All About Fibre
What is fibre?
Plant cellulose (yellow), magnified
Dietary fibre is a non-digestible polysaccharide, which means it’s a complex form of carbohydrate (poly = “many”; saccharide = “sugar”). (See All About Carbohydrates for more on the various types of saccharides.)
These polysaccharides give plants their structure — think plant cell walls.
Types of dietary fibre
We can divide fibre into two general categories, based on their structure and what they do in our bodies.
- Soluble fibres are viscous and fermentable, and can lower our blood cholesterol.
- Insoluble fibres help to bulk up stool volume and improve motility.
We need both types of fibre in our diets.
However, within these general groups, there are many types of dietary fibre, including:
| Fibre type | Found in |
| Beta-glucans | Baker’s yeast, some mushrooms, some grains, seaweed |
| Cellulose / hemicellulose | Plant cell walls, especially plants with a rigid structure (e.g. trees) |
| Chitin | Fungi, exoskeletons (e.g. crab shells) |
| Chitosan | Produced as a chitin derivative |
| Fructans | Many vegetables and grains, such as chicory, Jerusalem artichoke, barley, and the Allium group (onions, leeks, garlic, etc.) |
| Gums | Seaweeds, barley bran, some tree saps and seeds |
| Lignins | Plant cell walls, especially xylem (nutrient-transporting) cells |
| Non-digestible dextrins | Plant starches |
| Non-digestible oligosaccharides (the prebiotic fibres) like inulin, fructo- and galacto-oligosaccharides | For inulin and fructo-oligosaccharide, see fructans. Galacto-oligosaccharides are derived from lactose in milk. |
| Pectin | Fruits such as apples, apricots, quince, guava, and citrus. Citrus peels are a very high source of pectin (30% of weight). |
| Polydextrose | Synthesized from dextrose (combined with citric acid and sorbitol), used as a starch replacer in commercial food products |
| Resistant starches | Seeds, legumes, whole grains, potato, corn, green bananas (especially if these foods are cooked then cooled) |
As you can guess from the table above, grains, vegetables, legumes, fruits and nuts account for 85% of the fibre in the U.S. food supply. These same plant foods provide lots of other nutrients.
However, since we live in a world of refined and fortified foods, there are now “functional dietary fibres.” These are the isolated, non-digestible carbohydrates that fortify foods not usually containing fibre — stuff like “fibre-ed up” Splenda and Apple Jacks. This allows many unhealthy products to claim that they are “healthy”.
| Fibre found in nature | |
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| Fibre NOT found in nature | |
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Why fibre is so important
In 1972, scientists proposed a “dietary fibre hypothesis”, which suggested that common GI diseases like colon cancer, diverticulosis and appendicitis were due, in part, to lack of dietary fibre. It’s not hard to see why:
- Americans consume about 15 grams of fibre per day, on average (17.8 g for males and 13.6 g for females). I eat that much before 8:00 am.
- Refined sugars, oils, dairy products and alcohol contain no fibre and comprise 48% of the energy in the average U.S. diet.
- When asked about their dietary fibre consumption, 73% of individuals with a fibre intake below 20 grams/day think the amount of fibre they ingest is “about right.” The Institute of Medicine recommends 19 to 38 grams of fibre per day (varies based on age and gender).
- White flour and white potatoes provide the most fibre to the American diet. This isn’t because these foods contain lots of fibre, it’s because we eat lots of these foods. Legumes only provide about 6% of the fibre in the U.S. diet.
In other words, people eating a standard Western diet aren’t getting anywhere near the fibre that they should.
A low-fibre diet is associated with many health problems, including:
- Cardiovascular diseases and high blood fats – fibre helps bind and eliminate blood cholesterol/fat
- GI disorders, cancers and poor bowel function – fibre helps keep the GI tract clean and can ease constipation and diverticular disease
- Diabetes – fibre controls blood sugar, insulin and body fat
- Excess body fat – fibre contributes to satiety and dilutes energy density
- High blood pressure – see all the above
A study with over 500,000 people in 10 European countries showed that people who ate more than 30 grams of fibre per day had approximately half the risk of colon cancer as those who ate 12-15 grams of fibre daily.
