Several sources including IFLScience have set the Social Meeja Intertubes alight with the fascinating suggestion that your gut microbiota control your appetite.

[NOTE: This article was written in the United Arab Emirates where is a banned website; in the UAE homosexuality is illegal, rape victims are jailed, it’s customary to wipe your arse with your hand but bacon is unclean. Go figure. The inevitable typos will be corrected on Recto Fossa’s return to more liberal climes.]

This is fascinating, biologically plausible and has to be the best excuse for refractory lack of dietary self-control yet, beating classics like “it’s not me, it’s hormonal” or “I’m big boned” into a cocked hat. And isn’t it funny how people who describe themselves as being ‘big boned’ always seem to have prodigious amounts of fat attached to them? My femur is about 20″ long – which is a pretty big bone by any standards – yet I have a BMI of 21.5 which is apparently reasonably healthy. (And yes, I did have to work that out as BMI isn’t a metric I really bother about much. My body’s a temple, I tell you. It’s just on a long lease to drunken, sarcastic demons).

So, what’s the score, then?

The central premise in ‘Is eating behaviour manipulated by the gastrointestinal microbiota? Evolutionary pressures and potential mechanismsis that our those of Our Microbial Overlords that inhabit the gut manipulate us into eating what they like, and if it’s pizza we crave it’s because our gut microbes are producing neurochemicals that make us remember Domino’s phone number. The article argues that – essentially – we have little free will regarding food choice, it’s Our Microbial Overlords telling us what they want to eat. I’ve written before how bugs carried by the Fluffy Vermin will reprogram your brain to crash your car – but there are other fascinating examples our being unwitting stooges for Our Microbial Overlords. This goes beyond just providing them with food and social care and this is a subject that is bound to be the subjects of future posts.

It’s true that our gut bacteria do speak to each other, they do send out behaviour altering chemicals and there are plenty of animal models where this has been demonstrated. But I think there are some logical flaws in this article and some of the assertions are stretchy, to say the least. It’s still fascinating but let’s look at the abstract – and, as ever, I haven’t found any journalists who’ve read beyond that bit. It says:

Microbes in the gastrointestinal tract are under selective pressure to manipulate host eating behaviour to increase their fitness, sometimes at the expense of host fitness.

Are they under a selective pressure? You pick up your gut bacteria shortly after birth in a very orderly sequence and their makeup remains very, very constant throughout your life – unless a major catastrophe like carpet-bombing them with broad-spectrum antibiotics occurs. That our gut microbiota stay soooooooo constant does not reek of selective pressure to me.

Yes, their proportions change depending on diet. And there are various illnesses which correlate with a disturbed microbiome – but it’s complicated and very easy to put the causative cart before the horse – but correlation is not causation as any fule kno. I’ve written before about the proportions of Firmicutes and Bacteriodetes in the gut of the obese and the svelte; in this article they do get this bit arse backwards, though. They correctly note that Bateriodetes prefer fat as a food but ignore the inconvenient fact that if you have a lardy arse it has fewer Bacteroidetes up it – which kind of scuppers their position a little. Another good example is the correlation between autism and gut symptoms in children which some (including Andrew Jeremy Wakefield) have used to peddle dietary ‘cures’ for autism – as if bringing up an autistic child isn’t challenging enough without getting them to eat a restrictive diet too.

(As a side point it some have argued that we have ‘stopped evolving’– this is Utter, Utter Bollocks (µ²B); we have managed – in the developed world at least – to control many of the selective pressures that would usually drive evolution by our use of technology, medicine etc: this is not the same thing.)

Back to the point. They continue:

Microbes may do this through two potential strategies: (i) generating cravings for foods that they specialize on or foods that suppress their competitors, or (ii) inducing dysphoria until we eat foods that enhance their fitness. We review several potential mechanisms for microbial control over eating behaviour including microbial influence on reward and satiety pathways, production of toxins that alter mood, changes to receptors including taste receptors, and hijacking of the vagus nerve, the neural axis between the gut and the brain.

This smacks of begging the question (in the logical, not casual sense). Their assumption their premise is true (see their first para) is taken as evidence for it. And yes, some claims here are true, it’s their interpretation that goes a bit far. More:

Because microbiota are easily manipulatable by prebiotics, probiotics, antibiotics, faecal transplants, and dietary changes, altering our microbiota offers a tractable approach to otherwise intractable problems of obesity and unhealthy eating.

Errrrmmmmm… no. Your microbiota are not easily manipulated (I assume that is what the authors mean by ‘manipulatable’) except by the antibiotic means described above. The best argument I’ve seen from the probiotics lobby ‘proving’ probiotics manipulate our microbiota is that if you feed people large amounts of Lactobacillus you get more Lactobacillus in their poop.

This assumes (a) that microbiological supplantation is beneficial (no data for that anywhere), and (b) the fact you poop more Lactobacillus shows they have taken over – when it actually proves the opposite. If you take in a bolus of Lactobacillus and a load of them subsequently come out of your Chocolate Starfish this is not only entirely unremarkable – it also tends to demonstrate your microbiota are singularly untroubled by your efforts to reprogramme your internal ecosystem and if anything they have not taken hold. Given the thousands of species of bacteria in your gut microbiome (most of which you can’t even grow in a lab) taking probiotic supplements is like planting a few daisies in a rainforest. Yes, there is data suggestive that live yoghurts are beneficial if you’re in hospital being fed a ton of antibiotics but this is a very different circumstance.

So, I think they are over-egging the pudding somewhat here – which is fine because they are presenting a hypothesis. It is a very interesting paper (to a geek like me, at least) but don’t take it as fact; it’s just an interesting piece of microbiological omphaloskepsis.

The idea that one can manipulate one’s weight using probiotics or eat oneself healthy using supplements is an attractive one – all gain, no pain – but isn’t new and one I’ve written about before here and elsewhere. I foresee the probiotics woo-peddlers citing this discussion piece as ‘fact’ and using it to con the <ahem> ‘well upholstered’ among us into buying their Utter Utter Bollocks (µ²B).

This slant that it’s the bugs that are manipulating us is, as I say, interesting and there’s a lot of fascinating and plausible-sounding circumstantial evidence. But it’s just circumstantial for now. Also the authors are not microbiologists, they medics with a taste for ‘evolutionary psychology’– which is at best ‘interesting’ and at worst pure Confirmation Bias Bollocks Woo (CFB²W – new FLA! Cool!).

They talk about ‘the struggle to resist cravings for foods that are high in sugar, fat and salt’. This is driven by physiology and we are hard-wired to go for high-energy, high salt foods. This is why a big, juicy burger with bacon is sooooo appealing – even to many Chop Dodgers – but these resources are no longer scarce. The ‘struggle’ to resist them is more down to a lack of self- control in a time of abundance. I’m usually more than happy to accredit Our Microbial Overlords for most things – but in this case I’m still to be convinced. As I say, well worth a read and an interesting synthesis – but not proven!

And if anyone tries to persuade you their ‘probiotic’ supplement will make you thin, just politely punch them and go on your way.

Again, this is a really interesting article and the whole notion if manipulating one’s microbiota has real potential. But don’t believe the hype. Yet.