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Nutrition and Metabolism


What is fiber?

Dietary fiber refers to a group of macronutrients that cannot be digested by human body. The most common dietary fibers include cellulose, dextrins, inulin, lignin, chitins, pectins, waxes and non-digestible starches.  Most of them are carbohydrates. But unlike other carbohydrates, fibers cannot be digested by human body. So they provide no energy. However, dietary fibers provide many other health benefits.

There are two types of dietary fibers: insoluble fibers and soluble fibers. Insoluble fibers, found in whole-wheat bread, barley and brown rice, promotes bowl movement. So they are particularly beneficial to people struggling with constipation, hemorrhoids, irritable bowel syndrome etc. Soluble fibers, found in oatmeal, peas, beans, apples and strawberries, help lowering blood cholesterol and glucose levels. Therefore, soluble fibers are particularly beneficial to your cardiovascular system. In addition, both types of fiber make you feel satiated without adding calories to your diet, preventing you from gaining weight.

How is fiber beneficial to human body?

Fibers are not digestible by human body.  All the physiological impacts of fibers are through the interactions within the digestive system. Both insoluble and soluble fibers can increase the fecal volume and reduce the fecal transit time, promoting bowl movement. As a result, your body waste stays brief in your colon, leading to a better colon health. Insoluble fibers, although are not digested by human body, are degraded partially by beneficial microbial in the colon to produce lactate and short chain fatty acids, which inhibit the growth of harmful microbial, reducing the risks for many potential diseases such as irritable bowel syndrome and colorectal cancer. Soluble fibers interfere with lipid absorption, leading to improved serum parameters such as low cholesterol and low triglycerides, and ultimately a healthier heart.

How much fiber do I need in my diet?

The AI (Adequate Intake) for total fibers by the Institute of Medicine of the National Academies is age and gender dependent (Table 1).  For men and women between 19-50 years old, the AIs are set at 38 and 25 g/day respectively, roughly equivalent to the fibers in 3 and 2 cups of mature kidney beans. There is no Tolerable Upper Intake Level (UL) for fibers.

Table 1. AI for total fiber intakes by the Institute of Medicine of the National Academies.

Age Total fibers (g/day)
Male Female
1-3 years 19 19
4-8 years 25 25
9-13 years 31 26
14-18 years 38 26
19-50 years 38 25
51 years or older 30 21
Pregnant   28
Lactating   29

Should I take fiber supplement?

The best way to get sufficient fiber is from fiber rich foods such as whole food, beans and nuts, fruits and vegetables. Fiber fortified foods such as cereals are the next alternatives.  Fiber supplements are the shortcuts for sufficient intake.  But you need to take caution when taking supplements. Too much fiber supplements may potentially interfere with mineral and vitamin absorptions. If you have intestinal problems or take certain medicines, talk to your doctor before adding any fiber supplement to your diet.

Why dietary fiber is more important for people carrying certain gene variants?

The IL-10 gene is involved in many signal transduction pathways. A variant of the IL-10 gene, distributed in 40% of the general population, influence the risk for colorectal cancer by interacting with dietary fiber intake.  When eating less than 17 g/day of fiber, the colorectal cancer rate in the variant carriers is significantly higher than in the normal gene carriers. Therefore for people who carry this IL-10 gene variant, higher fiber intake reduces the risk for colorectal cancer. Another gene TCF7L2 is well known for the development of type 2 diabetes. A variant at a particular position of the gene called SNP rs7903146 is additionally associated BMI (body mass index). For the roughly 47% of CC alleles carriers, high fiber intakes (31.4 g/day or more) lead to a significantly greater weight loss. Therefore, for these people, high fiber intake is an effective way to prevent weight gain.

Which foods contain high level of fiber?

Bran (in whole foods), beans, nuts and seeds, fruit and vegetables are all natural fiber-rich sources (Table 2). Some fortified foods, for example certain brands of breakfast cereals, also contain very high levels of fibers. You need to check the Nutrition Facts on the labels to know the amount of fiber you will be getting per serving when you do your grocery shopping.

Table 2. Common fiber-rich foods.

Corn bran
Per 100 g (g) 79
Per Serving (g) 60
Serving Size 1 cup (76 g)
Fortified Cereals
Per 100 g (g) 50
Per Serving (g) 13
Serving Size 1 NLEA serving (26 g)
Dried Fruits
Peppers, hot chili, sun-dried
Per 100 g (g) 28.7
Per Serving (g) 10.6
Serving Size 1 cup (37 g)
Dried Fruits
Tomatoes, Sun-dried
Per 100 g (g) 12.3
Per Serving (g) 6.6
Serving Size 1 cup (54 g)
Figs, dried
Per 100 g (g) 9.8
Per Serving (g) 14.6
Serving Size 1 cup (149 g)
Apricots, dried
Per 100 g (g) 7.3
Per Serving (g) 9.5
Serving Size 1 cup (130 g)
Beans, Navy
Per 100 g (g) 10.5
Per Serving (g) 19.1
Serving Size 1 cup (182 g)
Beans, white
Per 100 g (g) 10.4
Per Serving (g) 18.6
Serving Size 1 cup (179 g)
Beans, French, mature seeds
Per 100 g (g) 9.4
Per Serving (g) 16.6
Serving Size 1 cup (177 g)
Beans, Soybeans
Per 100 g (g) 8.1
Per Serving (g) 13.9
Serving Size 1 cup (172 g)
Beans, Kidney
Per 100 g (g) 7.4
Per Serving (g) 13.1
Serving Size 1 cup (177 g)
Dark Chocolate
Chocolate, dark
Per 100 g (g) 11
Per Serving (g) 11
Serving Size 1 bar (101 g)
Nuts and Seeds
Almonds, dry roasted
Per 100 g (g) 12.2
Per Serving (g) 17.4
Serving Size 1 cup (142 g)
Seeds, Sunflower
Per 100 g (g) 11.1
Per Serving (g) 14.2
Serving Size 1 cup (128 g)
Pistachios, dry roasted
Per 100 g (g) 9.9
Per Serving (g) 12.2
Serving Size 1 cup (123 g)

How to get sufficient dietary fiber intake?

First you are recommended to use the GB Food Calorie and Nutrition Calculator to get the most accurate estimate of your current fiber intake.

To increase fiber intake, the best way is to eat fiber-rich foods as those shown in Table 2. Many fortified foods listed the fiber content in the Nutrition Facts.  Choose the ones with high fiber content while shopping.

For people who cannot get enough fiber from diet, fiber supplements are alternatives. However, you need to be very careful when shopping for dietary fiber supplements.  There are many different types of fiber supplements targeted for specific populations.  For example, inulin supplement is a natural soluble fiber made from chicory roots. It functions as a prebiotic to stimulate the growth of beneficial bacteria in the colon. As the result, it prevents the growth of harmful bacteria and reduces the risk for infections. In contrast, psyllium based supplements are more suitable for treating constipation.

Anyway, too much fiber supplements may cause gas, bloating, diarrhea, and may interfere with the absorption of certain nutrients or medicine. You should always consult your health providers before taking any dietary supplements.

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