Fibre in common foods
Fibre and cardiovascular disease
In one study, researchers followed more than 16,000 middle-aged men from around the world for 25 years. The higher the consumption of beans, the lower the risk of death from heart disease — up to 82% reduction in risk! Dietary fibre from whole grains seems to also have a strong protective effect against cardiovascular diseases.
Fibre and blood lipids
In randomized controlled trials, compared to baseline values, vegetarian diets with additional fibre, soy, and nuts were associated with a 20% decrease in total cholesterol and a 35% decrease in LDL cholesterol.
Fibre and GI function
Fibre can stimulate bacterial fermentation (prebiotic fibre is fuel for colon cells) in the large intestine or pass through the GI tract unfermented. Since GI health is synonymous with immune function, fibre seems to play an important role in immunity. See All About Probiotics for more.
Without adequate stool bulk, the colon will panic and respond with excessive contractions. This leads to the creation of diverticula, which are herniations of the mucosal layer through weak colon muscles (essentially little inflamed intestinal pouches). Consuming sufficient fibre can prevent this.
Fibre and diabetes
Consuming more fibre, especially from legumes, seems to help prevent type 2 diabetes. This is likely due to the moderation of blood sugar and insulin release. (See All About Insulin for more.) Oh – and fibre can help control body fat (see next section).
Fibre and body fat
Fibre thinks body fat is a joke. Fibre increases dietary bulk, decreases energy density and reduces energy intake. This is helpful for the 75% of Americans who are overfat. (On the other hand, because fibre increases satiety, eating lots of fibre isn’t helpful for those trying to maintain or increase body weight. We talk about this in All About Raw Food. Also see All About Appetite Regulation.)
More fibre means more fecal energy losses, since GI transit time speeds up. Rapid GI transit leads to less time for digestion and absorption of nutrients. This can be a problem if mineral intake is lacking. Up to 32 grams of fibre each day doesn’t seem to diminish mineral balance in adults. Same thing is noticed with up to 25 grams in kids.
Resistant starches aren’t degraded in the small intestine; they act as a fibre source in the large intestine. Legumes are the big boys of resistant starch. Nearly 35% of legume starch escapes digestion because it’s resistant. American youth and adults are estimated to consume approximately 3 to 8 grams of resistant starch per day.
What you should know
Eat more plants
Plant foods contain fibre. The best sources are legumes, whole grains, vegetables, fruits and nuts/seeds. (See All About Fruits and Vegetables.)
Plant-based diets tend to be higher in fibre – with vegan diets being the highest. People eating these types of diets tend to have lower rates of cardiovascular disease, cancer, lower blood fat, lower blood pressure, and lower body weight. This could be in part due to a higher fibre intake, or because they’re biking all over the city to protest.
Remember that whole grains are whole grains, as in the entire thing — not whole grain flour, whole grain pasta, whole grain “nutrition bars”, or Fruit Loops with whole grains. (See All About Grains and The Safe Carbs: Whole Grains.)
Kids need fibre too
Adequate fibre during youth can reduce the risk of cardiovascular disease, cancer and diabetes. Fibre goals for kids:
- All children 1 to 3 years, 19 g/day
- All children 4 to 8 years, 25 g/day
- Boys 9 to 13 years, 31 g/day
- Girls 9 to 13 years, 26 g/day
The adequate intake (AI) of fibre for children and adolescents is based on the data cited for adults, which showed that 14 grams per 1,000 calories reduced the risk of heart disease. The 2005 U.S. Dietary Guidelines recommend a fibre intake of 14 grams per 1,000 calories as well.
However, too much fibre in kids can reduce overall food intake (due to its satiating effect), increase fecal energy loss, and decrease mineral absorption. In children consuming a standard Western diet, this is rarely a problem, though.
Also, grain and legume intolerance (most commonly to wheat, corn, soy, and peanuts) is common in children, so parents should look for other sources such as vegetables, fruit, other seeds/nuts, and other beans/legumes if necessary. (May we suggest a kid-friendly, fruit-filled Super Shake? You can sneak all kinds of things in there!)
