What is Fiber?

Fiber was first referred to by Hipsley in 1953 as "nondigestible constituents that make up the plant cell wall."¹ Although our understanding of fiber has increased over the past five decades, there is still no universally accepted definition for fiber. In 2002, the Institute of Medicine (IOM) published a new set of proposed definitions for fiber suggesting that:

"Dietary Fiber would describe the nondigestible carbohydrates and lignin that are intrinsic and intact in plants, whereas functional fiber consists of the isolated nondigestible carbohydrates that have beneficial physiological effects in human beings. Total fiber would then be the sum of dietary fiber and functional fiber."^{2, 3}

Dietary fiber is categorized into two different groups: soluble (the fiber is soluble in water) and insoluble(the fiber is not soluble in water).

Soluble Fiber

Often times soluble fiber forms a gel-like substance when it combines with water in the digestive tract and is therefore referred to as "viscous fiber." Some soluble fibers are also "fermentable" in the large intestine, creating byproducts which include carbon, hydrogen, methane, and short chain fatty acids. These fermentable fibers include resistant starches, inulin, oligosaccharides, and guar gums. These types of soluble fiber are very important to a normally functioning body and provide several widely accepted health benefits.

Health Benefits of Soluble Fiber

• Up to a 40% reduced risk of coronary heart disease⁴ due to decreased LDL and total cholesterol levels
  • Soluble fiber decreases the amount of bile salts that are reabsorbed in the intestines. In order for the liver to produce more bile salts, it must pull LDL cholesterol from the blood stream, thus decreasing the amount of LDL, and therefore total cholesterol levels.
• Improved glycemic control among people with diabetes
  • Soluble fiber mixes with water in the stomach and turns into a viscous gel, which causes gastric emptying to be delayed. This delayed gastric emptying leads to delayed digestion of the carbohydrate, causing a smaller, slower rise in blood glucose rather than the fast peak often seen after a person with diabetes consumes carbohydrates.
• Prebiotics are "nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, especially Lactobacilli and Bifidobacteria."⁵ In other words, prebiotics provide food for the healthy bacteria (probiotics) in the gut. As dietary fiber, prebiotics have positive effects on stool production excretion. Consumption of prebiotics such as fructooligosaccharides has been shown to significantly increase fecal biomass and promote regularity.
• Other potential benefits of fermentable soluble fiber include the following:
  • Increased mineral absorption-especially calcium and magnesium. Although the exact mechanism is not fully understood, one proposed mechanism suggests that short chain fatty acids, produced as fermentable soluble fibers are metabolized, lower the pH of the intestine, allowing for increased mineral absorption. ⁶ However, there is little evidence at this time to show that this increased calcium and magnesium absorption has a positive effect on bone health over the long term.
  • Improved immune function. Preliminary studies show that fermentable soluble fibers may enhance the gut mucosa, decreasing the risk of bacterial colonization and translocation.⁷ However, more research is needed to determine whether this improved immune function reduces risk of disease.
  • Appetite regulation. As fermentable soluble fiber mixes with water in the stomach and forms a viscous gel, gastric emptying is delayed, possibly leading to increased satiety. Introductory data suggests that fermentable soluble fiber may also affect appetite-regulating hormones grehlin and glucagon-like-peptide-1.⁵

Food Sources of Soluble Fiber

Soluble fiber is found in many fruits, vegetables, and whole grains. Some of the most common food sources of viscous soluble fiber include:

• Oatmeal
• Legumes such as beans and peas
• Carrots
• Apples
• Pears
• Nuts
• Barley
• Flax seed
• Oranges
• Psyllium husk-often found in whole grain breakfast cereals

Fermentable soluble fiber (resistant starches, inulin, oligosaccharides, and guar gums) is also found naturally in many foods commonly eaten. Some of these foods include:

• Navy beans
• Raw banana
• Lentils
• Whole grain bread
• Garlic
• Onion
• Agave
• Chicory root
• Asparagus
• Tomatoes

Fibers that function as prebiotics, such as inulin, are a common fiber source being added many food products including:

• Snacks and bars
• Ice cream
• Jellies and jams
• White bread
• Yogurt

Insoluble Fiber

Health Benefits of Insoluble Fiber

While insoluble fiber does not form a viscous gel, nor is it fermentable, it plays a crucial role in the functioning of digestive health. Insoluble fiber adds weight and volume to fecal matter, decreasing transit time and improving regularity. This property of insoluble fiber helps to prevent and manage constipation. There is some evidence to show that consuming insoluble fiber decreases risk of developing diverticulosis (the presence of pockets in the intestine which can cause gastrointestinal distress and inflammation). The exact mechanism of this decreased risk is unknown. Some theories suggest that this protection against diverticulosis stems from the added bulk which insoluble fiber provides to fecal matter. The increased bulk and decreased transit time leads to decreased force in muscle contractions along the large intestine.²

Food Sources of Insoluble Fiber

Insoluble fiber is present in many foods--commonly the cell walls of plants:

• Whole grains including barley, brown rice, whole grain bread, whole corn, and oatmeal
• Flaxseed
• Green Beans
• Zucchini
• Dark leafy vegetables
• Fruit skins
• Seeds and Nuts

Daily Fiber Recommendations

According to the IOM reference adults 50 years and younger should consume 38 grams/day for men and 25 grams/day for women, while for men and women over 50 it is 30 and 21 grams per day, respectively, due to decreased food consumption.²

Despite the marked health benefits of adequate fiber consumption, Americans are only, on average, consuming around 15 grams of fiber each day, according to the American Dietetic Association.

A one ounce serving of SunChips can help contribute to the daily recommendation, with 3 grams per serving.

References

1. Hipsley EH. Dietary "fibre" and pregnancy toxaemia. Brit. Med. J. 2:420, 1953.
2. http://www.nap.edu/openbook.php?record_id=10490&page=7
3. Position of the American Dietetic Association: Health Implications of Dietary Fiber. J Am Diet Assoc 2002; 102:993-1000.
4. Rimm EB, Ascherio A, Giovannucci E, Spiegelman D, Stampfer MJ, Willett WC. Vegetable, Fruit, and Cereal Fiber Intake and Risk for Coronary Heart Disease among Men. JAMA. 1996; 275:447-451.
5. Roberfroid, MB. Prebiotics and Probiotics: are they functional foods?. American Journal of Clinical Nutrition 2000; 71:1682-1687.
6. Coxam V. Current data with inulin-type fructans and calcium, targeting bone health in adults. The Journal of Nutrition. 137:24935-2502S; Nov 2007.
7. Seifert and Watzl. Inulin and Oligofructose: Review of Experimental Data on Immune Modulation. The Journal of Nutrition. 137: 2563S; Nov 2007.