Probiotics . . .
A Common Sense Discussion

Overview
Probiotics are most simply defined as naturally occurring health-enhancing microorganisms consumed as a food component or dietary supplement. Other common terms for probiotics are "friendly" or "healthy" bacteria.

For centuries people have unknowingly consumed probiotics as a natural component of their food supply. Fermented milks have historically been the most common foods containing probiotics. However it was not until 1908, when Elie Metchnikoff published his book entitled "The Elongation of Life" that health benefits were first attributed to the consumption of milk fermented with specific types of bacteria.

The human digestive tract is a delicate balance between beneficial and potentially pathogenic microorganisms. Under conditions, such as stress, traveling and use of antibiotics and birth control pills, this natural balance is disrupted and the number of pathogens increase while the number of beneficial organisms decrease, resulting in gut abnormalities.

Bacteria promoted as probiotics should have an established history of safe use and documentation of efficacy. They should be normal inhabitants of the human body. Delivery in an effective dose, with viability guaranteed through shelf life under recommended storage conditions, is critical. A minimum dose of billions of probiotic bacteria per day is generally regarded as necessary to observe health benefits. Storage under refrigerated conditions is recommended to assure viability.


How Do Probiotics Work?
To understand how probiotics work, it is important to know a little about the microbiology of the human body. Humans, like animals, are highly colonized by microbes. Microbes live on our skin, in our mouths, in women's vaginal tract and throughout our gastrointestinal (GI) tract (Fig. 1). It is estimated that there are 1014 microbes associated with our bodies. This is 10 times greater than the total number of cells our bodies contain.

Since probiotics are consumed as food and dietary supplements, it is also helpful to review some basic information of the human digestive system. 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 the digestive enzymes, including those in saliva. Smaller food particles also travel more easily (and therefore more quickly) throughout the GI tract. 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 chyme, and the breakdown of dietary proteins, fats and carbohydrates is completed. Most nutrients are absorbed in the small intestine. Within about 4 to 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.

Various studies have shown that healthy bacteria may improve our well-being via: o Manufacture and assimilation of B vitamins. o Enhancement of dairy product digestion. o Reduction of serum cholesterol levels. o Improvement in digestive and bowel functions. o Stimulation of the immune system. o Exclusion of harmful microbes associated with diarrhea, yeast infections, urinary tract infections and small bowel bacterial overgrowth. o Reduction of fecal enzyme activities postulated to play a role in colon cancer. o Reduction of antibiotic side effects. o Reduction of lactose intolerance. o Binding of mutagens by cell wall components (in vitro).

Gastrointestinal Tract (Fig. 1)
Microbes are not very prevalent in the stomach or upper small intestine. The high acid and bile concentrations coupled with the rapid transit time of contents is not favorable to microbial growth. However, toward the lower small intestine, microbes begin to attain higher populations (106-108/ gram of small intestine contents) and in the colon they constitute about 1011-1012/ gram of colon contents. This is a huge number of microbes.

Considering the multitude of microbes in the intestinal tract, what are the effects of their presence? 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 soluble fibers which are resistant to enzymes present in the small intestine. But there is evidence of non-digestive microbial activity 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 mutagenic compounds. Depending on the specific microbe, mutagenic activity can be either increased or decreased.


Health Effects of Probiotics
Using probiotic cultures to promote health has an inherent advantage in that it is a natural approach...

There are hundreds of papers published on many health benefits of probiotic cultures. Described in these publications are in vitro studies, studies in animal models and human clinical studies, all designed to determine how probiotic cultures may influence a variety of health conditions. Of course, these are very complicated questions, and research is still actively being conducted to further define the role of probiotics in human health. But what can be said is that an evaluation of the body of research done on probiotic cultures suggests that certain strains consumed at high levels positively influence human health. Food sources of probiotics, like milk and yogurt can be healthy additions, but due to intolerances and lower culture levels, supplemention becomes a more viable option.

Using probiotic cultures to promote health has an inherent advantage in that it is a natural approach which does not disturb the normal colonizing flora of the human body.

Perhaps the most compelling use of probiotic cultures is to decrease the degree of disturbance of the normal intestinal microflora which comes with antibiotic use. The purpose of antibiotics is to kill harmful bacteria. Unfortunately, they kill normal bacteria also. Instead of wiping out bacteria, probiotics promote the growth of friendly flora, which in turn fight disease and infectious-causing bacteria. The following describes some of the further proposed health benefits of consumption of probiotic cultures.


Immune System Stimulation
The immune system provides an important defense against microbial pathogens which 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) can also occur due to misdirected immune system activity.


How are our Gut and Immune System Related?
The gastrointestinal tract is the primary interface between us and our outside environment. The total mucosal surface area of the adult GI tract is about 300 square meters, and is by far the largest body area exposed to "foreign" substances like pathogens, harmless bacteria and food.

