Turnips green, buttermilk and potatoes. Obesity Explained episodes 8 and 9.

Obesity explained episoeds 8 and 9 are out. They are based on a trio of really great papers that have come out since 2018. Two are in the journal Nature and one is in PNAS, but remarkably all three are publicly available. The papers are (Kim, 2018), (Kim, 2020) and (Huang, 2022).

Kim (2018) shows that the aryl hydrocarbon receptor (AhR) is activated by insulin signalling and translocates to the nucleus to initiate gene transcription after a meal. Later, it’s transcriptional activity is suppressed by bile acid signalling. Bile acids absorbed in the intestine post-prandially activate the FXR, which releases SHP which directly binds to and inactivates the AhR.

This finding turns our understanding of the AhR inside out. It was long thought that the AhR’s main role was to identify and metabolize toxins such as TCDD. This paper places the AhR directly at the center of our endogenous metabolism.

Furthermore, the paper goes on to show that when bile acid signalling breaks down, the AhR can run amock in the liver, leaving PEMT permanently active in the liver. This results in overproduction of phosphatidylcholine, which uses up methyl groups that are needed elsewhere, leading to fatty liver.

The paper further shows that bile acid signalling breaks down in humans with fatty liver, leading to upregulated PEMT.

Kim (2020) goes on to show that vitamin B12 and folate – two B vitamins involved in the methylation cycle – both actively bind to and inactivate the AhR. They show that, in fact, symptoms traditionally associated with folate and B12 deficiency – anemia, fatty liver and neural tube defects – can be caused by activating the AhR with TCDD and prevented with the small bits of B12 and folate that inhibit the AhR.

This puts the AhR at the center of the discussion of methylation and vitamin deficiency.

Lastly, Kim (2022 (different Kim)) shows that the development of obesity in both mice and humans is associated with low vitamin B6 levels. Low B6 leads to a buildup of the tryptophan metabolite kynurenine which is a well known activator of the AhR. The AhR upregulates IDO1 in adipose tissue, leading to more tryptohpan being converted to kynurenine, a positive feedback loop leading to fatness.

Three different approaches are used with varying effect to break mice given a fattening diet out of this feedback loop: adipose specific deletion of IDO1, adipose specific deletion of the AhR or vitamin B6 supplementation. Vitamin B6 supplementation turns out to be the best strategy!

This nexus of B vitamins and AhR biology give me a perfect platform to discuss traditional diets and the ancestral wisdom inherent in balancing B vitamin intake.

Here are the historical documents I’ve taken the data about traditional Diets from:

Cornell Extension Bulletin 478. “Living From The Farm.”

“Present Day Diets in the United States” by Hazel K. Stiebeling and Callie Mae Coons. From the 1939 USDA Yearbook of Agriculture

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Please enjoy, “Turnip Greens and Buttermilk” and “Meat and Potatoes?”.

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