Fiber Up! Dietary Fiber and Mortality Risk

We hear quite a bit in the lay press about the potential benefits of dietary fiber including eating multiple daily servings whole grains,  fruits and vegetables.  I tend to be skeptical of many of these recommendations.  But after following some of the research related to the Mediterranean Diet,  I at least have my antenna (and my Google Reader) up on this issue.  A previous post on basics of the Mediterranean diet is here and a review of a study linking the diet to slower cognitive decline in older adults is here.

My Google Reader recently retrieved an online first article from the Archives of Internal Medicine by Dr. Park and colleagues from the National Cancer Institute entitled "Dietary Fiber Intake and Mortality in the NIH-AARP Diet and Health Study". 

This is a longitudinal correlational study of the association between self-reported dietary fiber intake and cause of death from the National Death Index Plus registry.  I wasn't aware of this study that includes cooperation with the American Association of Retired Persons or AARP.  AARP collaborated with the National Institute of Health on a survey completed by 567, 169 AARP members in 1995 and 1996.

The survey collected detailed personal dietary habit information and then scanned the National Death Index for deaths in those who had completed the survey.   The daily dietary fiber intake was calculated for each participant and split into quintiles (five groups each comprised of 20% of the sample) with those with the highest fiber intake (top 20%) compared to those with the lowest fiber intake (lowest 20%).  Rates of death for these two groups were examined.  Since people who tend to eat more nutritious diets tend to also have other healthy lifestyle behaviors, a variety of potential confounding variables were controlled, i.e. smoking status, age, race, education, marital status, baseline health status, alcohol consumption, etc.

During the follow up period, here is the estimated reduced death rates associated with being in the highest fiber intake groups (compared to lowest) with women in bold first followed (men):

• Death from cardiovascular disease: 34% lower (24% lower)
• Death from cancer: 4% (not statistically significant) lower, (17% lower)
• Death from respiratory disease: 46% lower, (41% lower)
• Death from infectious disease: 69% lower, (56% lower)

Risk of death from accidents did not differ by dietary fiber ranking.  However, the study numbers are pretty impressive.  Although there is pretty good data supporting dietary fiber and cardiovascular disease risk, there is limited study in other causes of death.  This study supports association between dietary fiber and lower cancer deaths in men, but not women.  The authors point out that many of the types of male-predominant cancers (i.e. head and neck, esophageal, liver, bladder and kidney) may be those where dietary fiber plays a role.  Another potential interpretation is that breast cancer is not influenced by dietary fiber.

I was surprised by the finding of reduced death in the respiratory and infectious disease categories.  There were many fewer deaths in these categories so the results need to be interpreted cautiously.  However, there is at least one study (Iowas Women's Health Study) showing dietary fiber reduced death from inflammatory disorders.  The authors point out something I didn't know, that high dietary fiber is associated with reduced levels of endogenous (the body's own) inflammation markers including lower C-reactive protein, interleukin 6 and tumor necrosis factor alpha receptor.  Inflammation may play a role in deaths due to asthma and other lung inflammatory conditions.

This study provides some additional intriguing data.  The authors broke down the source of dietary fiber into whole grains, vegetables, fruits and bean sources.  What they found was that the dietary fiber from grains (but not fiber from vegetables, fruits or beans) contributed the largest effect of the association of high fiber intake with reduced mortality.  And this is where the figure above comes in.  The figure shows that in a whole wheat kernel, the wheat bran and wheat germ pack the most fiber, minerals and chemicals called phytochemicals.  The authors propose that high dietary grain fiber is likely to include high intake of whole grain components such as wheat bran and wheat germ.

Again, these findings are only correlational.  They do not prove a cause and effect.  There may be some other factor contributing to this reduced mortality in the high fiber group.  This study does not prove putting people with a low fiber diet on a high fiber diet will reduce their risk of death during a period of follow up.  But my skepticism is fading so please excuse me now as I'm heading to the store for some bran cereal and a box of rolled oats.

Disclosure: I had not reached the minimum age for AARP membership in 1995 and did not participate in this survey.  I am now an AARP member primarily for motel/hotel discounts for AARP members.  I received no money from AARP, the American Bean Council or other source for writing this post. 

The figure in this post comes from Wikipedia Commons file authored by jkwchui.

Park, Y., Subar, A., Hollenbeck, A., & Schatzkin, A. (2011). Dietary Fiber Intake and Mortality in the NIH-AARP Diet and Health Study Archives of Internal Medicine DOI: 10.1001/archinternmed.2011.18