Gut

Digestion and the microbiome

Promoted into the core roster in this version. It outranks four tracks that were ahead of it in your saves, and it was the one filed under later.

The honest limit

The microbiome is real, important, and the field is much earlier than the products built on it suggest. Most of the striking findings are in mice. Most of the human work is correlational. Probiotic products are largely sold ahead of the evidence, with a few specific strain-and-condition exceptions. Fibre and fermented food are the boring parts that hold up best.

All 6 stepstrack 10 of 11

Your verdict

When the track is done, write what you are keeping and what you are dropping. A verdict of "none of it, and here is why" is a real result and the most useful kind. It shows up on the Health page with the others.

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Where this practice came from

The Bulgarians

Health 10: The Second Brain · Episode 1 · the first of eight stories

Élie Metchnikoff, who lived from 1845 to 1916 and shared the 1908 Nobel Prize in Physiology or Medicine for his work on immunity, spent a considerable part of his later career on a theory about yoghurt, set out in his 1907 book The Prolongation of Life.

His argument, in outline, was that the large intestine was a liability. It held a mass of putrefying material and bacteria whose by-products, he thought, slowly poisoned the body and caused ageing. He used the phrase intestinal autointoxication, and it was a serious position held by serious people.

And he noticed, or believed he noticed, that certain rural populations in Bulgaria and the Caucasus contained an unusual number of very old people, and that these populations consumed a great deal of fermented milk.

So the theory assembled itself. The lactic acid bacteria in the soured milk colonise the gut, displace the putrefying organisms, reduce the poisoning, and extend life.

There are two things I want to draw out of this, because they set up the entire season.

The first is that the core intuition was not wrong. The idea that the bacteria in your intestine matter to your health, that they are not passive inhabitants but active participants, and that what you eat shapes which ones are there, is now mainstream and well supported. Metchnikoff got the direction right a century before the tools existed to check.

The second is that the specific claims were built on almost nothing. The longevity observation rested on reported ages in populations without reliable birth records, in a period when age exaggeration among the very old was common and sometimes systematic. The mechanism was speculative. And the recommendation, eat this specific fermented product, ran far ahead of anything demonstrated.

That combination, a correct general intuition plus dramatically overstated specifics plus a product, is the shape of this field for the next hundred and twenty years.

It also produced an immediate commercial response. Fermented milk products were marketed on the strength of the theory within Metchnikoff's own lifetime, and the yoghurt industry has never really stopped citing some version of it.

The gut has been a place people sell things about for a very long time.

Next episode: animals with nothing living inside them.

Wren

The record says this much; the scenes between are imagined. The rest of the season, and the sourcing behind it, runs a few times a week at Wren. One page for the whole practice: /practice/.