A recent study published in the prestigious journal “The Lancet” brought to my mind the interesting subject of dietary fiber and started me on a quest to discover our present state of understanding of the health benefits of this rather unglamorous food component. Fiber has been mentioned many times before in this blog but has never been the focus of an article. It’s about time to shine a well-deserved light on this important carbohydrate.
First of all, let’s define what fiber is. Fiber encompasses all the forms of carbohydrate in foods that are not digestible or only very minimally digestible by human digestive enzymes in our digestive tract. Fiber is found ONLY in plant-based foods and these include fruits and vegetables, whole grains, beans and other legumes, nuts and seeds. Animal-based foods such as meat and dairy products contain absolutely NO fiber at all. Different types of fiber are often categorized into two groups based on their solubility. First is insoluble fiber, plant parts such as cellulose and lignins that do not dissolve or even change shape appreciably as they travel through the human digestive system. Benefits of insoluble fiber include faster passage of food through the digestive system, increased stool bulk with more regular bowel movements and prevention of constipation. Soluble fiber, on the other hand, dissolves in water to become a gel-like substance. Examples of soluble fibers are pectin, beta-glucan, inulin, gums and mucilage. Soluble fiber is associated with decreased blood sugar levels, lowered blood cholesterol and feeding the beneficial microorganisms living in our digestive tract. Both soluble and insoluble fibers are linked to decreased incidence of many chronic diseases such as cardiovascular disease, diabetes and some cancers (1,2).
The soluble/insoluble classification works well for isolated fibers but we eat whole foods in which different types of fibers exist together in a structured package, changing the way they perform in real life. Recently it has been suggested that fiber is better categorized by its viscosity or its thickness when mixed with water. Generally insoluble fiber is non-viscous fiber and soluble fiber is viscous but some insoluble fibers do fit into the viscous category. Viscous fiber becomes gel-like when mixed with water while non-viscous fiber can absorb some water but does not thicken considerably. Viscous fibers are the ones preferred as food by our friendly gut bacteria (2,29).
THE STUDY: CARBOHYDRATE QUALITY AND HUMAN HEALTH: A SERIES OF SYSTEMATIC REVIEWS AND META-ANALYSES
The new study on fiber mentioned at the start of this article was completed in New Zealand and published in The Lancet on February 2, 2019 (3). It was a breakthrough of sorts in that it used fiber as a marker for high-quality carbohydrate and then examined the association of healthy carbohydrates with health outcomes and risks. Numerous studies have pitted high-carbohydrate diets against high-fat and/or high-protein diets. Regrettably, the actual content and quality of the carbohydrate food studied is often completely ignored, despite the vast quality differences that exist among foods of the carbohydrate group. Often the high carbohydrate comparison diet is very high in calories and fats and low in nutritional value so that the high protein/fat diet it is being compared to comes out looking okay, a case of comparing one poor diet to another. Even with such a handicap, the abundant health benefits of high-carbohydrate diets are well known and backed by solid evidence. However this new study was able to come to better defined conclusions because it identified high-quality carbohydrates using the presence of fiber as a measure of this quality.
The study is a review and meta-analysis of previously completed prospective studies and randomized, controlled trials which had identified carbohydrates of high quality and reported their effects on the incidence of death, risks of chronic disease and the presence of risk factors for diseases. The investigators did not include trials that focused on weight loss or supplement use or those that reported on participants suffering from a chronic disease at the start of the study. Within these parameters 185 prospective studies and 58 clinical trials were chosen and analyzed. They included 4635 adult participants and almost 135-million-person-years of data.
Here are the study findings (3):
Observational data suggests a 15 to 30% decrease in…
– early death from all-causes
– cardiovascular related death
– incidence of coronary heart disease
– incidence of stroke and stroke related death
– incidence of type-2 diabetes
– incidence of colorectal cancer
…in participants eating the most dietary fiber when compared with those eating the least.
Clinical trials showed…
– significantly lower body weight
– lower systolic blood pressure
– lower total cholesterol
…with higher fiber intake when compared with low fiber intake.
Reductions in the risks of experiencing a range of critical diseases was greatest when daily intake of fiber was between 25gm and 29 gm.
Data suggested that increasing fiber intake further would confer even greater benefit against cardiovascular diseases, type-2 diabetes and colorectal and breast cancer.
Whole grain intake showed the same findings as for other quality carbohydrates studied.
The researchers conclude that their study’s striking dose-response evidence suggests that too low an intake of dietary fiber, including that from whole grains, could be a major cause of the development of many chronic disease conditions (3).
EVIDENCE FROM OTHER STUDIES
Of course, this information is not new, but it firmly strengthens what we already know. Many studies have found that higher intake of dietary fiber and whole grains are associated with lower rates of cardiovascular disease, diabetes and metabolic syndrome, cancer and mortality from all causes (4,5,6,7,8,9,10,11).
