Nutritional And Health Aspects Of Yogurt

The nutritional value of yogurt is derived from milk. The value of milk and its products was recognized long ago. An example is a reference to the Promised Land as a land ''flowing with milk and honey'' (62). Yogurt has a higher nutrient density at 13-18% milk solids compared to milk at 12.3%. In literature, a number of healthful benefits have been assigned against various disease states (16,19,62). These disease states are colitis, constipation, various kinds of diarrhea, gastric acidity, gastroenteritis, indigestion, intoxication (bacterial toxins), diabetes, hypercholesteremia, kidney and bladder disorders, lactose intolerance, liver and bile disorders, obesity, skin disorders, tuberculosis, vaginitis and urinary tract infections, cancer prevention, prevention and treatment of Helicobacter pylori gastritis, and irritable bowel syndrome. Yogurt is a good source of calcium. One 8 oz serving will provide about 400 mg of calcium (62). It is recommended that milk and yogurt be consumed with oxalate-rich foods such as spinach and soy products. A diet of products rich in calcium decreases urinary oxalate and helps in the prevention of kidney stones (62).

In 1908, in an explanation of his longevity-without-aging theory, Metchinkoff stated that lactic acid bacteria in sour milk displaced toxin-producing bacteria, thus promoting health. At about the same time (1899), Tisser isolated bifidobacteria from the stools of infants and recommended administration of the same to infants suffering from diarrhea (63). Since that time, numerous studies on the use of lactic cultures in foods have continued. These studies have yielded variable results with regard to the health benefits of probiotics. In the past few years, the use of probiotics in yogurt and fermented products have exploded.

A. Definition of Probiotics

Probiotics are a live microbial feed supplement that beneficially affects the host animal by improving its intestinal microbial balance (64). Considerable work is going on in which combination of molecular biology techniques and phenotypical analysis is being used in deciphering the participation and the role played by the probiotics. This will certainly challenge our current concepts.

B. Benefits of Probiotics

Health benefits imparted by probiotic bacteria are strain-specific and not species-or genus-specific. The strains Lactobacillus rhamnosus GG (Valio), Saccharomyces boulardii (Bio-

codex), Lactobacillusparacasei Shirota (Yakult), and Bifidobacterium BB12 (Chr Hansen), have the strongest human health efficacy data, against some or all of lactose intolerance; rotaviral diarrhea; antibiotic-associated diarrhea and the associated Clostridium difficile diarrhea; and some other bacterial diarrheas and infections. Traveler's diarrhea may also be alleviated in some people by Lactobacillus GG. It is not yet possible to relate probiotic intake to prevention of bowel cancer in humans. However, there is stronger evidence for effects of the Shirota strain on bladder cancer in the Japanese population (65). Probiotics may also help in atopic eczema, irritable bowel syndrome, inflammatory bowel disease, Helicobacter pylori infections, and possibly in arthritic conditions (65). For early evaluation and comparison of probiotics in vitro, laboratory tests and standards for good clinical tests must be followed. These are listed in Tables 5 and 6. Over the years, many studies were done with different strains but there are only a few strains for which peer-reviewed published evidence from human clinical trials is available. The evidence from these trials is presented in Table 7 (65). In a 15-month-long study with human subjects consuming probiotic milk products containing Lb. rhamnosus DR20, it was observed that population of Lactobacillus and enterococcal content of the feces was transiently altered without markedly affecting biochemical or other bacteriological factors (66). Based on their work, the authors have proposed a new definition: ''Probiotics contain microbial cells which transit the gastrointestinal tract and which in doing so, benefit the health of the consumer.'' This definition certainly calls into question some of the criteria set forth in Tables 5 and 6 for an organism to qualify as a probiotic.

It is generally agreed that Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, the yogurt bacteria, do not adhere to the mucosal surfaces of the intestinal tract during their transit through the gut (5,67,68). These organisms do not survive in the human gastrointestinal tract because of their low tolerance for bile salts (69,70). A combination of molecular methods have been used for the detection of lactic acid bacteria in the human gastrointestinal tract (41). Lactobacillus delbrueckii group was detected but not Streptococcus thermophilus (41). Recent feeding trials with Gottingen minipigs appear to indicate that these yogurt organisms do survive the passage to the terminal ileum (71). The numbers detected (106 to 107 cfu/g of chyme) are considered to be high enough for these organisms to be considered as potential probiotics. It is believed that the gut is home to 400-500 species of organisms. Recent studies have indicated that the

Table 5 In Vitro Tests for Probiotic Bacteria as Indicators of Human Health

Acid/pepsin (pH 2.0 for 105 min) and bile tolerance (physiological concentrations) Fast growth rate and active metabolism

Acid production (rate of production, types of acids, and regio-specificity)

Production of bacteriocins

Immune response (local and systemic)

Vitamin production (e.g., folate, B group)

Colonization and adhesion (Caco2 and HT29 cells, mucus)

Antibiotic sensitivity patterns

Bile deconjugation

Inhibition of pathogens (e.g., Salmonella typhimurium, Escherichia coli,

Clostridium difficile, Clostridium perfringens, Candida albicans) Fecal enzyme concentrations (e.g., h-glucuronidase) Safe history

Table 6 Standards for Good Clinical Trials on Probiotics

Each strain identified by molecular methods and properties fully documented. Extrapolation of data from strains of the same species is not acceptable.

Double-blind, placebo-controlled randomized studies, with a ''crossover'' design where possible. Adequate number of treatments to avoid "confounding" of data by other ingredients and organisms.

Study end points should be unequivocally stated prior to commencement. Currently acceptable statistical tests must be used. Confirmation of findings by one or more independent research groups. Publication in peer-reviewed journals of international standing.

Efficacy of the probiotic strain in different products should be assessed separately for each product against each health condition.

total microbiota of each adult individual had a unique pattern reflecting differences in composition that are partly dependent on the host genotype (41). What happens to the balance of these in different human beings with different dietary habits and ages is not known. How the organisms become established in the gut in different segments is also not known. Much useful information is coming forth as this matter is studied seriously.

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