By David H. Rahm, M.D.

Q: I’ve been taking a probiotic for years but lately I’m hearing more about prebiotics. What’s the difference between prebiotics and probiotics? And, should I be taking a prebiotic supplement?

Emerging research over the past decade, has given us a greater appreciation for the role that probiotics – the beneficial microorganisms that line our digestive tract – play in health and wellness.

While the preference would be to obtain probiotics (beneficial bacteria) and prebiotics (the food source for beneficial bacteria) from the foods you eat, this can be a challenge for many of us.

To determine if you should add a prebiotic supplement to your healthy lifestyle, let’s first clarify the difference between probiotics and prebiotics. Then let’s review the types, benefits and sources of prebiotics.

What are Probiotics?

As a Greek word, probiotic means "for life." In sharp contrast to antibiotics, probiotics are derived from probiosis which means a symbiotic relationship between two organisms. These microorganisms play an important role in our health, helping us digest food, manufacture vitamins and defend against pathogens. While most probiotics are bacterial, these beneficial organisms also include yeast and fungi. By taking a probiotic supplement, you are directly adding these beneficial organisms to your digestive tract. Think of probiotics as planting new flowers in your intestinal microbial flower bed.

What are Prebiotics?

Using the garden analogy, if probiotics are the flowers, then prebiotics are the compost or the fuel source for the good bacteria. Prebiotics act as a food for these microorganisms which helps keep a healthy balance of bacteria in the digestive system.

What are Synbiotics?

A synbiotic is a food or supplement that contains both probiotics and prebiotics. Synbiotics not only directly add the beneficial bacteria to your digestive tract but provide them with a ready available food source to thrive.

Nature’s premier synbiotic is breast milk which inoculates the infant with probiotics (primarily Bifidobacterium) and prebiotics (galacto-oligosaccharides or GOS). The prebiotic nourishes B. infantis, a bacterial species crucial in establishing the digestive tract and immune system during the baby’s first year of life.

Fermented dairy products like yogurt and kefir are examples of synbiotic foods. A capsule that contains both probiotic strains and a prebiotic like inulin or FOS is a synbiotic supplement.

What Characterizes Prebiotics?

Prebiotics are non-digestible carbohydrates that nourish the beneficial bacteria. These carbohydrates can’t be digested or absorbed in the small intestine by human cells but are passed along to the colon where gut microbiota ferment (metabolize) them.

Both soluble fiber and insoluble fiber are non-digestible carbohydrates which we can’t process or digest. As it turns out, insoluble fiber or “roughage” can’t be digested by gut bacteria either but this fiber still plays a key role in digestive health by keeping us regular.

Only soluble fiber can be metabolized by the gut bacteria and is therefore considered a prebiotic. Soluble fiber dissolves in water and is associated with lowering cholesterol and regulating blood sugar levels.

Importantly, to be classified as a prebiotic, the carbohydrate must not only be metabolized by the gut bacteria but stimulate the growth or activity of the intestinal populations Lactobacillus or Bifidobacterium.

What are the Benefits of Prebiotics?

When the beneficial bacteria ferment (break down) prebiotics compounds called short-chain fatty acids (SCFs) are produced. SCFs not only nourish the cells that line the colon (colonocytes) but support health in a number of ways:

By lowering the pH level of the colon, enhance the absorption of minerals like calcium and magnesium.

By regulating the release of insulin by the pancreas and controlling the breakdown of glycogen by the liver, help stabilize blood sugar levels.

By suppressing cholesterol synthesis in the liver, help reduce levels of “bad” or LDL cholesterol and triglyceride levels.

By stimulating the production of cells, antibodies, proteins and other substances, support immune system function.

What are the Types of Prebiotics?

Prebiotics are a special type of carbohydrate which nourish the beneficial bacteria that reside throughout the colon.

The building block of carbohydrates is sugars and they can be further classified by how many sugar units or links are combined into one molecule. Prebiotics can range from one sugar molecule (monosaccharide), a collection of a few sugar molecules (oligosaccharide), to a large number of sugar molecules (complex carbohydrate).

Long-chain prebiotics have more links and are digested more slowly, on the left side of the colon. Short-chain prebiotics have fewer links and are metabolized much faster, on the right side of the colon. A full-spectrum prebiotic provides the full range of short- and long-chain prebiotics, nourishing the beneficial bacteria throughout the colon.

The monosaccharide fructose or fruit sugar is a prebiotic found in vegetables, fruit and honey. Chains of fructose sugars create the oligosaccharide fructo-oligosaccharides (FOS). FOS is naturally found in fruits, vegetables, and grains such as chicory, onions, asparagus, wheat, and tomatoes.

Galactose is another monosaccharide that is found in dairy products (yogurt, buttermilk and kefir) and sugar beets. Chains of galactose sugars create the oligosaccharide galacto-oligosaccharides (GOS). Dietary sources of GOS include lentils, chickpeas, kidney beans, black-eyed peas, broccoli, and soy-based products.

