You Decide: A Little Gas or Premature Death?
Abstract & Commentary
By Barbara A. Phillips, MD, MSPH, Professor of Medicine, University of Kentucky; Director, Sleep Disorders Center, Samaritan Hospital, Lexington. Dr. Phillips is a consultant for Cephalon, and serves on the speakers bureaus for Resmed and Respironics.This article originally appeared in the February 28, 2011 issue of Internal Medicine Alert. At that time, it was peer reviewed by Gerald Roberts, MD, Assistant Clinical Professor of Medicine, Albert Einstein College of Medicine, New York, NY; he reports no financial relationship to this field of study.
Synopsis: Dietary fiber intake appears to lower all-cause mortality, including death from cardiovascular, infectious, and respiratory diseases.
Source: Park Y, et al. Dietary Fiber Intake and Mortality in the NIH-AARP Diet and Health Study. Arch Intern Med 2011 Feb 14; doi:10.1001/archinternmed.2011.18.
This analysis was based on questionaires from 219,123 men and 168,999 women who participated in the NIH-AARP Diet and Health Study. This observational prospective cohort was made up of AARP members who were 50-71 years old at the time of the first questionnaire mailings in 1995 and 1996. At baseline, diet was assessed with a self-administered 124-item food-frequency questionnaire. Participants reported their average frequency (on a scale of never to > 6 times a day for beverages and a scale of never to > 2 times a day for solid foods) and portion size (on a scale of 1 to 3) over the previous year. The investigators used these questionnaires to calculate dietary fiber for each participant using the Association of Official Analytical Chemist method.1 The questionnaires also collected demographic, anthropometric, and lifestyle information, including history of smoking, physical activity, family history of cancers, menopausal hormone therapy use in women, and some medical conditions at baseline. Data on subsequent deaths were collected using standard techniques.
Energy-adjusted dietary fiber intake ranged from 13 g/day (10th percentile) to 29 g/day (90th percentile) in men and from 11 g/day to 26 g/day in women. The major sources of dietary fiber were grains, fruits, vegetables, and beans. People who ate more fiber were healthier in general; they were more likely to have higher education, to have self-rated their health as being very good/excellent, to have a lower BMI, and to be physically active, but were less likely to smoke, to drink alcohol, and to consume red meat. Women in the higher fiber intake categories were more likely to use menopausal hormone therapy.
Over about 9 years of follow-up, 20,126 men and 11,330 women in the cohort died. Dietary fiber intake was significantly inversely associated with the risk of overall mortality in both men and women. Both men and women in the highest quintile had a 22% lower risk of total death compared with the lowest quintile. Each increase in daily fiber intake of 10 g/day reduced the multivariate relative risk for total death by about 12% for men and 15% for women. This strong inverse relationship remained statistically significant even after adjusting for lifestyle factors including smoking, BMI, self-rated health, menopausal status, and age.
Dietary fiber intake was also specifically inversely related to risk of death from specific causes, including cardiovascular disease (CVD), cancer, infections, and respiratory disease. Comparing the highest with the lowest quintile of dietary fiber intake, men had a 24%-56% lower risk of death from CVD, cancer, and infectious and respiratory disease. Women in the highest quintile of fiber intake had a 34%-59% lower risk of death from CVD and infectious and respiratory disease, but no association with cancer death.
These relationships between fiber intake and death persisted after controlling for confounders including aspirin use, high blood pressure, and cholesterol. However, when the analyses were restricted to never smokers, the reduction in the relative risk of death remained significant only for deaths from cancer and infectious disease in men. Among the women who had never smoked, dietary intake remained statistically associated with reduced risk of death from cardiovascular disease and infectious disease.
Investigation of the relationship between different sources of fiber and risk of death was also undertaken. The relationship was strongest for fiber from grains, which correlated significantly inversely to the relative risk of total death and death due to CVD, cancer, and respiratory disease in both men and women. Fiber from vegetables and beans was also weakly associated with a lower risk of total death in both men and women, but fiber from fruits was not.
Commentary
Dietary fiber is the part of food that resists digestion and absorption in the small intestine and is then fermented in the large intestine.2 Dietary fiber has long been believed to reduce the risk of coronary heart disease, diabetes, some cancers, obesity, and premature death because it reduces transit time of feces through the bowel, increases excretion of fecal carcinogens, lowers serum cholesterol, reduces glucose absorption, lowers blood pressure, promotes weight loss, inhibits lipid peroxidation, and is anti-inflammatory.3,4 Truly, what is a little gas compared with all these benefits? Unfortunately, empiric evidence that dietary fiber affects mortality is limited and inconsistent. In the Scottish Heart Health study, dietary fiber intake was inversely related to total mortality in men, but not in women.5 The Zutphen Study (men only) reported a 9% lowered risk of total death per 10 g/d of dietary fiber intake.6 The National Health and Nutrition Examination Survey I Epidemiologic Follow-up Study found no association between dietary fiber intake and total mortality.7 Studies of dietary fiber in relation to cause-specific death other than from CVD are sparse. The current study adds quite a lot to our understanding of the likely benefits of dietary fiber. These investigators found that fiber intake was significantly inversely associated with the risk of total death and death from CVD, infectious diseases, and respiratory diseases in both men and women. Dietary fiber intake also was related to a lower risk of death from cancer in men but not in women. And it appears fiber from grains (as compared with vegetables, beans, and fruits) showed the most consistent inverse association with risk of total and cause-specific deaths.
It's important to note that this report is not the result of a randomized, controlled trial. And, since fiber-eaters in this study were generally healthier than those who ate less fiber, it's possible (even likely) that their better outcomes resulted from some healthy lifestyle trait that was not measured and reported here. On the other hand, fiber consumption, unlike taking a prescription medication or undergoing a surgical procedure, is unlikely to be associated with adverse outcomes if these findings are erroneous.
The Mayo Clinic provides a chart that includes fiber content of common foods (see Table on page 47). Remember, 10 g/day was enough to make a difference.8
References
1. Prosky L, et al. Determination of total dietary fiber in foods and food products: collaborative study. J Assoc Off Anal Chem 1985;68:677-679.
2. Dietary Fiber Definition Committee. Report of the Dietary Fiber Definition Committee to the Board of Directors of the American Association of Cereal Chemists: The Definition of Dietary Fiber. St Paul, MN: AACC International; March 2001:112-126.
3. Spiller GA, ed. CRC Handbook of Dietary Fiber in Human Nutrition. 3rd ed. Boca Raton, FL; CRC Press; 2001.
4. Anderson JW, et al. Health benefits of dietary fiber. Nutr Rev 2009;67:188-205.
5. Todd S, et al. Dietary antioxidant vitamins and fiber in the etiology of cardiovascular disease and all-causes mortality: Results from the Scottish Heart Health Study. Am J Epidemiol 1999;150:1073-1080.
6. Streppel MT, et al. Dietary fiber intake in relation to coronary heart disease and all-cause mortality over 40 y. Am J Clin Nutr 2008;88:1119-1125.
7. Bazzano LA, et al. National Health and Nutrition Examination Survey I Epidemiologic Follow-up Study. Dietary fiber intake and reduced risk of coronary heart disease in US men and women: the National Health and Nutrition Examination Survey I Epidemiologic Follow-up Study. Arch Intern Med 2003;163:1897-1904.
8. The Mayo Clinic. High Fiber Foods. Available at: www.mayoclinic.com/health/high-fiber-foods/NU00582. Accessed Feb. 17, 2011.
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