What is Dietary Fiber
“Dietary fiber”, as it is commonly called, is a broad term which actually encompasses a broad group of substances from plant foods. All cereal grains, fruits and vegetables contain some dietary fiber. And some are excellent sources while others supply only negligible quantities. In foods made from cereals, fiber value depends to a large degree on how much has been stripped away in the refining processes.
Loosely defined, fiber can be described as the cell wall material from plants. The definition is “loose” because it also includes complex substances associated with the plant cell walls and indigestible cellulose rich material – again largely derived from cell walls. Among them are lignins, gums, pentosans, pectins, hemicelluloses, celluloses, alginates and others.
Cellulose is a major constituent of plant cell walls. It is a complex carbohydrate that humans cannot digest. We lack the enzyme essential to breaking down cellulose, which is quite logically called cellulase. Cellulose it is the unique material that allows plants – from tulips to maple trees – to stand either supplely (the tulip) or stiffly (the maple tree) upright. It is the major constituent of the tough outer hull of most grains. Wheat bran is a prime example. But…
Fiber is More Than Bran
Listening to marketers of breakfast cereals, you may think all that’s needed to compensate for the loss of fiber in our modern, refined, low fiber diet is to add bran to many of the foods you eat. But to make that assumption would be naïve. Though the addition of small amounts of the outer layers of cereal grains might help a few disorders of the colon (e.g. constipation and diverticular disease), there are many more benefits to be gained from a fiber-rich diet that includes the other fibers too.
Consider cellulose. There are thousands of varieties of cellulose. Some are the tough, fibrous kinds we associate with bran. In laboratory evaluation of fiber in food, celluloses and their tough “woody” cousins, the lignins, are most easily detected and labeled as “crude fiber”. Crude fiber is only the portion of total fiber that resists dissolution by the acids and alkalis in laboratory tests meant to mimic human digestion. But acids and alkalis will dissolve out the hemicelluloses, pentosans, and pectins. Crude fiber then is not an accurate measure of total dietary fiber. Thus the ranking of foods by their crude fiber content alone is insufficient for determining the total value of dietary fiber in any food.
What Fiber Does
Dietary fiber has comprised a significant portion our ancestors’ and our diets throughout the more than 500 million years of evolution that brought us to where we are today. Prior to the advent of agriculture 10,000 to 23,000 years ago, Paleolithic diets included 3.2 pounds of plant based foods. Those foods were fibrous roots, leaves, stems, seeds and flowers, all with very little starch but lots and lots of fiber. Our Paleolithic ancestors would ingest roughly 150 g of fiber each day. That was a good thing. It swept their gastrointestinal tracts clean, helping move through the 2 pounds of wild meats that were also consumed each day, leaving the intestines and colon prepared to absorb the next round of nutrients that would come passing through. The bottom line is that the human digestive tract is designed to accommodate, and we have been eating, a diet rich in the full variety of dietary fibers for most of human existence. It is only fitting that we continue – as modern health records seem to indicate. The 15 to 30 grams of fiber recommended today just won’t deliver sufficient health benefits
Coronary heart disease was considered a rarity at the beginning of the 20th century. Appendicitis became more common only after 1880. Diverticular disease, hemorrhoids, hiatal hernia and colonic tumors grew to major health problems from the middle of last century to today. It is during this period of time that we experienced the development and pervasive growth of modern food processing. Technology focused on our food supply in an effort to make it more palatable, salable, transportable, convenient and hygienic.
Although it would be incorrect to assign the increased prevalence of particular diseases solely to the loss of fiber in our diets, the disorders mentioned are peculiarly common to all economically developed societies. Yet they remain unknown or rare in rural Africa and among the few remaining hunter-gatherer societies. Cultures where full-fiber whole foods are consumed in place of the processed and refined foods we have come to associate with progress suffer far less of the “diseases of affluence,” including cancer, cardiovascular disease, diabetes, arthritis, and the intestinal disorders mentioned above.
Though our technology is advanced, our biochemistry is not. It is still Paleolithic. Evolution moves far more slowly than the human mind. The biochemistry then that runs us today is virtually the same as that which powered our predecessors as they cowered from the cold under mammoth skin robes in their caves. And our ancient biochemistry has the same requirements for celluloses, hemicelluloses, gums, pentosans, lignins and pectins.
Consider for instance the role of only one of the pectins, high-methoxy pectin. This pectin from-citrus fruits and others has the unique ability to moderate fluctuations in insulin secretions. If abnormal variations in insulin levels can be controlled, then precipitous decreases and increases in blood sugar can be avoided. The appearance of a hypoglycemia epidemic in the latter quarter of the 20th century and the inexorable rise in diabetes, which began in the 1950s and has taken off like a rocket since the 1980s, could be tied at least partially to the loss of dietary fiber from our diets.
The Diet Connection
Control of insulin is not just a factor in overcoming diabetes. It is also important to cardiovascular health. High insulin levels can drive cholesterol and other lipids into arterial tissue. We know that high-methoxy pectins, alginates, gums and assorted polysaccharides slow the absorption of carbohydrates. When carbohydrate is rapidly broken down into sugar and absorbed, the pancreas responds by pouring a large amount of insulin into the blood to remove the excess blood sugar. Insulin directs muscle cells and liver cells to first synthesize glycogen from excess blood sugar. Glycogen is then held in reserve to be released during fasting periods or intense physical exertion.
