Two 2022 papers on betaine’s effects, starting with a review:

“Rodent studies provide evidence that betaine effectively limits many diabetes-related disturbances.

• Betaine therapy improves glucose tolerance and insulin action, which is strongly associated with changes in insulin-sensitive tissues, such as skeletal muscle, adipose tissue, and liver.
• Betaine supplementation positively affects multiple genes, which expression is dysregulated in diabetes.
• AMP-activated protein kinase is thought to play a central role in the mechanism underlying anti-diabetic betaine action.
• Studies with animal models of type 2 diabetes have shown that betaine exerts anti-inflammatory and anti-oxidant effects, and also alleviates endoplasmic reticulum stress.

These changes contribute to improved insulin sensitivity and better blood glucose clearance. Results of animal studies encourage exploration of therapeutic betaine efficacy in humans with type 2 diabetes.”

https://www.sciencedirect.com/science/article/pii/S0753332222003353 “The anti-diabetic potential of betaine. Mechanisms of action in rodent models of type 2 diabetes”

Reference 31 was a human study:

“Few studies on humans have comprehensively evaluated intake composition of methyl-donor nutrients choline, betaine, and folate in relation to visceral obesity (VOB)-related hepatic steatosis (HS), the hallmark of non-alcoholic fatty liver diseases.

• Total choline intake was the most significant dietary determinant of HS in patients with VOB.
• Combined high intake of choline and betaine, but not folate, was associated with an 81% reduction in VOB-related HS.
• High betaine supplementation could substitute for choline and folate to normalize homocysteine levels under methyl donor methionine-restriction conditions.
• Preformed betaine intake from whole-grain foods and vegetables can lower obesity-increased choline and folate requirements by sparing choline oxidation for betaine synthesis and folate for methyl donor conversion in one-carbon metabolism.

Our data suggest that combined dietary intake of choline and betaine reduces the VOB-related HS risk in a threshold-dependent manner.”

https://www.mdpi.com/2072-6643/14/2/261/htm “Optimal Dietary Intake Composition of Choline and Betaine Is Associated with Minimized Visceral Obesity-Related Hepatic Steatosis in a Case-Control Study”

Increasing betaine intake to lower choline and folate requirements was similar to an idea in Treating psychopathological symptoms will somehow resolve causes? that:

“Such positive effects of taurine on glutathione levels may be explained by the fact that cysteine is the essential precursor to both metabolites, whereby taurine supplementation may drive metabolism of cysteine towards GSH synthesis.”

I came across this first paper by it citing All about the betaine, Part 2:

“This review focuses on biological and beneficial effects of dietary betaine (trimethylglycine), a naturally occurring and crucial methyl donor that restores methionine homeostasis in cells. Betaine is endogenously synthesized through metabolism of choline, or exogenously consumed through dietary intake.

Human intervention studies showed no adverse effects with 4 g/day supplemental administration of betaine in healthy subjects. However, overweight subjects with metabolic syndrome showed a significant increase in total and LDL-cholesterol concentrations. These effects were not observed with 3 g/day of betaine administration.

Betaine exerts significant therapeutic and biological effects that are potentially beneficial for alleviating a diverse number of human diseases and conditions.”

https://www.mdpi.com/2079-7737/10/6/456/htm “Beneficial Effects of Betaine: A Comprehensive Review”