Nutritional Benefits of Yogurt

One of great all-time television commercials aired in the late 1970s and featured an interview with a Russian centagenerian who claimed that his longevity was due to his daily consumption of yogurt. When asked who got him started on his yogurt regimen, he proudly stated it was his mother, who's smiling face then moves into the television frame. Indeed, the popularity of yogurt, as implied by this advertisement and as mentioned previously in this chapter (Box 4-1), has long been due, in large part, to the purported health benefits ascribed to yogurt consumption. In fact, this notion of yogurt as an elixir that fends off aging and promotes human health originated at least a century ago, when the Russian immunologist and Nobel laureate Elie (Elia or Ilya) Metch-nikoff published The Prolongation of Life in 1906.As Metchnikoff and many other microbi-ologists have since reported, it is the specific bacteria that either conduct the fermentation (i.e., S. thermophilus and L. delbrueckii subsp. bulgaricus) or that are added to yogurt in the form of culture adjuncts (e.g., lactobacilli and bifidobacteria) that are responsible for the desirable health benefits of yogurt.

Of course, yogurt has nutritional properties other than those derived from the culture organisms. A single 170 g (6 ounce) serving of plain, nonfat yogurt contains about 170 calories and supplies 18% of the Daily Value requirements for protein, 30% for calcium, and 20% for vitamin B12. Still, these are the same nutrient levels one would get from milk (provided one had accounted for the milk solids normally added to the yogurt mix). Although there are reports that yogurt contains more vitamins than the milk from which it was made (due to microbial biosynthesis), these increases do not appear to be significant. Thus, if indeed yogurt has an enhanced nutritional quality compared to milk, those differences must be due to the microorganisms found in yogurt. As noted earlier, there are many health claims ascribed to yogurt, and especially the probiotic bacteria added as nutritional adjuncts. Among these claims are that these bacteria are anti-cholesterolemic and anti-tumorigenic, enhance mineral absorption, promote gastrointestinal health, and reduce the incidence of enteric infections. There are also some rather surprising health benefits that yogurt may provide (Box 4-4). Despite volumes of research on these claims, however, few have been unequivocally established (Box 4-5.).

Perhaps the health benefit that is most generally accepted is the claim that yogurt organisms can reduce the symptoms associated with lactose intolerance. Lactose intolerance is a condition that is characterized by the inability of certain individuals to digest lactose. Its specific cause is due to the absence of the enzyme p-galactosidase, which is ordinarily produced and secreted by the cells that line the small intestine (Figure 4-4). In individuals expressing p-galactosidase, lactose is hydrolyzed and the glucose and galactose are absorbed across the epithelial cells and eventually enter into the blood stream (in the case of galactose, only after conversion to glucose in the liver). If p-galactosidase is not produced in sufficient levels, however, the lactose remains undigested and is not absorbed. Instead, it passes to the large intestine, where it either causes an increase in water adsorption into the colon (via osmotic forces) or is fermented by colonic anaerobes. The resulting symptoms can include diarrhea, gas, and bloating, leading many lactose intolerant individuals to omit milk and dairy products from their diet. Lactose intolerance has a genetic basis, affecting African, Asian, American Indian, and other non-Caucasian populations far more frequently than Caucasian groups.These individuals could typically tolerate lactose (i.e., milk) when young, but lose this ability during adulthood. As many as 50 million people in the United States may be lactose intolerant.

It has often been noted that lactose-intolerant individuals could consume yogurt without ill effect. It is now known that the bacteria in

Box 4—4. Stressed out? Have a cup of yogurt and relax

It's exam time and you're feeling a bit stressed, maybe even depressed. How can you possibly relax so that you can study effectively? The solution is not have yet another cup of coffee, but to have a cup of yogurt (yes, yogurt) instead. At least, that's the suggestion made recently by a research group at the Consejo Superior de Investigaciones Cientificas research institute in Spain (Marcos et al., 2004).

In this study, two groups of college students were fed, on a daily basis, either plain milk or milk fermented with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus (i.e.,yogurt bacteria),plus a probiotic organism,Lactobacillus casei.The study began three weeks prior to the start of an annual college testing period and continued for the three weeks during which the tests were administered. Consumption of other fermented milk products was not allowed, although no other dietary restrictions were imposed.