Insulin may also direct the liver to synthesize triglycerides from the excess sugar, raising blood fat level. Most of those triglycerides will later be stored within fat cells of adipose tissue, but some are likely to infiltrate cardiovascular tissue, building fatty lesions within arterial walls and perhaps even in myocardial (heart muscle) tissue.
Fiber assists in the control of carbohydrate metabolism by setting up barriers. Carbohydrate (as starch) is always formed within the walls of plant cells. Human digestion can break down starch, but lacks the cellulose enzyme required to demolish plant cell walls. It is largely the resistance of plant cell walls to human digestion that delays the conversion of carbohydrate to sugar.
Refined, fiber-poor carbohydrates that we encounter in potatoes and other starchy vegetables, baked goods and confections are quickly converted to simple sugars. Digestive enzymes present in the microvilli making up the “brush border” of the intestinal lining are capable of splitting starches and more complex sugars into simple sugar at the moment they come into contact with the brush border. The villi and their enzymes are very efficient at guiding the conversion. Soluble fibers, pectins, gums, and alginates for example, temporarily bind to macro and micro nutrients including starches and sugars to slow their absorption to normal rates leading to a slower, more regulated, and hence, more healthful, release of insulin.
Water soluble fibers in oat bran are now believed to inhibit the absorption of cholesterol. Though the agents in oat bran have not been accurately identified, they have in barley. The sub-aleurone layers in barley, the outer layers of the grain immediately below the bran, contain a vitamin E analog d-alpha tocotrienol and the fatty acid 1,3-dilinoleoyl-2-gamma-linolenioylglycerol. These components of barley fiber have demonstrated cholesterol lowering ability in animals. Recent studies continue to expand on the efficacy of tocotrienols in the control of cardiovascular disease and atherosclerotic plaque.
Pectins are known to bind toxic heavy metals in the gut, keeping them from being absorbed. Eating whole foods rich in pectins, which usually means eating fruits, can cut down on the amount of pro-oxidant, pro-aging heavy metals that could find their way into delicate tissues and organs. Cadmium, for instance, is released into the air from the burning of fossil fuels and as automobile tires wear down during use. Inhaled cadmium can find its way to the kidneys where it may act to raise blood pressure.
Lead is known to impair immune response, and, especially in the young, can diminish intelligence. Whole foods rich in pectins could enables the body to more easily cope with the modern pollutants of today’s world. Gums are known to inhibit the absorption of excess calories, allowing the absorption of nutrients at a rate more easily managed by our metabolism. For this reason, guar gum has proved to be a valuable dietary adjunct in weight loss. It normalizes calorie intake, and can prevent unneeded blood sugar surges that could otherwise be converted to storage fat.
Some other fibers provide simple bulk. Cellulose is a moderate absorber of water in the bowel. Hemicelluloses are better. Alginates from seaweed are phenomenal absorbers. Lignins, though poor “sponges” may bind bile salts, and carry them out in the stool. Thus, they would work against the resorption of the cholesterol fraction of bile and the resorption of carcinogenic breakdown products from decomposing bile.
Arabinogalactan complex fibers from Larch trees are known to both stimulate the immune response and, in larger molecular form, act as pre-biotics, feeding friendly probiotic bacteria. Fructo oligosaccharides, fermentable fibers extracted from the roots of several plants likewise feed probiotics, enhancing their abilities to destroy cholesterol in the gastrointestinal tract, synthesize vitamin K, liberate nutrients from food, and kill off intestinal and colonic pathogens.
We are still learning about the varieties of fiber that exist in foods. Not all have been identified or isolated, and new roles are being identified for many that have been identified. But the early work of Dr. Benjamin Erschoff beginning in the 1950s and stretching through the 1970s and expanded upon others has at last demonstrated the critical importance of multiple forms of dietary fiber to the achievement of optimal health.
FIBER CONTENT OF COMMON FOODS
Following is a list of the top 20 sources of dietary fiber followed by a list of specific foods.
1.) Legumes – beans and peas including baked beans, kidney beans, split peas, garbanzo, pinto and black beans.
2.) Lima beans – fordhook and baby limas.
3.) Bran cereals.
4.) Peas – fresh and frozen
5.) Dried fruit – figs, apricots and dates are best.
6.) Raspberries, blackberries and strawberries.
7.) Sweet corn
8.) Whole grain cereals and cereal products.
10.) Baked potato with skin (NOT THE FLESH)
11.) Green snap beans, pole beans and broad beans.
12.) Plums, pears and apples – all high in pectin.
13.) Raisins and prunes
14.) Green leafy vegetables – spinach, beet greens, kale, collards, swiss chard and turnip greens.
15.) Nuts – especially almonds, Brazil nuts, peanuts and walnuts.
20.) Brussels sprouts.
©Mark Timon, M.S. Clinical Nutrition 2014