The growing awareness of the relationship between diet and health has led to an increasing demand for food products that support health above and beyond providing basic nutrition. Probiotics and prebiotics are components present in foods, or that can be incorporated into foods, which yield health benefits related to their interactions with the gastrointestinal tract (GI). While the benefits of prebiotics have come to light in more recent years, recognition of probiotic effects dates back to the 19th century when the French scientist Louis Pasteur (1822 –1895) postulated the importance of microorganisms in human life; this was further reinforced by work done by 1908 Nobel Prize-winner Elie Metchnikoff.
Slightly different definitions of probiotics exist. For example, a probiotic has been defined as “a live microbial food ingredient that, when ingested in sufficient quantities, exerts health benefits.”1 Similarly, the Joint Food and Agriculture Organization/World Health Organization Working Group on drafting “Guidelines for the Evaluation of Probiotics in Food” has recommended that probiotics be defined as “live microorganisms which, when administered in adequate amounts, confer a health benefit on the host.”2
Prebiotics are defined as “nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth of one or a limited number of bacterial species in the colon, such as Bifidobacteria and Lactobacilli, which have the potential to improve host health.”1 Prebiotics are, simply speaking, the “food” for beneficial bacteria.
Probiotics and Prebiotics Found in the Foods We Eat
Today, probiotic-containing foods are commonly found and consumed in Japan and Europe.3 In the US, several probiotic- and prebiotic-containing foods have recently been introduced into the marketplace. The benefits associated with pro- and prebiotics are strain- and substance-specific, respectively, and must be shown through adequate clinical trials reflective of the dose of pro- or prebiotics present in the food at the time of consumption.
Probiotic microorganisms can be found in both supplement form and as components of foods and beverages. These bacteria and yeasts have been used for thousands of years to ferment foods. Certain yogurts and other cultured dairy products contain such helpful bacteria, particularly specific strains of Bifidobacteria and Lactobacilli. Not all bacteria present in fermented milk products or yogurt have a probiotic effect. For this reason, in order to consider a Lactobacillus or Bifidobacterium a probiotic, the specific strains selected must exert a clinically established health benefit.4-6
Prebiotics are found naturally in many foods, and can also be isolated from plants (e.g., chicory root) or synthesized (e.g., enzymatically, from sucrose)—see below, “Examples of Probiotics and Prebiotics.” In order for a food ingredient to be classified as a prebiotic, it has to be demonstrated, that it: (a) is not broken down in the stomach or absorbed in the GI tract, (b) is fermented by the gastrointestinal microflora; and (c) most importantly, selectively stimulates the growth and/or activity of intestinal bacteria associated with health and wellbeing.7
Probiotic bacteria taken together with prebiotics that support their growth are called “synbiotics.” Both work together in a synergistic way more efficiently promoting the probiotics’ benefits.
Actions of the GI Tract
To understand the role that probiotics and prebiotics play in overall health, familiarity with the GI tract and the body is important. Human beings play host to many types of microorganisms on the skin, in the mouth, and in the GI tract. The human gastrointestinal environment, including the microflora, has a significant role in the health of its host. The normal gut microflora activity is complex and can be impacted by a number of factors. The gut microflora, which includes both potentially beneficial and potentially harmful bacteria, is important in maintaining a healthy intestinal tract and helps the intestine act as an effective barrier; allowing nutrients to be absorbed, and keeping out toxins and pathogens (foreign bacteria or viruses). The gut microflora breaks down vitamins and also ferments fibers and carbohydrates that are not digested in the upper GI tract. This breakdown produces fatty acids that are important for supporting a healthy intestinal barrier (particularly in the lower GI tract) and also inhibits the growth of harmful bacteria. Healthy intestinal flora is also associated with intestinal (stool) regularity.
Actions of Probiotics in the GI Tract
Consumption of probiotics, particularly certain species of Bifidobacteria and Lactobacilli, can help “balance” the flora, increasing the number of helpful, and reducing (inhibiting the growth of) harmful bacteria, in the intestine. Consumption of probiotics can also modify the gut immune response and improve its barrier function. For example, specific probiotic species can shorten or reduce the risk of certain infections, particularly those of the GI tract, such as intestinal viruses.1 More recently, probiotics have also been shown to modulate/adjust the activity of the immune system, helping to control or reduce the development of certain allergies.
