This short essay examines the evidence for diseases in which lack of fiber in the diet is implicated and what these imply for health education.
Both the National Academy of Sciences(NAS) Institute of Medicine and the British Nutrition Foundation have issued guidelines recommending from 12 to 18 grams intake of fiber daily for the average healthy adult. In fact, the American Dietetic Association enjoins a higher level of 20 to 35 grams a day while the NAS/ISM (2002) has gone so far as to recommend an “Adequate Intake” level of no less than 38 grams daily for Americans up to 50 years of age. What is the rationale for these recommendations?
The mechanical action and readily-established benefit of dietary fiber are well-known. Fiber adds bulk to food intake and hence, should make one feel full faster. The soluble types bind with bile acids and, by hindering absorption, help reduce cholesterol levels; as well, this type seems to hinder sugar absorption and thus helps regulate blood glucose levels better. These are obviously important benefits for diabetics, those watching their weight and striving to control cardiovascular disease.
With respect to diabetes, conventional wisdom about switching to whole-wheat bread or taking bran supplements alone is refuted by a meta-analysis that reviewed the evidence from nearly two dozen studies. Instead, the studies spotlighted the effectiveness of a large intake of a variety of fiber sources. All the studies were controlled clinical trials, certainly more rigorous for establishing cause and effect than the simple association or odds ratios other researchers are so fond of using in retrospective or prospective studies. Specifically, increased consumption of fiber from natural foods or supplements containing the viscous type reportedly prevented glycemic levels from spiking after meals and generally stabilized the serum lipid test results of diabetics (Anderson, Randles, Kendall and Jenkins, 2004).
For cardiovascular disease, there seems to be a protective effect against coronary heart disease and myocardial infarction (MI). However, the limitation of prospective studies reared its head once again. Is it the cholesterol-lowering action of dietary fiber that is responsible? Could it be the beneficial impact on systolic and diastolic blood pressure? Or does fiber work on C-reactive protein, a recognized marker for MI? Should nursing practice emphasize just the viscous type and supplements only or accept the evidence of epidemiological studies that increased consumption of whole grains, legumes, fruits, and non-starchy vegetables is at least as effective? Dare we say “cut back on fries, friend chicken and soda”?
As to other well-publicized claims about the health benefits of dietary fiber, the evidence is decidedly mixed or inconclusive (i.e. based on the relatively weak validity of prospective cohort studies) for breast cancer, weight loss, and colorectal cancer or adenomas (polyps). In fact, a three-year intervention employing psyllium as fiber source found an increase in the recurrence of colorectal polyps. After all, reliance on odds ratios and “higher risk” alone is on the same plane of scientific rigor as saying that being an African makes one more vulnerable to AIDS or that residing in San Francisco raises the odds that one is homosexual.
Diabetes and CHD are two of the biggest scourges today and account for a substantive fraction of health care costs. However, the threat of the White House’ “public option” seems to have receded for now. It is likely more productive to attack the predisposing factors and dietary behaviors for these two diseases. Our limited resources for health education and social marketing are more properly allocated to convincing Americans to cut back on Big Macs, steaks and sugary drinks. As the population ages, they belatedly gravitate towards healthy choices anyhow, e.g. salad bars. At the end of the day, evidence-based nursing practice learns to take all these considerations into account when helping with meal selections for confined patients and counseling those who are receptive to dietary recommendations on release to home or community care.
References
Anderson, J.W., Randles, K.M., Kendall, C.W., & Jenkins, D.J. (2004). Carbohydrate and fiber recommendations for individuals with diabetes: A quantitative assessment and meta-analysis of the evidence. J Am Coll Nutr., 23(1):5-17.
Institute of Medicine. Dietary, functional, and total fiber. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids. Washington, D. C.: National Academies Press; 2002:265-334.