Epigenetic reversal of some inhibitory histone marks at the promoter region of the enzyme? Driven by some yet-to-be-discovered signaling cascade induced by reintroduction of lactose and proliferation of lactose loving microbes?

Just spitballing, this is pure speculation, but is plausible.

I have noticed pseudosciences are becoming fashionable again.

Well, as you probably know, genes are regulated more or less according to environmental cues all the time, including being very much subject to presence and behavior of commensals/parasites/symbionts.

I don't know any specific data whatsoever about inducing lactase, but it seems perfectly plausible that microorganisms such as lactic acid bacteria could interact with the system.

This has been studied, but the results tend to argue that it's purely genetic, eg)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6669050/

> Studies that have measured changes in endogenous lactase activity after an intervention period consistently show a lack of enzyme induction, suggesting that lactose intake does not affect an individual's lactase activity. Although these studies are scarce and have relatively few subjects, data from cross-sectional studies support the theory of purely genetic regulation

That's interesting, and thank you for bringing it up.

What I was speculating about though, and what may be more relevant for the (Science) article, also includes things along the lines of a microbe manipulating the host gene expression to improve its living conditions. For instance a species that derives energy by fermenting glucose to lactate might under certain conditions have better benefit from that piece of metabolism if it can also make the host act as a sink for the product.

Again, not sure if that particular example is real or documented, but the idea has some consequences for the sort of adaptations and connections one might find.

I thought it would be that as well. But I went quite a while, probably almost two years, without ingesting milk at all. And I just had my 34th birthday two weeks ago. I figured there’s just no way I can still process milk and it turns out I absolutely can. No issues at all. Reading into it, it seems to be genetic.

https://medlineplus.gov/genetics/condition/lactose-intoleran...

You just either get the right genes or you don’t. I’m curious as to how long into my life I’ll be able to drink straight cow milk and be fine. Still got a bit of experimental time left. Hopefully, anyway.

Also both my parents can drink milk without issues.

You can't say something is "perfectly plausible" right after admitting you don't know any data supporting that hypothesis.

The negative symptoms of lactose intolerance is exactly due to microbes in the gut digesting lactose and producing metabolic byproducts such as gas. People who aren't lactose intolerant are free of such symptoms because lactose is digested with enzymes produced by the body before it reaches the gut thus depriving gut microbes of sufficient quantities of lactose to cause negative symptoms.

You could always ingest other bacteria that can digest and break down lactose and rely on probiotics to digest the lactose for you instead of having a gene.

That's what ruminents do anyways, you need a steady colony of these helpful bacteria or you lose the tolerance. While that seems troublesome, for many animals that's how metabolism works for their entire life.