Epidemiological studies have shown that whole-grain intake specifically is protective against cancer, cardiovascular disease, diabetes and obesity. This effect is likely related to the richness of whole grains in a wide spectrum of nutrients with known health benefits – fiber; resistant starch; oligosaccharides; antioxidants such as phenolic compounds and trace minerals; phytate; phyto-estrogens such as lignan, plant stanols and sterols; vitamins; and minerals (12).
Contributing to these multiple healthy outcomes are the “good bacteria” in our gut whose preferred food is fiber. Daily activities of these microbes not only inhibit the growth of yeast and harmful bacteria; beneficially affect the metabolism of fat and sugar; and enhance mineral absorption; they also can disable carcinogens and are associated with decreases in risks of many chronic diseases (29). Their fermentation of viscous fiber is “prebiotic”, stimulating the growth of more of these friendly microbes with their multiple health-boosting effects (13,14). What’s more, the large variety of dietary fibers found in a diet consisting of whole plant foods supports a more diverse gastrointestinal microbial community than does a diet high in refined foods which is associated with much diminished species variety (15). A species-rich gut ecosystem remains stable against environmental influences so that diversity is a good indicator of the health of the gut microbiome (32).
In addition, fiber can help with weight control. Increasing fiber intake triggers the stretch receptors in the stomach before too many excess calories have been consumed resulting in reduced intake of calories yet still experiencing feelings of being “full” and satisfied (16).
A 2015 meta-analysis quantified the benefits of fiber intake as a 10% reduction in risk of early mortality from all causes for each 10 gm increase in the intake of fiber (17). Other studies report that each 10 gm of additional fiber intake is associated with a 14% decreased risk of coronary events and a 27% decrease in coronary death (30).
ARE WE EATING ENOUGH FIBER?
Though the prolific health advantages of eating fiber are well known, this information has not translated into increased fiber intake by people around the world. Recommendations for total fiber intake in Europe are 25 to 32 gm/day for adult women and 30 to 35 gm/day for men with slightly lower levels for children and elderly people. A recent review in Europe noted that average actual intake of fiber is much less than the recommended level and that few European countries provide any guidance on increasing fiber amounts or how to choose fiber-rich foods (18,19). In Canada, guidelines state that women need 25 grams of fiber daily and men need 38 gm. Most Canadians eat only about half that much (20). In the US, recommendations are for 25 to 30 grams of fiber daily for adults with that fiber being derived from food, not supplements. It is estimated that currently only about 5% of Americans achieve these goals. Additionally, US residents appear to understand the merits of fiber and believe that they are eating enough, despite evidence to the contrary. This situation is now a “public health concern” in the US (1,21).
HOW CAN WE INCREASE OUR FIBER INTAKE?
Both insoluble fiber and soluble fiber have health benefits and so we want to include them both in our daily diets. This is not difficult to do since fiber is plentiful in all whole foods derived from plants. In fact both types of fiber are present at some level in all plant foods.
The reason most people do not get enough fiber is simply that they do not eat enough unprocessed plant foods. This is exacerbated by eating too many animal-based foods, lessening the opportunity to ingest foods containing fiber.
SOLUBLE FIBER SOURCES:
The following are some foods that contain soluble fiber (22,23);
Grains: Oats, oatmeal, barley, brown rice, whole-wheat pasta, whole-wheat bread, rye bread, psyllium
Legumes: Lentils, dried beans, chickpeas, edamame (soybean)
Vegetables: Broccoli, Brussels sprouts, asparagus, peas, carrots, white potato, sweet potato, turnip, beets, collard greens
Fruits: Oranges, nectarines, grapefruits, apples, pears, peaches, plums, apricots, banana, avocado, mango, passion fruit, guava, eggplant, prunes, dried figs
Nuts and Seeds: Flax seeds, sunflower seeds, hazelnuts
INSOLUBLE FIBER SOURCES:
Insoluble fiber is found in all plant-based foods – whole grains, beans and legumes, vegetables, fruits, nuts and seeds. However foods highest in insoluble fiber include the following (24,25):
Whole grains such as whole wheat, bulgur wheat, buckwheat, rye, barley, rolled oats, brown rice; bran from wheat, corn and rice; quinoa
Fruits with edible peels such as apples, peaches, pears, plums, apricots; berries such as raspberries, strawberries, blackberries, blueberries
All vegetables but especially vegetables with skins; leafy green vegetables; cauliflower, green beans, peas, potatoes
Beans and legumes
Nuts and seeds
TIPS FROM “DIETICIANS OF CANADA” FOR INCREASING FIBER INTAKE (25):
Eat at least six servings of whole grain products every day.
Choose whole-grain grain products for breads, pitas, bagels and pasta.