The gut-associated lymphoid tissue (GALT) makes the GI tract our largest immune organ.

It is through this mechanism that probiotics are thought to influence the immune response. 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 probiotic bacteria on enhancing levels of certain immunoreactive cells (e.g. macrophages, lymphocytes) or factors (e.g. 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 when ingested, probiotics organisms are exposed to the mucosal layer of the GI tract, providing an additional tool to help your body protect itself.


Hypertension
About 50-60 million people in United States are estimated to have hypertension, or elevated blood pressure. Evidence suggests that some decrease in blood pressure may result from consumption of certain lactobacilli. Some studies done with hypertensive rats have shown a positive effect. Studies with human subjects are limited. However, one study conducted with subjects with hypertension showed a decrease of 10-20 mm Hg in systolic pressure. Attempts to isolate the component causing the antihypertensive effect have suggested that in at least one case it is due to a part of the bacterial cell wall. This implies that the cells need not be alive to mediate this effect. Other research demonstrated that a compound produced when probiotic bacteria were grown caused an antihypertensive effect. These results suggest that consumption of certain lactobacilli, or products made from them, may reduce blood pressure in hypertensive subjects, providing patients another tool in managing elevated blood pressure.


Vaginitis
The vagina and its microflora form a finely balanced ecosystem. Disruption of this ecosystem can lead to a microbiological imbalance and symptoms of vaginitis. Vaginitis used to be considered a mere annoyance, but now is being examined for a role in serious conditions including pelvic inflammatory disease, pregnancy related complications (e.g. low birth weight babies), and increased susceptibility to AIDS infection. Vaginitis can be caused by several different organisms, and in many cases, the causative agent may not be identified. What is known is that lactobacilli predominate in the healthy vagina, and a lack of lactobacilli (especially those producing hydrogen peroxide) is correlated with vaginitis. The lactobacilli are thought to maintain a favorable, vaginal pH in the acidic range and to inhibit pathogens via the production of hydrogen peroxide. Intravaginal applications of lactobacilli have been somewhat effective in treating bacterial vaginitis. One study done with 13 women showed that consumption of Lactobacillus acidophilus decreased the incidence of Candida yeast infections. Research suggests that lactobacilli may be helpful in controlling the incidence and duration of vaginal infections.


Diarrhea
Many types of diarrheal illnesses, with many different causes, exist. These can result in a bacterial imbalance, leading to diarrhea. Replenishing the flora with normal bacteria during and after antibiotic use can improve the microbial balance until the normal flora is reestablished. Probiotics have also been tested for effectiveness against Clostridium difficile colitis. In general, the results suggest that consumption of high levels of a quality probiotic may shorten the duration or decrease the incidence of certain diarrheal illnesses.


Small Bowel Bacterial Overgrowth
Under certain conditions, such as production of low stomach acid or in patients with kidney disease, microbial populations in the small intestine can increase significantly beyond normal levels. This is termed small bowel bacterial overgrowth. The high population of these microbes can produce by-products from their growth which can be toxic. Researchers have found that feeding high levels of Lactobacillus acidophilus can control the toxic effects of these microbes. This is another example of the ability of probiotic strains fed in high numbers to modulate the activity of other intestinal bacteria.


Lactose Intolerance
The inability of adults to digest lactose, or milk sugar, is prevalent worldwide. People of northern European descent are unique in retaining the ability to produce the lactose digesting enzyme, lactase, into adulthood. Consumption of lactose by those lacking adequate levels of lactase can result in symptoms of diarrhea, bloating, and flatulence. These symptoms are due to the undigested lactose reaching the large intestine and being fermented by the colonic microbes. These microbes can produce gases and products which lead to watery stool.

The approach to coping with lactose digestion problems can be multifaceted, including moderating intake of dairy products, ingesting lactase prior to eating dairy products, and consumption of products containing pre-digested lactose. Many consumers have also noticed that they are better able to consume fermented dairy products, such as yogurt, with fewer symptoms than the same amount of unfermented milk, even though yogurt contains about the same amount of lactose as milk. Yogurt was found to aid digestion of lactose because the lactic acid bacteria used to make yogurt produce lactase and digest the lactose before it reaches the colon. In addition to yogurt starter bacteria, Lactobacillus acidophilus and bifidobacteria have been shown to improve digestion of lactose.


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 maintenance of serum cholesterol levels, although the extent of influence varies significantly from person to person. Probiotic cultures have been evaluated for their effect on serum cholesterol levels. Although clinical studies on the effect of lowering of cholesterol or low density lipid levels in humans have not been conclusive, there have been human studies which suggest that consumption of probiotic-containing dairy foods in people with elevated blood cholesterol can reduce this level.