Research on Probiotics’ Health Effects
Perhaps the most compelling evidence for probiotic efficacy is in the areas of anti-diarrheal effects and helping with stool regularity. In the case of yogurt containing sufficient amounts of live and active yogurt cultures (Streptococcus thermophilus and Lactobacillus bulgaricus), there is significant evidence for the alleviation of the symptoms associated with lactose intolerance.3,7 A study in young children showed a clear beneficial effect of two Lactobacillus strains on the improvement of acute rotavirus diarrhea.8 In studies of children attending day care centers, changes in severity and duration of diarrhea after consumption of specific strains, were also seen.9-11 A meta-analysis of 18 studies also reported that bacterial probiotic therapy shortened the length of acute diarrheal illness in children.12 In adults there are limited data to support an effect on either travelers’ diarrhea or that caused by antibiotic use.13
Some probiotic strains can modulate the immune system and enhance immune system defenses of the intestinal mucosal cells through both specific and non-specific effects.14-15 Certain probiotic strains have been shown to have a favorable effect on markers of the immune response to stress.15 A study among the elderly found an enhancement of immune function following consumption of milk supplemented with a Bifidobacterium lactis strain.16 Bifidobacteria are the predominant bacterial species in the intestinal tracts of breastfed infants. Some experts believe higher levels of Bifidobacteria in the gut of breastfed infants may be one reason why they are considered to be generally healthier than formula-fed babies.17 There is also a growing body of new evidence that some probiotic strains may have a beneficial impact on allergy.14,15,18 Some probiotic strains may also reduce the severity of microbe-induced gut inflammation, acute gastroenteritis, inflammatory bowel disease, and may also reduce the risk of colorectal cancer.5,19,20
While research is ongoing, current evidence with several probiotic strains in animal and human studies suggests a moderate cholesterol-lowering effect from cultured dairy products such as yogurt and milk products.21-22
Actions of Prebiotics in the GI Tract
The principal characteristic and effect of prebiotics in the diet is to promote the growth and proliferation of beneficial bacteria in the intestinal tract, and thus, potentially yield or enhance the effect of probiotic bacteria. Prebiotics have also been shown to increase the absorption of certain minerals (such as calcium and magnesium).4 Prebiotics may also help inhibit the growth of lesions, such as adenomas and carcinomas in the gut, and thus reduce the risk factors involved in colorectal diseases.1
Research on Prebiotics’ Health Effects
Prebiotic effects on mineral absorption have been investigated, and recent human studies have confirmed that specific prebiotics, such as non-digestible oligosaccharides (NDOs) enhance calcium absorption.23 However, this effect varies according to the individual NDO and particular human population studied, and is thought to be due to its specific fermentation profile and the amount consumed.20 Preliminary studies suggest that prebiotics may have a favorable effect on the immune system and provide improved resistance against infection.24-25
For prebiotic substances, little data pertaining to lipid-lowering effects are available and come mostly from studies with inulin and oligofructose. In hyperlipidemic subjects, when a prebiotic effect is seen, it is a reduction in cholesterol; whereas in normal-lipidemic subjects, any noted effects are on serum triglycerides.26
While some of the pro- and prebiotic beneficial effects on the function of the human gut have been established and their favorable impact on health widely supported, further scientific research is ongoing to substantiate their direct relationship to disease risk reduction.27
For more information about pro- and prebiotics and other foods with health promoting benefits visit Background on Functional Foods.
|Examples of Probiotics and Prebiotics|
|Certain species and strains
of Lactobacilli, Bifidobacteria,
|Certain yogurts, other cultured
and non-dairy applications
|May improve gastrointestinal health
and systemic immunity
|Inulin, Fructo-oligosaccharides (FOS),
|Whole grains, onions, bananas,
garlic, honey, leeks, artichokes,
fortified foods and beverages,
and other food applications
|May improve gastrointestinal health;
may improve calcium absorption
|Chart adapted from International Food Information Council Foundation: Media Guide on Food Safety and Nutrition: 2004-2006.
*Examples are not an all-inclusive list
The Bottom Line
The healthful effects of pre- and probiotics factor in their potential impact on the balance of the body’s microflora, and directly or indirectly in their enhancement of the function of the gut and systemic immune system. Although benefits vary, depending on the type and amount of a pre- or probiotic consumed, experts agree that daily consumption of foods containing these functional components is beneficial. In addition, effects of probiotics are strain-specific and must be demonstrated through appropriate clinical trials.
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