In a normal human being there can be anywhere from 40 trillion to 100 trillion microbial cells at any given time – most of which reside in the digestive tract. With around 30 trillion human cells, this means that your bacterial cells can outnumber your own significantly! With populations in such high numbers, you may be correct in assuming that taking a simple probiotic capsule once a day is not enough to actually benefit you. The key, according to most experts, is to take a therapeutic dose of probiotics – but just how much constitutes a therapeutic dose & does this change depending on the person?

Well, let’s start off by saying that the science is relatively unclear on this subject – & that’s probably because we’re dealing with so many unique individuals that it’s hard to give a blanket recommendation based upon a variety of animal studies conducted thus far.

Now, let’s play with the notion that your digestive system hosts around 10 trillion microbial cells (that’s around ten-million-million depending on the format you use for ‘trillion’). In order to have any type of actual impact on that population we can assume that some pretty high numbers of bacteria are needed. When measuring numbers of bacteria we use the unit CFU which means colony forming units – most probiotic formulations tend to deliver between 1 billion CFUs all the way up to 1 trillion CFUs, & when we put that into a percentage of your total microbes within the gut – we’re only looking at between 1 & 10% of that.

The true test of a probiotic it seems is the long term alteration of the gut microbiome. If we’re only taking in 1% of its total numbers, we may not see an actual change at all hence the popular saying now that probiotics are a waste of money. When we start looking at doses of around 10% of our gut microbiome, that’s when the potential for a more permanent & clinical effect becomes promising. This is where we start to enter the realm of a therapeutic dose of probiotics – that is, an amount high enough to see a clinical benefit to the host.

More science is definitely needed on the topic, with factors such as mode of delivery, the time of day when administered, the specific strains & mix of strains used, & the health status of the subject all having a strong influence over the outcomes.

Dr Natasha Campbell McBride, creator of the GAPS diet, maintains that those with significant health issues are much better off starting small when it comes to probiotic treatment as even healthy bacteria, if given the opportunity to move into the bloodstream, will cause a negative immune reaction.

So what can you take away from this?

At this stage, if you’re only interested in taking probiotics prophylactically as opposed to treating an illness, its safe to say that those probiotics with higher CFU counts are much more beneficial to take.

If you do have an illness, try & find a good integrative GP to work out how much of a given probiotic will work for you.


**update: scientists are now deliberating over the exact ratios of bacterial cells to human cells – rather than the originally though 10-1 ratio – it could actually be a lot smaller**

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Resources for this article include

O’Hara A.M., Shanahan F. (2006) The gut flora as a forgotten organ EMBO Rep 7: 688–693 [PMC free article]  [PubMed]