Proper nutrition is essential to maintaining a healthy status, however, Western diets often lack healthful ingredients such as fruit, fiber, and whole grains. A key to maintaining good health includes nurturing the body's naturally occurring microorganisms for proper digestive-tract function.
Diets that are high in fat and red meat, alcohol, drugs, and stress can alter the delicate balance between good and bad bacteria in the digestive tract. Not only do antibiotics kill harmful bacteria, they kill beneficial bacteria as well.
Adding probiotics or live beneficial bacteria to the diet, such as acidophilus bacteria and bifidobacteria, helps modulate gut microflora and improve the intestinal microbial balance. Probiotics, increasingly, are being added to dairy products such as yogurt. For maximum benefit, the bacteria should be viable.
A (Healthy) Gut Response
While consumption of probiotics is recommended by the nutrition community, prebiotics are also recommended. Prebiotics are food sources that are preferentially chosen by beneficial bacteria. The result is enhanced intestinal health, i.e., a balance between the beneficial and harmful microbes. Prebiotics are nondigestible carbohydrates that pass through the small intestine undigested and are fermented in the colon. The body does not produce the enzymes necessary for their digestion, therefore, they serve as a source of fiber or bulk.
Recently, the American Association of Cereal Chemists revised the definition of dietary fiber, which means that the nondigestible carbohydrates, such as inulin and resistant starch, may be classified as such by AACC.1 Prebiotic fermentation leads to health benefits such as increased fecal biomass and, consequently, increased stool weight and/or stool frequency. Colonic fermentation of prebiotics by bifidobacteria produces short-chain fatty acids (SCFA), mainly acetate, propionate, and butyrate, hydrogen and carbon dioxide gas, and bacterial cell mass.2 Production of SCFA leads to a decrease in pH in the colon that produces a favorable environment for the friendly bacteria and a poor environment for harmful bacteria, such as coliforms. In addition, lower intestinal pH facilitates absorption of minerals such as calcium, magnesium, and zinc. Lowering intestinal pH may also help lower the risk for colon cancer.
The most well-researched prebiotics include inulin and fructo-oligosaccharides (FOS), which are found in approximately 36,000 plants at various levels.
Sidebar: Potential Prebiotic
- Oligosaccharide (example): Chemical composition Fructo-oligosaccharides (Raftilose P95): 95% oligosaccharides b (2-1) fructan: 60% glucose, fructose (n), 40% fructose (n) dp 2-8, average 4-5
- Inulin: >99% oligosaccharides b (2-1) fructan; average dp 10-12
- Pyrodextrins: Complex mixture of glucose-containing oligosaccharides Transgalactosylated oligosaccharides (Oligomate 55): Mainly 6' galactosyllactose, dp of oligosaccharide fraction 2-5 (primarily dp 3); 55% pure
- Galacto-oligosaccharides: Oligogalactose (85%), small amounts of glucose, galactose, and lactose
- Soya oligosaccharides: Stachyose (fructose, galactose, galactose, glucose) and raffinose (fructose, galactose, glucose), dp 3-4
- Xylo-oligosaccharides: b (1-4) linked xylose; 70% pure, dp of oligosaccharide fraction 2-4
- Isomalto-oligosaccharides: Mixture of a (1-6) linked glucose oligomers (isomaltose, panose, isomaltotriose)
- Lactulose: Galactose and fructose-containing disaccharide dp = degree of polymerisation
Source: Medical Research Council - Dunn Clinical Nutrition Centre, Cambridge, UK; www.bmj.com/cgi/content/full/318/7189/999/T1 (8/2001)
Types of Prebiotics
In order for an ingredient to be classified as a true prebiotic, it has to fit the following definition as defined by Gibson and Roberfroid: "A prebiotic is a non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon to improve host health."3
Currently, the criteria used for classification of a food component as a prebiotic includes: resistance to digestion, hydrolysis and fermentation by colonic microflora, and selective stimulation of growth of one or a limited number of bacteria in the feces.4 In addition, a prebiotic may repress the growth of pathogens for overall beneficial health.
