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Are we absolutely or relatively sure that increasing dietary fiber reduces breast cancer risk?

Boston, MA – Women who eat more high-fiber foods during adolescence and young adulthood–especially lots of fruits and vegetables–may have significantly lower breast cancer risk than those who eat less dietary fiber when young, according to a new large-scale study led by researchers at Harvard T.H. Chan School of Public Health.

The study will be published online February 1, 2016 in Pediatrics.

“Previous studies of fiber intake and breast cancer have almost all been non-significant, and none of them examined diet during adolescence or early adulthood, a period when breast cancer risk factors appear to be particularly important,” said Maryam Farvid, visiting scientist at Harvard Chan School and lead author of the study. “This work on the role of nutrition in early life and breast cancer incidence suggests one of the very few potentially modifiable risk factors for premenopausal breast cancer.”

The researchers looked at a group of 90,534 women who participated in the Nurses’ Health Study II, a large long-running investigation of factors that influence women’s health. In 1991, the women–ages 27-44 at the time–filled out questionnaires about their food intake, and did so every four years after that. They also completed a questionnaire in 1998 about their diet during high school. The researchers analyzed the women’s fiber intake while adjusting for a number of other factors, such as race, family history of breast cancer, body mass index, weight change over time, menstruation history, alcohol use, and other dietary factors.

Breast cancer risk was 12%-19% lower among women who ate more dietary fiber in early adulthood, depending on how much more they ate. High intake of fiber during adolescence was also associated with 16% lower risk of overall breast cancer and 24% lower risk of breast cancer before menopause. Among all the women, there was a strong inverse association between fiber intake and breast cancer incidence. For each additional 10 grams of fiber intake daily–for example, about one apple and two slices of whole wheat bread, or about half a cup each of cooked kidney beans and cooked cauliflower or squash–during early adulthood, breast cancer risk dropped by 13%. The greatest apparent benefit came from fruit and vegetable fiber.

The authors speculated that eating more fiber-rich foods may lessen breast cancer risk partly by helping to reduce high estrogen levels in the blood, which are strongly linked with breast cancer development.

“From many other studies we know that breast tissue is particularly influenced by carcinogens and anticarcinogens during childhood and adolescence,” said Walter Willett, Fredrick John Stare Professor of Epidemiology and Nutrition at Harvard Chan School and senior author of the study. “We now have evidence that what we feed our children during this period of life is also an important factor in future cancer risk.”


Other Harvard Chan School researchers involved in the study included Heather Eliassen, associate professor in the Department of Epidemiology, and Xiaomei Liao, research scientist in the Departments of Epidemiology and Biostatistics.

The study was supported by National Institutes of Health grants R01 CA050385 and UM1 CA176726 and a grant from The Breast Cancer Research Foundation. Farvid was supported by the Japan Pharmaceutical Manufacturers Association.

“Dietary Fiber Intake in Young Adults and Breast Cancer Risk,” Maryam S. Farvid, A. Heather Eliassen, Eunyoung Cho, Xiaomei Liao, Wendy Y. Chen, and Walter C. Willett, online February 1, 2016, Pediatrics, 137(3):e20151226 doi: 10.1542/peds.2015-1226

Visit the Harvard Chan School website for the latest news, press releases, and multimedia offerings.

Harvard T.H. Chan School of Public Health brings together dedicated experts from many disciplines to educate new generations of global health leaders and produce powerful ideas that improve the lives and health of people everywhere. As a community of leading scientists, educators, and students, we work together to take innovative ideas from the laboratory to people’s lives–not only making scientific breakthroughs, but also working to change individual behaviors, public policies, and health care practices. Each year, more than 400 faculty members at Harvard Chan School teach 1,000-plus full-time students from around the world and train thousands more through online and executive education courses. Founded in 1913 as the Harvard-MIT School of Health Officers, the School is recognized as America’s oldest professional training program in public health.

Higher dietary fiber intake in young women may reduce breast cancer risk

Our Review Summary

Dietary fiberThis news release from Harvard describes a study published in Pediatrics that found an inverse association between a high fiber diet during adolescence and a lower risk of developing breast cancer later on. Those consuming the most fiber were about 20% less likely to develop breast cancer in middle age, according to the release. The data analyzed was drawn from a long-term epidemiological study of factors that affect women’s health (the Nurses’ Health Study II). The release doesn’t note that the findings show a relationship, but not a cause. This study doesn’t provide proof that fiber caused the reduction in cancer risk, and there’s still uncertainty as to if and how fiber might “fight cancer.” Like the CBS and NPR news stories also reviewed by, the release does not tell us if the lowered risk refers to absolute or relative risk. Maybe those omissions in subsequent news coverage started here, with the news release.


