In the early 1900s, Nobel prize-winning scientist Elie Metchnikoff was the first to propose that acid-producing organisms in fermented dairy foods offered a prolongation of life by preventing fouling in the large intestine (34). From his idea evolved the idea of functional foods as a marketing term in the 1980s. This term is used to describe foods fortified with ingredients capable of producing health benefits. The trend for healthful foods is becoming increasingly popular with consumers because of the awareness that diet can influence health. Probiotic foods are defined as foods containing live microorganisms in significant concentrations that actively enhance the health of consumers by improving the balance of microflora in the gut when ingested (18). This restricts probiotics to products that contain live microorganisms, improve health and well-being of humans or animals, and can affect host mucosal surfaces including the mouth and gastrointestinal tract, the upper respiratory tract, or the urogenital tract (35). In Japan, a standard for probiotic foods was developed stipulating that a product contain greater than 1 x 107 viable Bifidobacteria per gram or milliliter of product (36). The probiotic must be viable and available at a high concentration to achieve health benefits (18). These factors are often overlooked or ignored, and many of the commercially available probiotic products have shown low populations of probiotics (37).
Fermented dairy products have evolved as the predominant carriers of probiotics in foods. They already have a healthful image that facilitates recommendations of daily consumption, and consumers are aware that fermented foods contain live microorganisms (38). Many of the fermented dairy foods have been optimized for survival of the fermentative microorganisms, which creates important technological advantages for the use of dairy products as probiotic carriers (39).
Foods that help prevent disease are becoming increasingly popular with consumers, with 70% of American shoppers believing that certain foods contain components that reduce the risk of diseases and improve long-term health (35). Additionally, 97% of shoppers surveyed in the United Kingdom were willing to change their eating habits to have an impact on their health. The purported benefits of probiotics include inflammatory disease control, the treatment and prevention of allergies, cancer prevention, immune stimulation, and respiratory disease reduction. Many of the suggested benefits of probiotic dairy foods are based on the involvement of the gastrointestinal microflora in resistance to disease. Unfortunately, few well-controlled studies have looked at clearly defined health effects, and many of the reported health benefits are based on unsubstantiated reports. However, the use of pro-biotics for the treatment of lactose maldigestion, diarrheal disease, lowering of serum cholesterol, and prevention of cancer or formation of carcinogens appear to be well substantiated (41). Lactose intolerance or maldigestion causing abdominal discomfort affects approximately 70% of the world's population to varying degrees (35). A probiotic bacterium, Lactobacillus acidophilus, has been clinically shown to alleviate the symptoms of lactose intolerance. The recent work in this area indicates that lactose in yogurt—possibly because of lactase-producing bacteria—can be utilized more efficiently than lactose in milk. Additionally, many of the probiotic strains that produce lactase have shown promise in alleviating symptoms of lactose intolerance (40).
Rotavirus is one of the leading causes of gastroenteritis worldwide (41). Gastroenteritis characterized by acute diarrhea and vomiting is a leading cause of death and illness among children, affecting approximately 16.5 million children annually (35). Ample evidence has shown that the probiotic strain Lactobacillus GG reduced the duration and severity of rotavirus infection. Oral administration of Bifidobacterium bifidum has shown potential for reducing the incidence of diarrhea in infants hospitalized for rotavirus infection
(41). Other research suggests that the probiotic bacteria Bifidobacterium longum and Saccharomyces boulardii prevent antibiotic-associated diarrhea (35).
Much attention has been given to the cholesterol-lowering potential of probiotics. However, the influence of lactic acid bacteria such as Lactobacillus acidophilus, although widely studied, remains controversial (42).
The role of lactic acid bacteria in reducing the incidence of DNA damage and other carcinogenic changes has also been investigated (43). In addition microflora and composition of the intestinal flora with probiotics might suppress the growth of bacteria that convert procarcinogens to carcinogens. An important index of carcinogenic activity is the activity of enzymes that convert procarcinogens to carcinogens. Several studies have shown an inhibitory effect on these enzyme activities following the consumption of probiotics (41).
Additional research on the prevention or delay of tumor development by probiotic bacteria suggests that they might bind to mutagenic compounds in the intestinal tract. A reduction of mutagens in urinary excretions was found when meals were supplemented with fermented milk containing Lactobacillus acidophilus. This suggests that lactobacilli were binding to the mutagenic compounds, thereby reducing their absorption in the intestine. It is inconclusive whether this will lead to a decreased incidence of cancer but research looks promising (41).
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