Fiber is one of the most popular nutrients that intrigues scientists, yet we learn that there is continuously more to know. We know it’s good for us, but what is it? Where can we find it? Does it help with fat loss? What else is there? Here we try to answer these and other important questions.
What is Fiber?
Unfortunately, there is not a universal definition of fiber that is accepted throughout the world. Why? Because so many types of fiber behave differently under various conditions that scientists cannot agree on what is a definition that would cover all of the physiological and scientific behaviors of fiber.
The “accepted” definition of dietary fiber is “the plant polysaccharides (i.e. long-chain sugars) and lignin which are resistant to hydrolysis by digestive enzymes of man.” In layman’s terms: undigested long-chain sugars and lignin from plant cell walls. The problems with this definition are:
• It fails to include the types of fiber that may reach the large intestine.
• It uses the ability to be digested as the basis for the definition when undigested food reaching the colon does not necessarily lack the ability to be digested, nor is it necessarily unavailable to the body.
• Fiber actually can be broken down, and used as an energy source for the body.
We will see later how this definition does not cover those types of fiber that reach the large intestine intact, but are broken down into byproducts made available by the local normal flora (i.e. the bacteria normally found there).
Viscous and Fermentable Fiber
In the US, fiber is typically categorized as “soluble” and “insoluble” fiber, but even these terms are now starting to fade away. Soluble fiber were once characterized to be viscous in the stomach or small intestine, attenuate blood glucose after eating, and lower LDL cholesterol (the “bad” cholesterol). Some still do, but not all soluble fibers are viscous.
Insoluble fiber was once thought to be those that do not dissolve in water, contribute to fecal bulk and improve laxation. However, not all insoluble fibers improve laxation.
So the terms “soluble” and “insoluble” are being phased out as more physiologically meaningful definitions such as “viscous” and “fermentable” are preferred. Below is a list of some of the many types of fiber found in plants and on ingredient labels:
- guar gum
- xanthan gum
The effects of the above types of fiber vary based on the types and concentrations of ingested fiber in each meal. Significant characteristics of dietary fiber that affect its physiological and metabolic roles include:
- water solubility
- hydration or water-holding capacity and viscosity (thickness of fluid)
- adsorptive attraction or ability to bind organic and inorganic molecules
- degradability or fermentability by intestinal bacteria
As such, we should get our fiber from various sources to take advantage of viscous and fermentable fiber.
Soluble fiber (mostly viscous) can be found in oatmeal, oatbran, nuts and seeds, legumes, apples, oranges, pears, strawberries, and blueberries.
Fermentable fiber can be found in whole grains, fruits, beans and veggies.
Physiological Effects of Fiber
When you hear fiber and the body, you have to think about the GI tract. The effects of fiber on the body and GI tract are largely dependent on the type and concentration of fiber eaten.
Many, not all, studies have shown viscous fibers in the stomach can delay the rate of emptying ingested foods from the stomach into the duodenum. The discrepancies in the studies could be as a result of the other food components and the amount or type of fiber consumed.
In the small intestine, the viscous, gel-like material produced by fiber has been shown to:
• Inhibit enzyme activity associated with fat, protein and carbohydrate digestion. This results in reduced intestinal absorption of these macronutrients as well as cholesterol.
• Reduce the glycemic index of a meal
• May reduce the risk and treat upper small intestine ulcers (specifically fruit or veggie fiber, not cereal fiber).
The effects of fiber on the large intestine (colon) are very dependent on the type of fiber’s fermentability, which depends on the chemical behavior of the fiber and the concentration of natural bacteria in the colon. Highly fermented fibers are oat bran, pectin, psyllium and guar gum, whereas cellulose and wheat bran may be poorly fermented. In general, fruits and veggies (rich in hemicelluloses and pectins) contain more fermentable fiber than do cereals (rich in celluloses). Fiber also may attract water while traveling through the colon. The greater the attraction of water into the colon (e.g. wheat bran), the greater the laxative effect. As fermentable fiber passes through the colon, they produce large amounts of short-chain fatty acids (SCFAs) (including butyrate, acetate and propionate) which is the primary energy source of the colon and hypothesized to be protective against colon cancer.
Fiber and Weight Loss
Viscous fiber has been reported to reduce hunger. This has partially been attributed to the delayed emptying of food from the stomach, causing an extended feeling of fullness. However, this is dependent on the amount of fiber consumed.
It has been repeatedly shown adding fiber to a weight-reducing diet results in greater weight loss when calories remain the same between a control and experimental group. One interesting study demonstrating this used a resistant starch. Resistant starch (RS) is another term used to describe any starch that is not digested in the small intestine but passes to the colon. During this study, the researchers discovered that following eating a meal consisting of 5.4% RS, stored fat, not meal fat, was the predominant source of fat utilized for energy, contributing approximately 80% of the total fat metabolized. The researchers suggested the increase use of endogenous fat may be due to an increase in circulating SCFA’s produced by the bacterial metabolism of RS reaching the colon.
Therefore, for those trying to lose weight, fiber may help to burn more stored body fat and help individuals achieve successful fat loss.