Why This Matters

The way that studies are framed in news releases matters because it affects resulting news coverage from journalists. In our reviews of CBS and NPR stories on this study, we found that the coverage had many of the same problems that we identified in this release, namely:

  • Use of active verbs in the headline to describe the effects of fiber on cancer (e.g. fiber “may cut,” “may help protect”) when this type of study can’t prove a cause-and-effect relationship.
  • Reporting only of misleadingly high relative risk reductions when the absolute difference is very small.
  • No discussion of alternative lifestyle approaches that may help reduce risk.

Better news coverage of health studies starts with better, more complete news releases that communicate study results accurately and with appropriate descriptive language.



Does the news release adequately discuss the costs of the intervention?

Not Applicable

It seems so simple. Add more fiber to your diet to improve health. But one thing many health groups forget to consider when making recommendations for changing one’s diet is the inconsistency in the cost and availability of healthful foods. We’ll rate the release Not Applicable here because most people have a general idea what high-fiber foods will cost. But we wish the release had provided at least a ballpark figure on what the weekly, monthly or annual cost would be to adding more fruit and vegetables to one’s diet. In addition, it needs to be acknowledged more often that there is a large segment of the U.S. population that does not have ready access to grocery stores that sell fresh fruits and vegetables.

Does the news release adequately quantify the benefits of the treatment/test/product/procedure?

Not Satisfactory

We recognize that it would be challenging for anyone writing a news release about this study to report on the absolute risks seen in the study. The study does not report absolute risks, although it is possible to calculate them based on the data provided in the tables. One could look at the number of cases in each quintile of dietary fiber (found in Table 2) and divide that by the number of people in each quintile (which is found in Table 1). There are several steps involved, but for the example of early adulthood dietary fiber and breast cancer risk in this cohort, you could do the following calculation:

– 613 cases in quintile 1 / 18,364 women in quintile 1 = 3.3% later developed breast cancer (pre or post menopause)

.– 567 cases in quintile 5/ 18,167 women in quintile 5 = 3.1% later developed breast cancer (pre or post menopause).

– The difference is 0.2 percentage point, which is quite a small absolute risk reduction.

When absolute risk numbers aren’t reported in a study, we think press officers should ask the researchers to provide these numbers so that they can be disseminated to the public. The issue of absolute risk reduction is key to helping readers understand the impact of changing dietary behaviors. We think it’s important enough to rate any news release Not Satisfactory when these numbers aren’t provided.

Does the news release adequately explain/quantify the harms of the intervention?

Not Applicable

No harms are mentioned but we wouldn’t expect any in a release touting the benefits of a high fiber diet.

Does the news release seem to grasp the quality of the evidence?

Not Satisfactory

The study relies on data collected from women who had to recall details about their food intake from when they were in high school. In addition, they were surveyed on their diets every four years. Many people may have a hard time recalling what they had the previous day, let alone what their diet was like years ago. Participants may also try to appease the surveyors with answers they think they want to hear. There’s a great potential for “recall bias” in these types of questionnaire-based studies.

The news release doesn’t mention this or other important study limitations. Nor does it touch upon the fact that this was an observational study incapable of proving cause and effect. So the headline for the release — “Higher dietary fiber intake in young women may reduce breast cancer risk” — is not accurate since it implies that fiber may have caused the observed reduction in risk. It would have been better for the release to state that fiber intake was “associated with” reduced risk.

Does the news release commit disease-mongering?


The release doesn’t disease monger about breast cancer.

Does the news release identify funding sources & disclose conflicts of interest?


The National Institutes of Health and The Breast Cancer Research Foundation are named as funders. An existing financial arrangement between a lead researcher and the Japan Pharmaceutical Manufacturers Association is also disclosed in the release. It’s not at all clear how this trade group was involved with the study or how its interests dovetail with a fiber-rich diet.

Does the news release compare the new approach with existing alternatives?

Not Satisfactory

The release doesn’t discuss other risk factors for breast cancer at all, much less other lifestyle choices that can reduce risk, such as maintaining a healthy weight, exercising, limiting alcohol consumption or getting enough sleep. Perspective is needed about ways we can stay healthy overall and reduce later cancer risk.

Does the news release establish the availability of the treatment/test/product/procedure?

Not Applicable

We’ll give this a Not Applicable since it’s commonly assumed that fresh dietary fiber is widely available in various products at many grocery stores.  We discussed problems with this assumption under the “costs” criteria.

Does the news release establish the true novelty of the approach?


The news release suggests that this is the first analysis of how adolescent diet affects future risk for developing breast cancer. It states that previous studies on dietary fiber and breast cancer have not looked at diet during adolescence or early adulthood. “This work on the role of nutrition in early life and breast cancer incidence suggests one of the very few potentially modifiable risk factors for premenopausal breast cancer,” according to lead author Maryam Farvid.

Does the news release include unjustifiable, sensational language, including in the quotes of researchers?


While the headline and some of the quotes from researchers suggest there’s a cause-effect relationship between high fiber and breast cancer, we’ve already commented on that. They may have alluded to a breakthrough, but did not explicitly say it. We’ll give them a pass.

Total Score: 4 of 7 Satisfactory


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