Probiotics were defined by a group of experts convened by the Food and Agriculture Organization of the United Nations (FAO) as “live microorganisms administered in adequate amounts which confer a beneficial health effect on the host”. Most probiotics are bacteria, which are small, single-celled organisms. Bacteria are categorized by scientists with genus, species and strain names. One yeast - Saccharomyces boulardii - also has been evaluated as a probiotic.

Most probiotic products contain bacteria from the genera Lactobacillus or Bifidobacterium, although other genera, including Escherichia, Enterococcus, Bacillus and Saccharomyces (a yeast) have been marketed as probiotics. Some commercial probiotic products which contain Bacillus are incorrectly labeled with a name not recognized by the scientific community, ‘Lactobacillus sporogenes’.

At a minimum, probiotic products should be safe, effective, and should maintain their effectiveness and potency until they are consumed. This requires a responsible approach both by the producer and the consumer.

There is some debate about whether or not yogurt starter bacteria should be considered probiotics. The yogurt starter cultures Lactobacillus bulgaricus and Streptotoccus thermophilus are used to ferment milk and turn it into yogurt. But these cultures are not very resistant to conditions in the stomach and small intestine and generally do not reach the gastrointestinal tract in very high numbers. Therefore, they cannot mediate some probiotic effects. But these starter bacteria have been shown to improve lactose digestion in people lacking lactase and have demonstrated some immune enhancing effects. For these reasons, they are often considered to be ‘probiotic’.

Probiotics’ Role in Health

For centuries, folklore suggested that fermented dairy products containing live active cultures are healthful. Recent controlled scientific investigation supports these traditional views, suggesting that probiotics are a valuable part of a healthy diet. In addition, the emergence of some new public health risks suggests an important role for effective probiotics in the mitigation of illness. For example, the ability of probiotic bacteria to support the immune system could be important to the elderly or other people with compromised immune function. (It is important that immune compromised individuals ask their doctor before taking any dietary supplement, including probiotics.)

Infections are another area with potential for probiotics. - Some infections, once thought self-limiting or readily treatable with antibiotics, are now recognized as more serious health threats. Vaginosis used to be considered just an annoyance. Now we know it is associated with low birth weight and increased risk of sexually transmitted diseases. New foodborne pathogens have emerged as prevalent and life threatening, including Shiga-like Escherichia coli strains. Multiple antibiotic resistances are a continual threat in the battle against once-treatable infections. And in non-industrialized nations, infections such as rotavirus, claim the lives of hundreds of thousands of infants yearly. Prevention of infections before they occur is clearly the better alternative. Probiotics may be a safe, cost-effective, “natural” approach that adds a barrier against microbial infection.

To understand how probiotics work, it is important to understand a little about the microbiology and physiology of the human gastrointestinal tract.

Human beings, like all animals, play host to many types and high numbers of microbes on our skin, in our mouths, in women’s vaginal tracts, and all the way through our gastrointestinal tract. In fact, it has been estimated that there are more microbes associated with the human body (about 1014, or 100,000,000,000,000 bacterial cells) than there are human cells in it (about 1013). In addition to this very large number of bacteria, there also is a very large diversity of bacteria. It has been estimated that more than 400 different species, or types, of bacteria make their homes on humans.

Taking this into consideration, it is not surprising that microbes have been found to play an important role in human health. Most of these bacteria are not harmful, and in fact contribute positively to normal human growth and development. But some of these bacteria can have negative influences. It is therefore important that the balance of microbes be maintained to favor the beneficial bacteria over the potentially harmful ones.

Human Gastrointestinal System Review

The digestive process begins as soon as food enters the mouth. The process of chewing increases the surface area of food particles, making the food more susceptible to digestive enzymes, including those in saliva. Smaller food particles also travel more easily (and therefore more quickly) throughout the small and large intestines. In the stomach, food is mixed with gastric juices, containing digestive enzymes and hydrochloric acid. This mixture, known as chyme, is then actively pumped out of the stomach and into the small intestine. There, more enzymes and bile are mixed with the chyme, and breakdown of dietary proteins, fats and carbohydrates is completed.

Most nutrients are absorbed in the small intestine. Within about 4-6 hours of eating, what is left of the food passes into the large intestine, or colon. Waste material accumulates, water and electrolytes are absorbed and fecal matter is stored until it passes out through the rectum every 24-48 hours.

The microbes present in the gastrointestinal tract have the potential to act in a positive, negative or neutral manner. Due to unfavorable conditions, microbes are not very prevalent in the stomach or upper small intestine. However, toward the lower small intestine, they begin to attain higher populations (106-108/gram of small intestinal contents) and in the colon they constitute about 1011-1012/gram of colon contents (a very large number).

