Are dead probiotics still good for you?

This post isn’t, strictly speaking, about store-bought probiotic pills, but rather how the dead microbes present on fermented foods seem to continue to confer benefits when eaten, despite being dead. That said, the main paper I cite below may give you some confidence that a pill form of probiotics can confer benefits even if you’re not totally sure about that “live culture” count on the label. Read on!

How vegetable fermentation works

When one submerges fresh vegetables into a brine or tamps shredded, salted cabbage down until it’s covered in its own juices, a relatively well-understood sequence of microbial activity follows. According to Holzapfel, Schillinger, and Buckenhuskes in Handbook of Fermented Functional Foods, Second Edition, the microbial succession of sauerkraut follows this path: 

  1. Leuconostoc mesenteroides “…initiates the fermentation, as it is present at sufficiently high numbers and is well adapted to the substrate [sauerkraut]. Under ideal conditions, e.g., as in cabbage juice, this species may achieve a maximum cell population of more than 108 colony forming units (CFU)/ml after 12 to 14 h incubation. Lc. mesenteroides produces lactic and acetic acids that quickly lower the pH.” Strains of Lc. fallax have also been identified in this initial stage of fermentation.
  2. As the pH falls, Leuconostoc species, which are not especially acid-tolerant, give way to Lactobacillus brevis. According to the authors, “Depending on the temperature, the first two stages of sauerkraut fermentation are completed after 3 to 6 d. During that time, the concentration of lactic acid will increase up to approximately 1%.”
  3. As the pH further drops and environmental factors like dissolved oxygen are used up, “homofermentative lactobacilli become the predominant organisms,” including minor populations of streptococci and pediococci. Dominant species at this stage include Lactobacillus plantarum, Lb. sakei and Lb. curvatus. Total acid content in this phase increases to 1.5% to 2.0%. 
  4. The dominant species in the fourth stage is Lb. brevis, which (with some related species) is able to metabolize some of the metabolic remnants of the fermentation process itself. Acid content in this stage increases up to 2.5%, and pH falls to as low as 3.4. 

As each stage rolls into the next, the dominant organisms in the preceding stage either die off or become dormant. These organisms are one of the components of the sedimentation that occurs at the bottom of a fermentation jar. 

Fun fact: These organisms are also believed to be probiotic, i.e., they have beneficial effects on the human body when consumed. So what happens when they’re already dead? 

Dead probiotics are probiotics too! (At least, it seems so)

There is a ton of research on “heat-killed” organisms and their effects on the immune systems of mice, humans, human cells, and so on. (Dive in here to start scanning papers if that’s your thing.) I’ll quote from one particular paper relevant to this question, The probiotic paradox: live and dead cells are biological response modifiers, by Clifford A. Adams, from Nutrition Research Reviews 23, published in 2010. 

There are specific gastrointestinal effects of probiotics such as alleviating inflammatory bowel disease, reducing acute diarrhoea in children, inhibiting Salmonella and Helicobacter pylori, removing cholesterol, secreting enzymes and bacteriocins and immunomodulation. However, many of the effects obtained from viable cells of probiotics are also obtained from populations of dead cells. Heat-killed cells of Enterococcus faecalis stimulate the gastrointestinal immune system in chicks. Dead bifidobacteria induce significant increases in TNF-a production. Administration of heat-killed E. faecalis to healthy dogs increases neutrophil phagocytes. The probiotic paradox is that both live and dead cells in probiotic products can generate beneficial biological responses.

Adams’s paper is complex for the non-scientist (raises hand), but looks at research that shows that heat-killed Lactobacillus can speed the elimination of certain pathogens from the gut of newly-hatched chicks, possibly by stimulating an “inflammatory reaction involving macrophages.” Another paper he cites found that heat-killed bifidobacteria “were able to induce pronounced increases, of up to several hundred-fold, in the production of TNF-α [tumor necrosis factor alpha, an immune cell regulator] compared with that of controls.” 

Adams cites a variety of other conditions where dead probiotics have shown benefit, including pain reduction by L. reuteri, allergy relief by various Lactobacillus strains, and cholesterol lowering effects from Lactobacillus strains. In his Conclusions, Adams writes, “Many of the biological responses found with both live and dead probiotics are not antimicrobial effects but are, rather, immunomodulating effects.” He continues, “The effect of probiotics could thus be a dual one where live probiotic cells might well influence the gastrointestinal microflora and have an immunomodulating effect, whereas the components of dead cells could exert an anti-inflammatory response.” 

Are dead probiotics probiotic, for dummies

I’m not going to lie, the paper is a little above my head. My high school science nerd-dom was decades ago, but what I will say is that based on my lay reading of papers, the picture that has emerged for me is that when we ferment foods, the corpses (or spores, perhaps) of early-stage fermentation organisms are seen by our immune systems and responded to in ways that are favorable to our overall health. 

And this makes sense. Our bodies have a keen interest in every molecule that comes into them from any pathway, and our guts are arrayed with sensors to identify and react to cellular material, chemicals, and more. This is a survival mechanism, and since humans spent most of our evolutionary history eating wild foods that were often probably half-fermented (half-rotted, in another view) anyway, our bodies have learned to recognize these non-pathogens, dead or alive, and react to them in what generally appears to be immune-boosting ways. 

So don’t get hung up on live vs. dead (or raw vs. cooked) when it comes to fermented foods. Prepare healthy foods in whatever way you and yours enjoy. Your body knows what to do with them. 

(cover image by Riccardoariotti)

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