Various indicators of stress and anxiety were measured at the start (baseline) and at the end of the study. Anxiety self-report tests and analyses for lymphocytes, phagocytic activity, and serum cortisol levels were included.The latter analyses can be used to assess the effect of stress on the immune response of the subject. Although no significant differences were reported between the anxiety scores for the control (milk) and treatment groups (fermented milk), differences were observed for other measures. Specifically, the total lymphocyte numbers were higher in the treatment group, as was the number of CD56 cells (a specific subset or type of "killer cell" lymphocytes).

Because lymphocytes, and CD56 cells in particular, are part of the host defense system for fighting infections, the lower levels in the control group (compared to treatment subjects) may indicate that these subjects could be exposed to a greater risk of infection. Conversely, the yogurt-eating group may have a more resilient immune system, and, therefore, are better prepared to tolerate stress.

Recently, it was suggested that probiotic bacteria may also be effective at alleviating symptoms related to major depressive disorder or MDD (Logan and Katzman, 2005).This hypothesis is based, according to the authors, on several observations. First, there appears to be a relatively high correlation between the number of individuals who suffer from MDD and who also have intestinal disorders, atopic disease, endometriosis, malabsorption syndromes, small intestine bacterial overgrowth syndrome, and other chronic diseases. Second, the intestinal flora in these patients often contain lower levels of lactobacilli and bifidobacteria, and stress can lead to even greater long-term reductions in these organisms.Third, as the authors note, the intestinal tract is "a meeting place of nerves, microorganisms, and immune cells."Thus, the microflora, by virtue of their ability to produce various neurochemicals and to influence host production of cy-tokines and other bioactive compounds, may have intimate interactions with the immune and central nervous system.

Therefore, while more studies are certainly needed to establish the relationship between pro-biotic bacteria and stress and depression, students may, nonetheless, wish to consider the "chilling" effects of yogurt before the next big test.

References

Logan,A.C., and M. Katzman. 2005. Major depressive disorder: probiotics may be an adjuvant therapy. Med. Hypotheses. 64:533-538.

Marcos,A.,J.Warnberg, E. Nova, S. Gomez,A.Alvarez, R.Alvarez,J.A. Mateos, and J. M. Cobo. 2004.The effect of milk fermented by yogurt cultures plus Lactobacillus casei DN-114001 on the immune response of subjects under academic stress. Eur. J. Nutr. 43:381-389.

Box 4—5. Sorting Through the Pros and Cons of Probiotics

Few research areas in the food and nutritional sciences have received as much worldwide attention and study as the subject of probiotics. Since the late 1980s, there has been a rapid rise in the amount of research being conducted on probiotic microorganisms (Figure 1). Included in the scientific literature are descriptions of the biochemical and physiological properties of pro-biotic bacteria, their taxonomical classifications, criteria for their selection, and data from in vitro and in vivo studies in which physiological effects on hosts have been tested. In addition, there is a plethora of information in the popular press, trade magazines, and especially the Internet, on probiotics.

The challenge for probiotic researchers has been to establish mechanisms and degrees of efficacy of probiotics to prevent or treat particular diseases or conditions.These efforts are complicated by many factors, including:(1) the complexity of microbial communities of the alimentary canal; (2) the variety of potential probiotic organisms to evaluate; (3) the diversity of possible diseases or conditions that might be influenced by probiotics; and (4) the need for plausible mechanisms. Furthermore, there is a need for validated "biomarkers"—biological characteristics that predict underlying health or disease and that can be objectively measured.

Finally, clinical studies, designed like those in the pharmaceutical industry, have not (until recently) been done.In an effort to reconcile these issues, several scientific organizations (e.g., International Scientific Association for Probiotics and Prebiotics, the Canadian Research and Development Centre for Probiotics, and Proeuhealth) have been formed to provide structure to the debate, assess the state of the science, coordinate research, and, ultimately, formulate recommendations.

Which organisms are probiotic?

In truth, the probiotic field is replete with mis-named, mis-characterized, and mis-represented organisms. In some cases, an organism was long ago assigned to a particular species and the name stuck, even though the particular species was no longer officially recognized. As a result, some probiotic-containing foods and formulations contain microorganisms whose value, as a probi-otic, at least, is rather dubious. Moreover, the lack of appropriate regulations or standards (with regard to probiotics in foods) means that for cultured dairy products and other foods that profess to contain probiotic bacteria, there may or may not be organisms present at sufficient levels and viability (assuming they are alive), or that have the expected functional characteristics.

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