Considering the high number of microbes in the intestinal tract, what are their effects? It is known that microbes in the large intestine complete the digestion process on any food components that were not digested in the small intestine, such as lactose in lactose intolerant people or fibers resistant to the enzymes they encounter in the small intestine. There is evidence of non-digestive microbial activities as well. Certain intestinal microbes are known to produce vitamins. Also, in studies done with special microbe-free laboratory animals, evidence is strong that without normal microbial populations, the immune system functions poorly, and resistance to pathogenic bacteria is greatly reduced. Other evidence suggests that intestinal microbes might act on pre-carcinogenic or mutagenic (capable of inducing genetic mutation) compounds. Depending on the specific microbe, mutagenic or carcinogenic activity can be either increased or decreased.

It is apparent that there are advantages in skewing the balance of bacteria toward beneficial ones. Both lactobacilli and bifidobacteria are normal inhabitants of the healthy intestine. Although they are not the dominant genera in either the small or large intestine of adults (bifidobacteria are generally the dominant flora of breast-fed infants), they are non-pathogenic and their presence is correlated with a healthy intestinal flora. The metabolic end products of their growth are organic acids (lactic and acetic acids) that tend to lower the pH of the intestinal contents, creating conditions less desirable for harmful bacteria.

The gastrointestinal tract also serves to bridge the gap between “inside the body” and “outside the body”. Along this interface, microbes and foreign antigens colonizing or passing through the GI tract interact with important components of the immune system. This interaction serves to prime or stimulate the immune system for optimal functioning. Normal microbial inhabitants of the GI tract also reinforce the barrier function of the intestinal lining, decreasing passage of bacteria or antigens from the intestine into the blood stream. This function has been suggested to decrease infections and possibly allergic reactions to food antigens.

Probiotics and Dairy Products

Probiotic bacteria are typically, but not always, chosen from bacteria that normally inhabit the gastrointestinal system of humans, and belong to a species that is known to be safe. These bacteria are purified, grown to large numbers, concentrated to high doses and preserved. They are provided in products in one of three basic ways:

  • as a culture concentrate added to a food (usually a dairy product) at medium levels, with little or no opportunity for culture growth
  • inoculated into a milk-based food (or dietary supplement) and allowed to grow to achieve high levels in a fermented food
  • as concentrated and dried cells packaged as dietary supplements such as powders, capsules, or tablets

Probiotic bacteria have a long history of association with dairy products. This is because some of the same bacteria that are associated with fermented dairy products also make their homes in different sites on the human body, including the mouth, the gastrointestinal tract and the vagina. Some of these microbes, therefore, can play a dual role in transforming milk into a diverse array of fermented dairy products (yogurt, cheese, kefir, etc.), and contributing to the important role of colonizing bacteria.

Dairy products can provide a desirable “probiotic delivery vehicle” for several reasons.

Dairy foods can protect the probiotic bacteria
Traveling through the human digestive tract can be dangerous for bacteria. High acid levels in the stomach and high bile concentrations in the small intestine can lead to the injury and death of many members of the probiotic population. Although some bacteria are more resistant than others to this stress, consumption of probiotics with food, including milk, yogurt and other dairy products, buffers stomach acid and increases the chance that the bacteria will survive into the intestine.

Refrigerated storage of dairy products helps promote probiotic stability
Although the lactic acid content of yogurt can be a barrier to culture stability, short-term refrigeration generally promotes stability.

Live cultures in dairy foods carry a positive image
The consuming public may have a generally negative image of bacteria in foods, but they are aware of “live, active cultures” in fermented dairy foods, and these cultures convey a positive, healthful image. Probiotic bacteria in dairy foods can be an extension of the comfortable association of cultures in dairy products, and make it easier to communicate health messages to the public.

The healthful properties of probiotic bacteria blend with the healthful properties of milk products

A dairy product containing probiotics makes a healthy, “functional food package.” In addition to the vitamins, calcium, other minerals, and protein obtained from milk products, modern research has suggested healthful properties of fermentation-derived peptides and butyric acid found in some dairy products. Dairy products have recently been shown to be important components for a healthy diet, for more than the prevention of osteoporosis. Consumption of three or more servings of dairy products each day has been associated with lower levels of obesity among Americans. Obesity is associated with diabetes, hypertension and heart disease. The DASH (Dietary Approaches to Stop Hypertension) diet also recommends three servings of lowfat dairy products. Considering all these findings, dairy products combined with probiotic bacteria may translate into improved long-term health.

Health Effects of Probiotics

There have been hundreds of papers published on the health benefits associated with probiotic cultures. The field of probiotics is accelerating rapidly. A recent search of the bibliographic database, Medline, is a perfect example of how the science has grown over the past ten years. Prior to 1990, only five citations appeared in a search for “probiotic” with zero citations for probiotic when the search was limited to clinical trials. This is a stark contrast to a search of publications from 2002 through March 16, 2004, which returned 774 probiotic citations and 77 citations limited to clinical trials.