Fiber and Its Many Preventive Abilities
Metabolic Syndrome and Diabetes
Researchers at the US government science agency Agriculture Research Service say the results from a study of over 2000 people suggest at least three or more servings of wholegrain foods each day could reduce chances of developing “metabolic syndrome.” Metabolic syndrome is a condition marked by a combination of abdominal obesity, high blood pressure, poor blood sugar control, low HDL “good” cholesterol and high blood fats. Collectively, these increase the risk of developing type-2 diabetes and heart disease.
The ability of fiber to be beneficial for type 2 diabetics comes from the list of how viscous fiber behaves as previously mentioned: decreases meal glycemic index, delayed gastric emptying, and slower rate of digestion and absorption. Included in this list is an increase in insulin sensitivity. The cells of diabetics have difficulty identifying insulin. Bodily cells that are more insulin sensitive respond to insulin better than cells that do not; this is called insulin resistance. It is believed that people who ingest whole-grain foods and cereal fiber have a lower type 2 diabetes risk. However, it is most effective for diabetics to totally eliminate starch.
Blood and Heart
Large epidemiologic studies showed a protective effect of dietary fiber against coronary heart disease (CHD). The mechanism by which fibers may protect against CHD include lowering blood cholesterol, triglycerides (i.e. fatty acids in the blood), blood pressure, homocysteine and normalizing after-meal blood glucose levels.
What is nice here is that both viscous and fermentable fibers have been recorded to protect the heart and blood – one is not necessarily better than the other.
It was long thought that fiber was protective against colon cancer. However, three long-term studies of hundreds of people looking at polyp recurrence concluded that dietary fiber is not protective against colon cancer. Why? The following reasons have been proposed and suggest further research:
• Polyp recurrence may not be a good surrogate marker for colon cancer
• Insufficient fiber intake
• Wrong type of fiber used in the studies
• Confounding dietary factors may have been present
Fiber may be protective against breast cancer. The mechanism that has received the most attention is through decreasing serum estrogen concentrations. Estrogen is excreted via the GI tract, but may be reabsorbed in its broken form (i.e. unconjugated form). Fiber can bind to its unconjugated form, therefore preventing its reabsorption.
Finally, because of fiber’s ability to regulate blood estrogen concentration, some researchers have hypothesized a protective effect against hormone-related cancers, such as endometrial, ovarian and prostate cancer. Thus far the research is young. The research is questionable for endometrial cancer, potentially favorable for ovarian cancer, but there is no significant differences involving the risk of prostate cancer and dietary fiber.
The current adequate intake suggests that adults consume 25-38 grams (or 14 g/1000 calories) of dietary fiber per day, not necessarily one type over another. Men are at the upper end of this recommendation versus women.
The American Diabetes Association recommends diabetics should get 25-50 grams of dietary fiber (15–25 g/1000 kcal) per day.
Children over age 2 should consume an amount equal to or greater than their age plus 5 grams per day. The average American eats only 14-15 grams of dietary fiber a day.
Start making changes slowly, adding an additional 3 grams more per week until achieving the adequate intake. Initially, you may experience an increase in flatulence, laxation or feeling bloated. However, as the gastrointestinal tract begins to adapt to this overload of fiber, your body will become comfortable and adapt.
For those that may think they need fiber supplements, look for those that contain psyllium, a viscous fiber. This is the most comfortable form of supplementary fiber available. Others do exist. However, understand that many dietary fiber supplements require high doses in order to get a significant amount. It can be annoying taking capsules after capsule. Read the labels and know what type of fiber you are ingesting.
Eat more fiber-dense vegetables, such as broccoli, celery and asparagus. However, people with digestive disorders may still want to take a fiber supplment. On the good side for these people, there is no evidence that long-term use of fiber supplements, such as Metamucil, Konsyl or Citrucel, is harmful.
Many people take fiber supplements for constipation. Be sure to drink plenty of water or other fluids every day if you take fiber supplements. Fiber tends to absorb water. If your fluid intake is low and you increase fiber in your diet, stool can become hard — making constipation worse.
Various sources of fiber are an important part of a healthy diet. Some times, for those of us struggling with dieting, it is the missing link needed to reach our goals.
Get at least the 25-38 grams of dietary fiber per day for adults. For children over age 2, the recommended intake is the child’s age + 5 grams. The best sources are fresh fruits and vegetables, nuts and legumes, and whole-grain foods.
Understand there is a limit, and in some cases too much fiber can create poor absorption of certain minerals, such as calcium, iron, magnesium or zinc. However, this does not happen in the presence of adequate mineral and water intake.
Some tips for increasing fiber intake:
• Eat whole fruits instead of drinking fruit juices.
• Replace white rice, bread, and pasta with brown rice and whole-grain products.
• Choose whole-grain cereals for breakfast. Fiber One has the most on the market. Put in yogurt, smoothies, cottage cheese or on salads.
• Snack on raw vegetables instead of chips, crackers, or chocolate bars.
• Substitute legumes for meat two to three times per week in chili and soups.
• Eat the skins of vegetables, such as potatoes, cucumbers, grapefruit, etc.
• Wisely research and consult with a physician any fiber supplements before taking them.