Downsides of fibre
Lots of fibre can mean less absorption of nutrients and calories. In the U.S., this might actually be a positive thing.
Bezoars have been reported in the esophagus from glucomannan and the large intestine from other dense fibre foods.
Fermentation of fibre and other non-digestible carbohydrates by bacteria in the colon can produce hydrogen, methane and carbon dioxide gases. This may lead to distention and expired gas.
Many people are intolerant of some forms of fibre, such as grains. There is evidence that the lectins in grains and beans/legumes can cause health problems for susceptible people, because they affect the lining of the gut. (See All About Lectins for more.) However, there’s also evidence that prebiotic fibre sources such as inulin and glucomannan can help with inflammatory bowel disorders. If you have inflammatory bowel symptoms, review your fibre sources.
Summary and recommendations
Getting enough fibre is important for overall health and disease prevention. And it keeps your plumbing in good working order.
If you build your diet around vegetables, fruits, legumes, whole grains, nuts and seeds, you’re probably getting enough fibre.
If you want a goal:
- Women should aim for at least 30 to 35 grams of fibre per day.
- Men should aim for at least 40 to 45 grams of fibre per day.
You should get your fibre from whole foods. Fibre supplements (or fibre-supplemented foods) don’t provide the micronutrients, phytochemicals and water found in whole plant foods. Anyone who relies on a fibre supplement because they don’t get enough from food has a crappy diet (excluding those with unique medical situations).
When you eat enough fibre, you need to consume enough fluids. See All About Dehydration for more.
Extra credit
Eating one cup of beans = about 16 grams of fibre.
The fibre of apples and pears can be recovered from the juicing process and used as a food ingredient.
In those consuming adequate fibre, symptoms associated with hemorrhoids can substantially decrease.
Inulin, oligosaccharides, resistant starch, and other fibres have actually been found to enhance mineral absorption, particularly calcium.
There is a 1% increase in fecal energy loss for every 6 grams of dietary fibre added (check yourself disordered eaters – avoid over-consuming fibre supplements in the hopes of losing weight. You’ll likely end up with a bezoar and/or gut hemorrhage). This might explain why plant-based eaters tend to be lean.
3 additional grams of dietary fibre from oats can decrease blood cholesterol by 2%.
Further resources
Dietary Fibber: Don’t be fooled by polydextrose and other fiber additives
References
De Natale C, et al. Effects of a plant-based high carbohydrate/high fibre diet versus high monounsaturated fat/low carbohydrate diet on postprandial lipids in type 2 diabetic patients. Diabetes Care 2009;32:2168-2173.
Marcason W. What is the “Age + 5” rule for fibre? 2005;105:301-302.
Hiza H, et al. USDA fibre Fact Sheet. December 2007.
Position of the American Dietetic Association: Health Implications of Dietary fibre. J Am Diet Assoc 2008;108:1716-1731.
Williams CL. Important of dietary fibre in childhood. J Am Diet Assoc 1995;95:1140-1146,1149.
Craig WJ. Health effects of vegan diets. Am J Clin Nutr 2009;89:1627S-1633S.
Hu FB. Diet and lifestyle influences on risk of coronary heart disease. Curr Athero Rep 2009;11:257-263.
Ruottinen S, et al. Dietary fibre does not displace energy but is associated with decreased serum cholesterol concentrations in healthy children. Am J Clin Nutr 2010;91:651-661.
Anderson JW, et al. Health benefits of dietary fibre. Nutr Rev 2009;67:188-205.
Ferdowsian HR & Barnard ND. Effects of plant-based diets on plasma lipids. Am J Cardio 2009:104:947-956.
Mounsey AL & Henry SL. Which treatments work best for hemorrhoids? The Journal Of Family Practice 2009;58:492-493.
Robson AA. Preventing diet induced disease: Bioavailable nutrient-rich, low-energy-dense diets. Nutrition and Health 2009;20:135-166.
Weisberger L & Jamieson B. How can you help prevent a recurrence of diverticulitis? 2009;58:381-382.