Polyols, also known as sugar alcohols (isomalt, maltitol, sorbitol, xylitol, erythritol), are found in some fruits such as apples, pears, peaches, and cherries. Sugar alcohols are added as a sweetener to low-calories foods because they provide few if any calories. Sugar alcohols are neither sugar nor alcohol. Most sugar alcohols are made by adding hydrogen molecules to sugar. The addition of hydrogen makes the sugar harder to breakdown and they pass through the digestive tract which means they deliver fewer calories. An example is Zerose™ (erythritol) which is 70% as sweet as sugar but delivers zero calories.

Beta-glucans are a polysaccharide naturally found in whole grains such as oats, wheat, and barley, and mushrooms. These large sugar molecules are also found in the cell walls of bacteria, fungi, yeasts, algae and lichens.

What are Good Sources of Prebiotics?

Prebiotics are naturally found in foods, added to functional foods and formulated in supplements.

Prebiotic Foods

Inulin, one of the most studied prebiotics, is produced by plants as a means to store carbohydrates (or energy).

Inulin is found in a variety of plants including Jerusalem artichoke, asparagus, leek, onions, garlic, and banana. Dandelion root and chicory root are also good sources of inulin. A number of herbs including burdock, coneflower (Echinacea) and Leopard’s Bane (Arnica montana) have even higher concentrations of inulin. Smaller amounts can be found in a wide range of vegetables, fruits and whole grains.

While eating these foods is part of a healthy diet, you would need to eat large quantities of them to obtain an appreciable amount of inulin. For example, only about 1% of a banana contains prebiotic fiber. To obtain 6 grams of prebiotic fiber, you would need to eat almost a pound and a half of bananas! Foods such as raw chicory root have much higher concentrations (65%) but when was the last time you saw this vegetable in the grocery store, much less ate it?

Functional Foods

Foods fortified with prebiotics make it easy for those who can’t get sufficient levels of prebiotics from the foods they eat. Indeed, an increasing number of packaged and processed foods are formulated with fiber to give the product a health halo. When reading the label, look for inulin and FOS in the ingredient list.

Inulin, which is extracted from chicory root or Jerusalem artichoke, is often incorporated into drinks, dairy products, and baked goods and is used to replace sugar, fat, and flour and improve taste, texture and moisture.

Resistant starch is another type of prebiotic that is naturally present in foods such as whole grains and legumes and added to other foods. As the name implies, resistant starch “resists” digestion in the small intestine and passes to the large intestine where the fiber is broken down by gut bacteria. Maltodextrin, resistant starch, wheat dextrin, corn dextrin, and resistant dextrin indicate the food contains resistant starch (brand names Fibersol-2®, Nutriose®, Hi-maize®, Fortefiber®).

Due to their shorter chemical structure, inulin and FOS are broken down quickly by the beneficial bacteria and as a result can cause gas. Longer chain prebiotics like resistant starch, are not broken down as quickly and are less likely to cause the digestive upset.

An advantage of getting your prebiotics through a functional food is that it is much easier to obtain a larger dose per serving (5 to 8 grams) as compared with a food or supplement (1 to 3 grams).

Prebiotic Supplements

Taking a supplement is a convenient way to ensure that you’re getting sufficient levels of prebiotics to support a healthy digestive tract. Because many double as a good source of fiber, taking a prebiotic supplement also helps boost your dietary fiber intake.

Inulin is the most commonly found prebiotic supplement formulated separately or in combination with FOS. Most inulin is sourced from chicory root.

Aside from inulin, prebiotic supplements can also include other fiber sources such as acacia gum, glucomannan, guar gum, and arabinogalactans among others.

At over 85% prebiotic fiber content by weight, acacia gum is the richest source of prebiotics. Acacia gum (also known as gum acacia, arabic gum, acacia fiber) is a type of soluble fiber sourced from the sap of the Acacia senegal tree, a plant native to parts of Africa, Pakistan, and India.

Glucomannan is a soluble dietary fiber derived from the root of the konjac plant. Glucomannan absorbs water to increase its volume by up to 17 times and promotes a feeling of satiety (fullness).

A type of soluble fiber derived from the guar bean, guar gum is a type of plant in the pea family (legume). Used as a thickening agent in a wide variety of industries including foods, cosmetics, drugs and supplements.

Naturally found in variety of fruits and vegetables and the bark of the larch tree, arabinogalactans support the growth of Lactobacillus and Bifidobacterium.

How Much Prebiotics Do I Need?

Although a daily requirement for dietary fiber has been established (30-38 grams for men; 20-25 grams for women), the recommended amount for prebiotics is unknown. However, a regular source of prebiotics, whether from food or supplements, is required to feed the beneficial bacteria. Recommendations for prebiotic intake range from 4 to 8 grams daily, with higher levels for those with digestive disorders.