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Dairy Council Digest Archives
Functional Foods: An Overview
Probiotics, Prebiotics, and Synbiotics
Definitions. The use of probiotics, prebiotics, and synbiotics is a promising area for the development of functional foods (6b,21,22,23a). Dairy foods appear to be the preferred medium for introducing probiotic bacteria such as human-derived species of lactic acid bacteria (e.g., L. acidophilus, L. casei, L. gasseri, L. rhamnosus, L. reuteri, Bifidobacterium bifidum, B. breve, B. infantis, and B. longum) (6b).
Lactobacillus spp. (naturally found in the human small intestine) and various Bifidobacterium spp. (a major organism in the human large intestine) are the most commonly used probiotic cultures (8).
In the U.S., there is no separate regulatory category for functional foods. A functional food may be labeled as a conventional food or as a dietary supplement depending on its intended use, among other factors.
Probiotic (derived from the Greek word meaning ‘‘for life’’) generally refers to live bacteria that beneficially affect the host by improving its intestinal microbial balance (21,22). A prebiotic is a nondigestible food ingredient that benefits the host by selectively stimulating the growth and/or activity of health-promoting bacteria, such as Bifidobacterium in the colon, over undesirable or pathogenic microorganisms (2,22,23b,24–26). Starches, dietary fibers, other nonabsorbable sugars, sugar alcohols, and oligosaccharides are examples of prebiotics (25). The potential health benefits of non-digestible oligosaccharides such as inulin and oligofructose have received recent attention (4b,c,d,e,22,25).
A synbiotic is a product in which both a probiotic and a prebiotic are combined to have an additive or synergistically beneficial effect on the host by improving the survival and/or implantation of the probiotic in the intestinal tract (2,4f,21,24–26). In laboratory rats treated with a carcinogen, the combination of bifidobacteria (i.e., a probiotic) and oligofructose (i.e., a prebiotic) has been demonstrated to reduce colon cancer risk (4f). Most functional foods containing synbiotics are currently yogurt products found primarily in Europe (2,14).
Potential and Established Health Benefits. Few well-designed, well-conducted human clinical trials of probiotics have been conducted over the past 30 years (21). Only in recent years has the importance of choosing probiotic strains of demonstrated efficacy been recognized (21,22). With respect to health benefits of probiotics, research studies indicate the following:
- Improved intestinal health. Certain probiotic strains have been demonstrated to shorten the duration of diarrhea (i.e., antibiotic-associated diarrhea, acute infantile diarrhea,traveler’s diarrhea), suppress harmful intestinal microbes, and increase resistance to infectious diseases, particularly of the intestine (6b,21,22,23a,26–29). An investigation in Finland demonstrated that the duration of acute diarrhea in children between 4 and 45 months of age was shortened following intake of a specific strain of Lactobacillus (Lactobacillus rhamnosus GG) in a fermented drink or in a freeze-dried form (28). Dairy foods such as milk and yogurt with specific strains of probiotic cultures can favorably influence the intestinal microflora in experimental animals and humans(30–32). It has been reported that specific strains of probiotics may also help to alleviate constipation (23a).
- Modulation of the immune response. In both experimental animals and humans, probiotic cultures have been observed to influence local and systemic immune responses (4c,6b,21,22,23c,33,34). Various Lactobacillus and Bifidobacteria strains per se or in foods such as yogurt have been demonstrated to enhance nonspecific immunity by increasing macrophage activation and natural killer cell activity (22,33). In humans consuming a fermented milk product with either L. acidophilus LA1 or B. bifidum Bb12, there was no change in lymphocyte population, although macrophage phagocytosis of E. coli increased (34). Other studies report contradictory findings regarding the role of probiotics in immune function (4g,35). Nevertheless, researchers suspect that probiotics have a possible beneficial role in immune response (4g).