"While many ingredients can be utilized by the good bacteria, they cannot be utilized by the bad bacteria as well and be classified as true prebiotics," says Pam Galvin, vice president of sales and technical marketing with Imperial Sensus, Sugar Land, Texas. "For example, guar gum is a fermentable source in the colon "it stimulates good bacteria” but, technically, it is not a prebiotic because it doesn't meet the other criteria of being utilized by pathogenic bacteria."
Fructans, Inulin, FOS
Fructan refers to a group of fructose-polymers, most of which have a terminal glucose molecule. Inulin is a member of the fructans-group and is characterized by a ß(2-1) bond between the fructose-units. The chain length or DP (number of fructose units/degree of polymerization) of inulin can be anywhere from two to 60, depending on the plant from which it is extracted. Oligofructose and FOS are considered to be different types of inulin. While standard inulin extracted from chicory root typically has a DP up to 60, FOS (short-chain) is generally DP up to 20. Oligofructose ranges from DP two to eight.
FOS and inulin occur naturally in a variety of fruits, vegetables, and grains, especially chicory, Jerusalem artichoke, bananas, onions, garlic, asparagus, barley, wheat, and tomatoes. Commercially, most ingredient companies extract inulin and FOS from chicory or Jerusalem artichoke.
The stimulatory effect of inulin on the growth of bifidobacteria is well established. Daily intake of FOS over a few weeks leads to a selective stimulation of the growth of bifidobacteria. Recommended levels for prebiotic effect begin at five grams/day.
The experiments reported by Gibson, et al. demonstrate that the growth of bifidobacteria is accompanied by a reduction in pathogenic bacteria, such as bacteroides, clostridia and fusobacteria, leading to a major modification in the composition of the colonic microflora.3
Prebiotic ingredients vary in the microorganisms they select to ferment to various SCFA.
"Inulin stimulates certain types of beneficial bacteria that are more propionic producing with less butyrate production," says Galvin. Propionic acid can help mediate carbohydrate and lipid metabolism, reducing levels of LDL ("bad"?) cholesterol.
Unlike the other SCFA, acetate, a strong acid, lowers the pH more effectively, thereby eliminating pathogenic bacteria before they produce metabolites that can be precarcinogenic.
As we age, the epithelial cells that line the colon die off, and we lose the ability to absorb nutrients effectively. Prebiotics such as resistant starch and inulin help feed and restore these cells through the production of butyrate.
Besides inulin and FOS, a number of other compounds have potential as prebiotics such as soybean oligosaccharides (raffinose, stachyose), lactulose, isomalto-oligosaccharides, lactosucrose, gluco-oligosaccharides and palatinose. (See chart page NS 8.) Others may include tagatose, and lactitol.
While the Japanese and Europeans have introduced many prebiotic-containing products over the past years, the U.S. is quickly catching up.
Prebiotic ingredients are used in this country as bulking ingredients, sugar substitutes and, for inulin, as a fat replacer. In the U.S., we are seeing FOS and inulin on more ingredient statements. This year, BocaBear® Foods, Inc., Reno, Nev. launched three new products containing inulin, the first of which was Chill Jame spreads that are packaged in squeezable tubes. The other two products are baking mixes—Muffin Maniacse and Romancing the Sconese.
Last summer, Helios Nutrition, Ltd., Sauk Centre, Minn. introduced its New Organic Kefir that contains probiotic kefir and prebiotic FOS. Adding both prebiotics and probiotics will probably be the trend in the future, according to Martin Playne, Ph.D., of Australia-based Melbourne Biotechnology.5 Adding an oligosaccharide to the diet alone will not be the answer, he says. Researchers will have to identify "matching pairs” oligosaccharides that specifically enhance certain strains of bifidobacteria. "In addition, we will need to create symbiotic foods those with prebiotics and probiotics," says Playne.