AICR's Caribbean Black Beans

• Legumes such as chickpeas and black, kidney and pinto beans
• Oatmeal, oat bran, barley
• Apples, pears, oranges, grapefruit, prunes
• Carrots, peas, sweet potatoes, asparagus and some other vegetables
• Seeds, especially flaxseed
• Seaweed (as in seaweed salad)

Soluble fiber stands out in this analysis, but don’t zero in too tightly. The insoluble fiber we get from whole wheat and other whole grains, as well as many vegetables, seems linked to lower weight and waist gain (and other health benefits), so this is far from a call to give up on those foods. Instead, as you ramp up your use of high-fiber foods, simply make sure foods like dried beans are part of the picture.

2) Think big
With average dietary fiber consumption in the U.S. at 15 grams/day, most of us need to do more than just grab one extra piece of fruit each day.  In this most recent analysis, although statistical tests did not identify a particular minimum threshold of fiber necessary to reduce breast cancer risk, the authors noted that only studies that showed a large range or high level of fiber intake (at least 25 grams/day) found significant benefit. In studies in which almost everyone’s fiber intake was low, just getting a little more did not show protection.

Take it one step at a time if you prefer – probably the most comfortable way for your digestive system to get used to more fiber. But work toward a target of several “whole” plant foods (vegetables, fruits, whole grains) at every meal, and take the big step of making concentrated sources like dried beans a regular part of your eating pattern.

3) It’s not too early or too late
Fiber’s link to lower breast cancer risk does not seem any more strongly related to either premenopausal or postmenopausal disease.  Most breast cancer develops over many years, and some factors may act early in life influencing risk.  Yet in both recent analyses of the research, studies looking at risk over longer periods don’t show any stronger link to fiber, suggesting that fiber intake even in middle age and beyond may play a protective role.

4) Swap, don’t add
This analysis of research on dietary fiber and breast cancer risk was conducted by independent researchers at Imperial College in the UK as part of an ongoing process updating the database on which cancer prevention recommendations from the American Institute for Cancer Research (AICR) are based. The overall research picture shows that after menopause – by far the majority of breast cancer cases – excess weight and weight gain pose significant risk. To control calorie consumption, as you add in foods with more fiber, make sure they replace other foods (ideally some of your less nutritious choices).

Final thought:
As I noted regarding the previous analysis linking fiber consumption with lower breast cancer risk, it’s possible that “fiber” or “soluble fiber” is a marker for a whole range of protective compounds in high-fiber foods.  If further research identifies fiber (or any particular fiber type) as the active influence, then fiber supplements or foods with added isolated fibers could be protective.  For now, reasonable amounts of fiber from these sources can be healthful, but don’t count on them in your strategy to reduce breast cancer risk.  Keep your focus on a variety of foods rich in both fiber and nutrients.

Resources
Need some help working in more high-fiber foods? Check this free brochure from AICR on Beans and Whole Grains as part of the healthy eating pattern called the New American Plate.

References
Aune D et al. Dietary fiber and breast cancer risk: a systematic review and meta-analysis of prospective studies. Ann Oncol. Published online ahead of print, Jan 2012.

Dong J-Y et al. Dietary fiber intake and risk of breast cancer: a meta-analysis of prospective cohort studies.  Am J Clin Nutr September, 2011.