Most people are shocked – and a bit dismayed – to hear a basic fact about human biology.

The fairly amazing fact is that we all have about 10 times more microbes than cells in our bodies … 100 trillion bacteria versus 10 trillion human cells.
The term probiotic means "for life,” and refers to a living microorganism which, when consumed in adequate amounts, confers a health benefit.
The related term "prebiotic” refers to vegetable fibers such as FOS and inulin, which stimulate growth of one or more probiotic bacteria.

Probiotics and prebiotics are collectively called "synbiotics”, which simply means organisms and food factors that bring health benefits to we humans.

The beneficial Lactobacilli and Bifidobacterium cultures commonly used to ferment milk into yogurt – and to ferment raw vegetables – are prime examples of probiotic microbes found in traditional foods.
Now, an innovative, encouraging study in mice suggests that a specific strain of a common probiotic culture called Lactobacillus rhamnosus may help support healthy mood.
Probiotic alleviated anxiety and depression in mice
Researchers from University College in Cork, Ireland, report some intriguing results from a controlled trial in mice (Bravo JA et al. 2011).
The results, published in the Proceedings of the National Academy of Sciences, suggest that a specific probiotic bacteria strain may alter brain neurochemistry and thereby alleviate anxiety and depression-related disorders.
In their study, mice fed L. rhamnosus JB-1 displayed fewer behaviors related to anxiety and depression.
Moreover, mice fed the bacteria had significantly lower levels of the stress-related hormone corticosterone.
Chronically elevated levels of corticosterone raise levels of inflammation in the body, and, like its companion hormone cortisol, may harm the brain's memory capacity.
The authors noted the increasing, but largely indirect, evidence pointing to links between intestinal microbes and the central nervous system.
According to the lead author, Professor John Cryan, "These findings highlight the important role that gut bacteria play in the bi-directional communication between the gut and the brain … and opens up the intriguing opportunity of developing unique microbial-based  strategies for treatment for stress-related psychiatric disorders such as anxiety and depression.”
Cryan and his team showed that feeding mice the Lactobacillus strain caused changes in the expression of receptors for the neurotransmitter GABA in their brains – the first time that probiotics have been shown to exert a direct effect on brain chemistry.
As the Irish team wrote, "Together, these findings highlight the important role of bacteria in the bi-directional communication of the gut–brain axis and suggest that certain organisms may prove to be useful therapeutic adjuncts in stress-related disorders such as anxiety and depression” (Bravo JA et al. 2011).
The study also revealed the pathway through which the probiotic bacteria affected brain chemistry.
The neuro-chemical and behavioral effects were not seen in mice from which the vagus nerve – which runs between the gut and the brain – had been removed.
This finding pinpointed the vagus nerve as the major communication pathway between the brain and probiotic bacteria in the gut.
While these findings only apply to mice, and to one specific strain of probiotic bacteria, they seem to add another reason to enjoy yogurt, kefir, and cultured vegetables.

What do we know about probiotic health benefits?
Following a course of antibiotics designed to kill bad bugs, doctors often prescribe probiotic supplements to restore the patient's "good” bacteria.

Probiotics appear to improve the balance of intestinal microbes or "microflora” by competing with and reducing populations of pathogenic (disease-causing) bacteria or yeasts (e.g.,Candida albicans).

And there is substantial evidence that certain probiotic cultures exert specific health effects:
  • Reduce gut inflammation
  • Reduce the risk of colon cancer
  • Improve blood cholesterol profiles
  • Prevent and treat uro-genital infections
  • Alleviate irritable bowel syndrome and colitis
  • Prevent and treat "atopic” diseases such as eczema.
  • Reduce the risk of diarrhea with antibiotics and theC. difficilepathogen
We still lack conclusive proof for any of these health benefits. Due to insufficient evidence, the U.S. FDA and the European Food Safety Authority have so far rejected proposed health claims for probiotics (FAO/WHO 2011).

Recent research suggests that it's better to consume multiple strains of probiotic bacteria, because each kind populates a different part of the digestive tract and exerts distinct health effects.

We should note that probiotic cultures in foods or supplements probably do not alter a person's bacterial ecosystem or "enterotype” permanently, because its composition is guided by their personal genetic profile.
All people possess one of three well-defined enterotypes, according to landmark research published in the April, 2011 issue of the journal Nature.

Major yogurt makers have been running ad campaigns focused on claims that their active cultures can alleviate icky digestive ills.

Those claims rest on the results of clinical studies testing the effects of yogurts containing specific bacterial cultures on constipation.

But last year, Dannon agreed to pay 39 states $21 million in fines related to health claims made on the packaging for its Activia yogurt and DanActive dairy drink.

The Federal Trade Commission decided there wasn't enough evidence to back the claims that Activia helps relieve irregularity and that DanActive boosts immunity.

These and other claims surrounding live cultures usually involve one or more kinds of Lactobacilli (lactic acid bacteria) or Bifidobacterium … which research suggests are very important – and perhaps essential – to good health.

In fact, as the authors of a recent study wrote, "Breast milk contains viable lactobacilli and bifidobacteria that might contribute to the initial establishment of the [normal] microbiota in the newborn” (Solís G et al. 2010).

And the new study on mood adds to growing evidence that probiotic bacteria play critical roles in maintaining and enhancing human health.

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