As a part of food, fibers can enhance satiety, improve texture, reduce calories, slow the entrance of sugar into the bloodstream, trap cholesterol, encourage the growth of friendly bacteria and reduce colon exposure to potential carcinogens. All of this makes the study of fibers and their relative effects on disease risk complex.
We tend to take from this complexity the simple message that the soluble fibers tend to lower cholesterol and slow the movement of food through the small intestine and insoluble fibers act on the colon. But there's more overlap than we commonly think of, and insoluble fiber can also be effective at altering blood lipids.
In a study titled, "Effects of bamboo shoot consumption on lipid profiles and bowel function in healthy young women" (Nutrition, July-August 2009), researchers looked at the effects of bamboo shoots (which are 92 percent insoluble fiber) compared to cellulose and a fiber-free diet. Bamboo shoots were found to significantly lower both total and LDL cholesterol without affecting HDL cholesterol. Cellulose was associated with a decrease in serum triglycerides.
Results of an animal study reported in Journal of Agricultural and Food Chemistry showed insoluble fiber fractions (IFF) in carrot pulp were effective at lowering blood triglycerides and blood cholesterol. The point of this study was to see how much the beneficial effects of IFF and dietary cellulose would be improved by reducing the particle size of both insoluble fibers. The application of micronization treatment effectively reduced the average particle sizes of the carrot IFF (from 308 to 9.3 μm) and the cellulose (from 101 to 20.9 μm).
The benefits did, indeed, increase. The micronized insoluble fibers lowered the concentrations of serum triglyceride, serum total cholesterol and liver lipids about 15-20 percent over the standard preparations of these fibers. The mechanisms included enhanced excretion of lipids, cholesterol and bile acids. These results demonstrated that particle size is an important factor when considering the physiological effects of insoluble fibers.
Soluble and insoluble fibers act differently, but may achieve similar results, depending upon the application. A 2010 article in the Journal of Agricultural and Food Chemistry suggested "all the fibers tested — cellulose, pectin, konjac glucomannan (KGM) and inulin — exert chemoprotective effects in the colon cells, which may prevent the occurrence of colorectal tumors."
The soluble fibers exert these beneficial effects mainly through fermentation and reduction in the secondary bile acid level, whereas cellulose effects are mainly through inhibiting an enzyme called β-glucuronidase and providing bulk, which reduced secondary bile acid level. (Note: Bile acids are made from cholesterol, so trapping them forces the liver to pull more cholesterol out of the blood to make more bile, which lowers blood cholesterol).
In a large prospective cohort study published last year, dietary fiber intake was significantly and inversely associated with the risk of total death and death from cardiovascular disease, infectious diseases and respiratory diseases in both men and women. In other words, the more fiber the less risk. Among specific sources of dietary fiber considered in this study, the fiber from grains showed "the most consistent inverse association with risk of total and cause-specific deaths." Whole grains contain a variety of dietary fibers, both soluble and insoluble, depending upon the grain.
"To gain the health benefits of fiber, both types — insoluble and soluble — need to be consumed," confirms Laura Cooper, marketing manager for SunOpta Ingredients Group (www.sunopta.com), Chelmsford, Mass. "For many consumers, 'fiber is fiber.' But some are beginning to understand that there are different types and one might help with cholesterol reduction and another with laxation.
"There is still some confusion among consumers that 'whole grain' also means high fiber and this is not necessarily true, due to the varying level of fiber in whole grains," she continues.
"Consumers need to read labels of whole-grain products to see if they are also a good source of fiber — at least 2.5g per serving."
SunOpta Ingredients provides two cellulose fibers which are highly concentrated sources of insoluble fiber. Cellulose Fiber 210 has low water absorption and provides fiber enrichment for bread, baked goods, and tortillas. Cellulose Fiber 910 has high water- and oil-holding capacity and is preferred in ground and processed meats for moisture management and yield improvement.
"Consumers expect grain products to be good sources of fiber," adds Cooper. "This presents an opportunity for food processors to enrich cereals, breads, waffles, tortillas, cereal/nutrition bars, crackers and grain-based snacks. If a serving of food provides only 1g of fiber, addition of a concentrated fiber like cellulose can allow a food marketer to make a 'good' or 'excellent' source claim on the package."