Ryan Bradley, ND, MPH April, 2007
Many of us have heard that fiber is beneficial to our health, yet few know exactly how and why fiber is helpful. Unfortunately for us all, a relatively simple concept is mystified by advertisements announcing the miracle benefits of one fiber versus another. Even more unfortunately, very inexpensive fiber gets packaged, encapsulated and bottled for sale - making it very expensive to you. I find this marketing of expensive fiber unfortunate because it distracts you from focusing on the most important, and least expensive, sources of fiber - vegetables! I hope this article takes some of the mystery out of fiber for you and teaches you the importance of a high fiber diet in regulating blood sugar, blood cholesterol, and inflammation in the body.
Types of Dietary Fiber
Dietary fiber is generally considered indigestible to humans, and is broadly classified as water-soluble and water-insoluble fibers.
Water-insoluble fibers bind or attract water, becoming very viscous and add bulk to the stool. This bulking helps maintain normal bowel function by acting as a scouring agent in the bowel.
Water-soluble fibers actually dissolve in water and are further altered by the bacteria in our intestines.
How Fiber Works
All fibers can slow the absorption of sugar and fat from food, and therefore help prevent spikes in blood sugar and blood fat after eating, possibly reducing the inflammatory response to food. Fiber can also prevent the absorption of some fat and cholesterol all together, lowering blood triglyceride and cholesterol levels.
Calling dietary fiber indigestible is not entirely accurate. Although we do not produce the needed enzymes to digest many of the fibers we eat in our diets, many of the bacteria that live in our intestines (They belong there!), are able to break down, or ferment, fibers to the benefit of the bacteria and to us!
The bacteria that live in our intestines have important health functions, including helping to maintain normal nutrient absorption, immune function, and bowel function - so it is important to live with them and help them live to help us!
Fiber provides important nutrition for the bacteria to live and prosper, and so they are called pre-biotics. An example of a pre-biotic fiber is fructo-oligosaccharides (FOS) (say that fast ten times!), also known as inulin. A few examples of inulin-containing foods are: legumes, jicama, onions, and Jerusalem artichokes.
Fiber is further important in normal detoxification functions in the body. Our bodies are constantly breaking down (metabolizing) excess hormones, medications, synthetic food ingredients and environmental toxins. Much of this detoxification occurs in the liver (a large organ under the right-hand side of the rib cage). When the liver detoxifies these substances, the end products are frequently eliminated in the bile, a liquid substance made in our liver and secreted via the gall bladder into out intestinal tract. When we eat a high fiber diet, the fiber from our meals binds these toxins and allows us to eliminate these waste products. Without a lot of fiber in the diet, these toxins can be reabsorbed, go back to our liver and need to be processed again. Requiring the liver to reprocess these toxins requires more energy and may result in higher levels of these toxins in the bloodstream (the liver can only do so much at one time!).
How Much Fiber Should I Eat?
The United States Department of Agriculture (USDA) recommends adults eat 14 grams of dietary fiber per 1000 calories eaten in the diet . For must of us, this translates into 21-28 grams of fiber per day.
However higher fiber diets may have additional benefits for those people with diabetes, including reducing blood sugar, lowering insulin and lowering cholesterol. Chandalia et al. studied 50-grams per day of dietary fiber in 13 people with type 2 diabetes and recorded reduction in total cholesterol, triglycerides, pre- and post-meal glucose levels, and measures of insulin sensitivity  compared to a 24-gram fiber per day diet.
Some sources warn of gastrointestinal side effects (gas, bloating, cramping) with fiber intakes this high, however this has not been my experience except in people with sensitivities to specific types of fiber, e.g. psyllium or gluten-containing sources.
My typical recommendation to patients is 35-40 grams of fiber per day ideally achieved through the diet alone, with additional fiber intake (usually as a fiber supplement) for weight loss or to selectively target reduction in post-meal blood sugars.
Many people need to increase their water intake when they increase their fiber intake to avoid constipation because of the water-binding/bulking effects of water-insoluble fibers. Fiber, in combination with fish oil, has extra benefits on triglycerides and total cholesterol.
Food Sources of Fiber
Vegetables, whole grains, nuts and legumes (beans and peas) remain the single best sources of fiber in the diet - and not coincidentally - these same foods are recommended as the foundation for a healthy diet for people with diabetes.
High-fiber vegetables include many of the green leafy vegetables like kale, collard greens, chard, arugula, and even lettuces.
Whole-grain sources of fiber include quinoa, barley, oats and rye.
Legumes include beans like peas, soy, black, pinto and lentils.
Quick sources of supplemental fiber that I recommend include: ground flax seed (freshly ground to preserve the oils present in the seeds), store-bought psyllium supplements (without the added colors or artificial flavorings that are frequently added), chopped nuts, and/or oat bran. All of these fibers can be sprinkled over salads, mixed in protein-shakes or water, or added to yogurt, salads and vegetable medleys.
I recommend the World’s Healthiest Foods website http://www.whfoods.com/ for more detailed fiber content information.
Is There A Best Source ofSupplemental Fiber?
It is really difficult for me to make a recommendation about a “best” source of fiber for diabetes. Too few studies have been done and the studies have had different doses, types of fiber, duration of treatment and different outcomes being measured. Psyllium, oat bran, glucomannan (Konjac), corn bran, peas, and agar have all been studied in people with type 2 diabetes and these studies all demonstrated substantial reductions in blood glucose, hemoglobin A1c, triglycerides, LDL cholesterol and/or weight in study participants [4-11]. Wheat fiber has also been studied, but did not result in improvements in blood glucose or cholesterol in people with diabetes, though this was a very small and short study .
My recommendation remains to focus on dietary sources of fiber, add supplemental fiber when impossible to achieve through diet alone, and never buy fiber sold in a capsule. My experience also suggests some people cannot tolerate psyllium-based fiber supplements; gas, bloating, cramping and constipation are the signs of intolerance. Also, it is important to determine wheat/gluten sensitivity before choosing to supplement with oat, wheat, rye or barley bran as a fiber source.
Dietary fiber is important to human health and nutrition because of its role in regulating glucose and cholesterol absorption, detoxification/elimination, and promoting healthy bowel function by serving as a pre-biotic. Vegetables, nuts, whole grains and legumes are the best dietary sources of fiber - and are nutrient dense foods high in micronutrients, protein and antioxidants. Although some companies claim to have the “best” fiber (often sold in bottles), few studies have made thorough comparisons of one fiber versus another, and most studies of different fiber types show positive results in improvement of cardiovascular risk factors in diabetes.
The bottom line is: Fiber really is good for you. Eat a lot of it and often.
Ryan Bradley, ND, MPH
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13. King, D.E., et al., Effect of a High-Fiber Diet vs a Fiber-Supplemented Diet on C-Reactive Protein Level. Arch Intern Med, 2007. 167(5): p. 502-6.