The AhR Actively Manages the Gut Microbiome for Fatness

People think about the gut microbiome as something that is passively managed. Eat some fiber and pro-biotics and hopefully it will grow and bloom.

Whose microbiome is it anyway? Obesity Explained, Episode 5.

Watch the video version below if you prefer:

Hibernators adjust their microbiome to fatten

In the case of hibernating mammals, it is their biological imperative that they shift their microbiome in a way that allows them to fatten up for winter. They shift their microbiome in a way that minimizes diversity and that favors the firmicutes clan over the bacteroidetes clan. The following paper looked at dwarf lemurs. (Greene, 2022) These lemurs are tropical primates – the closest related mammals to us that are known to hibernate. The researchers actually captured the lemurs during the fattening season and again in the spring period when they are most active.

OK, well not actually spring since they’re tropical. It was actually after the dry season ended and they went about their business of leaning out and mating, etc. Lets think of it as spring.

The lemurs totally shift their microbiome between the lean season (emergence) and the fattening season. In the fattening season, diversity declines and firmicutes are favored over bacteriodetes.

The microbiome of obese humans mirrors that of fattening hibernators

This matches the types of bacteria seen in many obese human populations around the world. Consider this study from the Ukraine. (Koliada, 2017) The lean humans (BMI less than 18.5) have less bacteriodetes and more firmicutes than the obese humans (BMI>30). Just like the fattening lemurs.

The AhR actively manages the microbiome for fattening

If you were a hibernator, your biological imperative is to fatten for “winter”. (In the lemur case it’s for the dry season.) Would you take chances on whether your microbiome shifted to a fattening state?

You would actively manage it. I have made the argument that the aryl hydrocarbon receptor is the master regulator of fattening/torpor/hibernation. When the AhR is activated, it actively manages the gut microbiome to favor a fattening state: more firmicutes, less of everything else. (Korecka, 2016) Perhaps it does this by simply allowing the overgrowth of firmicutes.

Active vs. Passive Control of the microbiome

It is plausible that passive approaches at managing the microbiome – dietary fiber, pre-biotics, pro-biotics, etc. – could help control obesity. But the AhR is actively pushing you into a fattening state. Would you take pro-biotics and an antibiotic at the same time? No, you’d wait until the antibiotics were over and THEN you’d take the pro-biotic.

I suspect the case of the AhR works like this. The first step is to get the AhR in check. THEN you can think about pro-biotics and pre-biotics.

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