Some of these publications describe clinical studies designed to determine how probiotic cultures may influence a variety of health conditions. These are very complicated questions, and research is still actively being conducted to clarify the role of probiotics in human health.

When considering the health effects of probiotics, it is important to recognize that different strains, species and genera of bacteria may have different effects. For the most part, specific clinical studies on probiotics are done with one defined strain or a defined blend of strains. The following discussion, therefore, should be taken as a general description of probiotic activity, keeping in mind that any one effect may have been documented with only one or a limited number of probiotic strains.

Although any one study may not give a complete picture of effectiveness, an evaluation of the body of research done on probiotic cultures suggests that certain strains consumed at adequate levels positively influence human health. The following describes some of the proposed health benefits of consumption of probiotic cultures.

Scientific support for efficacy.

In the United States, it is essential to have scientific substantiation if a statement (known as a structure/function statement) about the effect of probiotics on the normal functioning of the human body is made on a food or dietary supplement product or during promotion of the product. The burden of proof rests with the manufacturer. Although the FDA does not require premarket approval of such statements, the manufacturer must provide scientific justification for use of any health statements if asked by the FDA.

The following provides a rationale for probiotic impact on a variety of health targets:

Diarrhea. Many types of diarrheal illnesses, with many different causes, disrupt intestinal function. The ability of probiotics to decrease the incidence or duration of certain diarrheal illnesses is perhaps the most substantiated of the health effects of probiotics. A paper published in 2002 reviewed nine studies on the effect of Lactobacillus as therapy for diarrhea in children. This paper (Van Niel, et al. 2002. Pediatrics 109:678-684) concluded that “Lactobacillus is safe and effective as a treatment for children with acute infectious diarrhea.” Although this meta-analysis can be criticized for combining data from different species and strains of Lactobacillus into one analysis, the positive nature of the conclusion suggests that at least for this indication and for these strains, positive results have been obtained.

Antibiotics. One group assessed for the impact of probiotics is people on antibiotic therapy. The purpose of antibiotics is to kill harmful bacteria. Unfortunately, they frequently kill normal bacteria as well, often resulting in disruption of the bacterial flora, leading to diarrhea and other intestinal disturbances. Replenishing the flora with normal bacteria during and after antibiotic therapy seems to minimize disruptive effects of antibiotic use. A paper published in 2002 reviewed seven studies (881 total patients) on the impact of probiotics (Lactobacillus rhamnosus GG or Saccharomyces boulardii) on antibiotic-associated diarrhea (Cremonini, et al. 2002. Aliment. Pharmacol. Ther. 16:1461-1467). The paper concluded that evidence suggests that probiotic can be used to prevent antibiotic associated diarrhea, but that no strong effect on the ability of probiotics to treat diarrhea exists. Not all studies have shown positive results in the prevention of antibiotic associated diarrhea or other symptoms associated with antibiotic therapy.

Hypertension. About 50-60 million people in United States are estimated to have hypertension, or elevated blood pressure. Antihypertensive effects have been documented in animal models and in mildly hypertensive adults for three compounds derived from the growth of certain lactobacilli: 1) fermented milk containing two tripeptides derived from the proteolytic action of L. helveticus on casein in milk; 2) bacterial cell wall components from cell extracts of lactobacilli; and 3) fermented milk containing fermentation-derived gamma amino butyric acid. Systolic blood pressure was decreased on the order of 10-20 mm Hg. These results suggest that consumption of certain lactobacilli, or products made from them, may reduce blood pressure in mildly hypertensive people. Viability of the lactobacillus is not required for the effect. Such fermentation-derived, but non probiotic products have been developed.

Cancer. In general, cancer is caused by mutation or activation of abnormal genes that control cell growth and division. (A substance that causes a mistake in genes is known as a mutagen). Most of these abnormal cells do not result in cancer since normal cells usually out-compete abnormal ones. Also, the immune system recognizes and destroys most abnormal cells.

Many processes or exposures can increase the occurrence of abnormal cells. Precautions that minimize these exposures decrease the risk of cancer. Among the many potentially risky exposures are chemical exposures. Cancer-causing chemicals (carcinogens) can be ingested or generated by metabolic activity of microbes that live in the gastrointestinal system. It has been hypothesized that probiotic cultures might decrease the exposure to chemical carcinogens by (1) detoxifying ingested carcinogens, (2) altering the environment of the intestine and thereby decreasing populations or metabolic activities of bacteria that may generate carcinogenic compounds, (3) producing metabolic products (e.g., butyrate) which improve a cell’s ability to die when it should die (a process known as apoptosis or programmed cell death), (4) producing compounds that inhibit the growth of tumor cells, or (5) stimulating the immune system to better defend against cancer cell proliferation.