- Reduced risk of cancer. Studies indicate that probiotics may reduce the risk of colon cancer (36–42) and possibly breast (43,44) and intestinal (45) cancers. For the most part, studies have been conducted in experimental animals and in vitro. Additionally, studies demonstrate that probiotic strains differ in their ability to reduce cancer risk (36,44). Probiotics may reduce the risk of cancer by several mechanisms. These include reducing fecal enzymes (e.g., ß-glucuronidase, azoreductase, nitroreductase) associated with the conversion of procarcinogens to carcinogens, stimulating the host’s immunological defenses, and altering the acidity (pH) of the colon rendering the environment less conducive to the development of cancer (2,6b,22,23a,36,37,41,42).
- Reduced risk of heart disease. Probiotics may potentially reduce risk of heart disease by lowering blood cholesterol levels, increasing resistance of low density lipoprotein (LDL) cholesterol to oxidation, and reducing blood pressure (2,22). In experimental animals, intake of yogurt or cultured milk has been demonstrated to reduce blood cholesterol levels (46–48). However, data from studies of the effects of milk and other dairy foods with probiotic cultures on blood cholesterol levels are contradictory (2,4g,49-51).
- Improved tolerance to milk. Probiotics may improve tolerance to lactose and reduce milk allergy (6b,21,22). Alleviation of lactose maldigestion symptoms is well established for some probiotics and dairy foods containing probiotics (21,52–57). Intake of yogurt made from milk fermented with Lactobacillus bulgaricus and Streptococcus thermophilus has been demonstrated to enhance lactose digestion in individuals with lactase nonpersistence (52,53). This beneficial effect of yogurt with probiotic strains is due primarily to the presence of the enzyme, ß-galactosidase, which digests the lactose in yogurt (52,53). Whether or not other dairy foods with probiotics aid digestion of lactose depends on the strain of the probiotics used and the concentration of the culture, among other factors (54–57). Bifidobacteria may improve lactose digestion because these probiotics have a relatively high level of ß-galactosidase activity and are stable under normal storage conditions (57). Probiotics, such as dairy foods with Lactobacillus spp., may also reduce food allergies including sensitivity to milk proteins, although additional investigations are necessary to establish this health benefit (58–62).
- Other. Limited scientific findings indicate that probiotic bacteria may help to protect against vaginal/urinary tract infections (27,63). However, many of the studies to date are uncontrolled and involve a small number of subjects (27,63). Probiotics may also reduce ulcers by decreasing the growth of ulcer-inducing bacteria (i.e., Helicobacter pylori) (64,65).
What lies ahead for probiotics?
Although several studies indicate that specific strains of probiotic cultures alleviate lactose maldigestion symptoms and reduce the duration of diarrhea, additional research is needed to scientifically substantiate many other alleged beneficial health effects of probiotic bacteria, as well as the mechanisms underlying these effects (21). To better understand the potential beneficial properties of probiotics such as Lactobacillus and Bifidobacterium, researchers are identifying the genetic characteristics of these probiotics (22,66). Genetic analysis and modification of probiotics can lead to the development of new probiotics with beneficial health effects, improved effectiveness of existing properties of probiotic strains in vitro and in vivo, and development of probiotic products for specific functional characteristics (22).
Dairy foods are a preferred medium for probiotics or health-promoting bacteria. Specific strains of lactic acid bacteria appear to alleviate symptoms of lactose maldigestion and reduce the duration of diarrhea, as well as possibly protect against several other diseases/conditions.
Before probiotics and in particular probiotic-enhanced dairy foods can become mainstream in the U.S., the quality and viability of these cultures in dairy foods must be ensured. Information is needed on what strain or combination of strains and what levels of specific strains of probiotics are required to achieve an intended health benefit. Also, the shelf life limitations of probiotics in foods such as dairy foods must be determined. Although most strains of lactic acid probiotics have a good safety record, the safety of new prebiotics and probiotics, including genetically modified bacteria, must be confirmed (21). Rigorous clinical trials in humans are necessary before probiotic therapy will be truly accepted.
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