Buy bread products that list whole grains as their first ingredient
Replace at least half of the white flour in recipes with whole-wheat flour. If you can replace all of it, that’s even better.
Add bran, chia seeds or ground flax seed to your morning cereal.
Add toasted nuts or pumpkin seeds to salads.
Eat at least seven servings of vegetables and fruit every day.
Eat the peels of your vegetables and fruits.
Eat raw vegetables and fruit instead of drinking fruit juice.
Add barley, peas, beans or lentils to your soups and stews
Use legume-based dips such as hummus or black bean dips for raw veggie snacks.
MINIMIZING THE EFFECTS OF GASTROINTESTINAL GAS (29)
Human beings pass gas an average of twelve to twenty-five times a day. There are two major causes for this gas, bacterial fermentation and, the one which is often forgotten, swallowing air.
You can limit opportunities to swallow air by eating slowly with the mouth closed, chewing food well and avoiding carbonated beverages.
To decrease gaseous components in beans;
Use fresh beans instead of dried ones.
Use up your dried beans within a few months. The longer they are stored, the more gas-producing they become.
Soak beans for twelve hours or overnight then discard the soaking water and rinse well before cooking the beans. This can be done twice for greater effect. If time is limited, boil the beans briefly, let sit in the water for an hour or so then discard the soaking water, rinse and cook.
Rinse canned beans well.
Remove white foam from the surface of your pot of boiling beans.
Make sure you cook your beans thoroughly until they are very tender.
ONE CAVEAT: MAKE SLOW CHANGES (15,24,26,27)
If you are increasing the amount of fiber in your diet, do it gradually. Remember that fiber is not digestible by humans. The fiber you eat reaches your large intestine undigested where the tiny microbes in your gut break it down through the process of fermentation. Our gut microorganisms can extract energy and nutrients from the fiber from which they produce the short-chain fatty acids such as butyrate, propionate and acetate that have been linked to decreased inflammation, improved immune function, decreased risk of obesity and reduced incidence of many chronic diseases.
If you suddenly send large amounts of fiber through your digestive system, your fiber-loving microbes will be very happy to do their best to ferment this fiber. The hitch is that the by-products of fermentation are gases such as methane, carbon dioxide and hydrogen. Sudden increases of these gases can result in bloating, excess gas, cramping and constipation. It is best to make small increases in fiber over a period of a few weeks so that your system has time to adapt to the changes. Your microbes will adjust as well. The population of beneficial microorganisms will rise as their favourite supply of food increases. Other species of microbes will flourish as well in a process called cross-feeding whereby the products of fermentation provide good growth opportunities for other desirable species. Remember that a gut microbial community made up of more distinct species is a healthier one.
OTHER HELPFUL TIPS (26,27)
Be sure to Increase the amount of water you drink as you ramp up your fiber intake. This will not only prevent constipation but will also provide extra water to replace what is absorbed by the soluble fiber as it passes through the intestines.
Exercise of any kind will promote movement of food through the digestive tract.
Spread your fiber intake throughout the day by eating fiber-rich foods at each meal. This makes it an easier task for your microbes to handle.
Once your body becomes accustomed to the extra fiber you will be able to eat all you want without experiencing any uncomfortable symptoms.
There doesn’t seem to be an upper limit for fiber intake. After all, our distant ancestors evolved eating many times more fiber than most people do in modern life with estimates of paleolithic DAILY fiber intake at between 70 and 150 grams (31). Even in this day and age there exist populations in rural South Africa that habitually eat more than 66 gm of carbohydrate every day and also have a rate of colon cancer that is sixteen times lower than that of African Americans who have moved to the US where a standard Western diet is the norm. Migrant studies of Japanese Hawaiians have demonstrated that it takes only one generation for an immigrant population to assume the colon cancer incidence of the citizens of their new country (28). Incidentally, the incidence of cardiovascular disease in some traditional African people, though once very low, is also increasing. Researchers attribute this change to the infiltration of Western lifestyle habits, especially the significant lowering of fiber intake and the slow adoption of a more sedentary lifestyle, both of which push aside their simpler and healthier traditional ways of living (33).
THE GOOD NEWS
The bottom line here is that you don’t have to worry about finding ways to get enough fiber in your diet. Fiber is plentiful in all plant-based foods. If the majority of the food you eat is in the form of whole plants you’ll end up eating much more than the minimum recommended daily amounts of fiber and in turn reap the numerous benefits that fiber provides.
1 Quagliani, D., Felt-Gunderson, P. Closing America’s Fiber Intake Gap: Communication Strategies From a Food and Fiber Summit. Am J Lifestyle Med. 2017 Jan-Feb; 11(1): 80–85.
3 – Reynolds, A., Mann, J., Cummings, J., Winter, N., Mete, E., Te Morenga, L. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019 Feb 2;393(10170):434-445.
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