Research suggests that the consumption of probiotic cultures may decrease cancer risk. Researchers testing the effect of the consumption of fermented milks, probiotic bacteria, components of bacteria or extracts of bacteria have found:

  • A reduction in the incidence of chemically induced tumors in rats.
  • A reduction of the activity of fecal enzymes (ß-glucuronidase, azoreductase, nitroreductase, and 7-^-dehydrogenase) postulated to play a role in colon cancer in human and animal subjects.
  • Degradation of nitrosamines.
  • A weakening of mutagenic activity of substances tested in the laboratory.
  • Prevention of damage to DNA in certain colonic cells.
  • In vitro binding of mutagens by cell wall components of probiotic bacteria.
  • Enhancement of immune system functioning.

Taken together, these results suggest that probiotic cultures may positively influence the gastrointestinal environment to decrease the risk of cancer. However, cancer reduction must be demonstrated in humans to confirm the significance of these observations and these studies are very expensive to conduct. In only one study was the impact of a probiotic preparation on cancer tested in humans. In this study, the effect of consumption of Lactobacillus casei fermented milk on recurrence of superficial bladder cancer was tested. The recurrence-free period for the Lactobacillus-consuming group was found to be almost twice as long as the control group.

Immune System Stimulation. The immune system provides the primary defense against microbial pathogens that have entered our bodies. The immune system is extremely complex, involving both cell-based and antibody-based responses to potential infectious agents. Immunodeficiency can result from certain diseases (e.g., cancer, AIDS, leukemia) or, to a lesser extent, from more normal conditions such as old age, pregnancy, or stress. Autoimmune diseases (e.g., allergies, rheumatoid arthritis, inflammatory bowel diseases) also can occur due to misdirected immune system activity.

Probiotic cultures have been shown in a variety of test systems to stimulate certain cellular and antibody functions of the immune system. Animal and some human studies have shown an effect of yogurt or lactic acid bacteria on enhancing levels of certain immunoreactive cells (e.g. macrophages, lymphocytes) or factors (cytokines, immunoglobulins, interferon). In addition, some studies have shown improved survival of pathogen-infected laboratory animals consuming probiotic cultures as compared to animals consuming a control diet. Results accumulated so far suggest that probiotics may provide an additional tool to help your body protect itself.

An exciting area of research has been documenting the ability of certain probiotic bacteria to modulate immune dysregulation. Studies have shown that probiotics are effective in decreasing the development of allergy and relapse of inflammatory bowel disease.

For more information, see effects of probiotics on intestinal immunity from Danone - In the News 2002

Elevated Blood Cholesterol. Cholesterol is essential for many functions in the human body. It acts as a precursor to certain hormones and vitamins and it is a component of cell membranes and nerve cells. However, elevated levels of total blood cholesterol or other blood lipids are considered risk factors for developing coronary heart disease. Although humans synthesize cholesterol to maintain minimum levels for biological functioning, diet also is known to play a role in serum cholesterol levels. The extent of influence varies significantly from person to person. Probiotic cultures have been evaluated for their effect on serum cholesterol levels. Clinical studies on the effect of lowering cholesterol or low-density lipid levels in humans have not been conclusive. There have been some human studies that suggest that blood cholesterol levels can be reduced by consumption of probiotic-containing dairy foods by people with elevated blood cholesterol, but in general the evidence is not overwhelming. It is likely that some strains may demonstrate this property while others do not.

Allergy. Allergy is on the rise in industrialized nations. It is estimated that the incidence of asthma in the United States doubled between 1980 and 2000. Scientists have proposed a hypothesis known as the ‘hygiene hypothesis’ to explain the rise in allergic conditions such as asthma and eczema. This hypothesis is based on observations that lower allergy incidence is associated with environments that have greater numbers of microbes, such as day care centers, farms, or in homes with siblings or pets. Sanitary living environments and the consumption of processed foods have limited the number of microbes in the diet. The hypothesis suggests that the exposure of infants to microbes before the age of six months helps the immune system mature to be more tolerant of exposure to allergens later in life (”Day Care May Boost Immunity To Asthma,” August 2000, Washington Post.)

Of course, increasing exposure to microbes must be done safely. This hypothesis led researchers in Finland to conduct a study evaluating the effects of a Lactobacillus strain on incidence of atopic eczema in 132 infants at high risk of developing eczema. The study was double-blinded and placebo-controlled. Pregnant mothers two-to-four weeks before delivery and newborn babies through six months of age were given Lactobacillus rhamnosus GG. Infants were followed through two years of age and incidence of recurring atopic eczema was recorded. The study reported a 50% drop in incidence of recurring atopic eczema in the group receiving the probiotic supplement. A follow up study of these same children indicated that these same trends were still present at 4 years of age. These results suggest that exposure to the right types of microbes early in life may decrease the risk of allergy.

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