Trimethylglycine (TMG): The Methyl Donor You Already Eat

Trimethylglycine, commonly known as betaine, is a naturally occurring compound found in beets, spinach, quinoa, and wheat germ. In the body, it serves two roles: it donates methyl groups to convert homocysteine back to methionine, and it acts as an osmolyte that protects cells from dehydration stress. TMG has gained attention in the supplement world for its ability to lower homocysteine levels and for its potential connections to liver health and exercise performance.

How It Works

TMG donates one of its three methyl groups to homocysteine via the enzyme betaine-homocysteine methyltransferase (BHMT), producing methionine and dimethylglycine.¹ This reaction occurs primarily in the liver and kidneys and represents an alternative to the folate-dependent remethylation pathway. When folate or B12 status is low, the BHMT pathway becomes more important for keeping homocysteine in check.

As an osmolyte, TMG accumulates inside cells to balance osmotic pressure, particularly in the kidney medulla. This protective function is well established in biochemistry, though its clinical relevance for supplement users is less clear.

Homocysteine Lowering: The Strongest Claim

Multiple controlled trials confirm that TMG lowers fasting plasma homocysteine in a dose-dependent manner. A meta-analysis of supplementation trials found consistent reductions, particularly at doses of 4 to 6 g/day over 6 to 24 weeks.² A dose-response study showed fasting homocysteine reductions of 12%, 15%, and 20% at doses of 1.5, 3, and 6 g/day respectively, with even larger attenuation of post-methionine-load homocysteine spikes.³

Confidence: Moderate-to-high for the biomarker. The homocysteine-lowering effect is reliable and reproducible. However, lowering homocysteine does not automatically translate to reduced cardiovascular events. The relationship between homocysteine and heart disease risk remains debated, and no TMG trial has used cardiovascular endpoints as primary outcomes.

Genetic Homocysteinuria: Medical-Grade Use

In patients with cystathionine beta-synthase (CBS) deficiency, betaine is an established adjunctive treatment when pyridoxine-responsive measures alone fail to reach target homocysteine levels. Expert guidelines recommend starting doses of approximately 3 g twice daily in adults, with individualized titration based on biochemical monitoring.⁴ This is a medical application requiring specialist oversight and regular lab work, not a supplement-store decision.

Liver Health: Disappointing Results

TMG's role in phosphatidylcholine synthesis and methyl donation gave it theoretical appeal for non-alcoholic steatohepatitis (NASH). However, a 12-month trial using 20 g/day of oral betaine found reduced steatosis grade but no significant improvement in overall NASH activity score, fibrosis, or metabolic markers like insulin and glucose.⁵ Confidence: Low. High-dose TMG does not appear to meaningfully alter the trajectory of liver disease.

Exercise Performance: Unproven

A systematic review of betaine supplementation for endurance performance found mixed results across small studies, all rated as high risk of bias.⁶ Some individual trials report modest improvements in power output or VO2 max, but no reliable consensus exists. Confidence: Very low. The evidence does not support TMG as an ergogenic aid.

Safety and Interactions

TMG is generally well tolerated. Nausea, diarrhea, and a fishy body odor (from trimethylamine production) are the most commonly reported side effects. The odor issue can be socially bothersome and may limit adherence at higher doses.

One important consideration: TMG raises methionine levels as a direct consequence of its mechanism. In people with remethylation disorders or those already taking high-dose methionine supplements, this could theoretically overshoot metabolic targets. Do not combine multiple high-dose methylation strategies without serial biomarker monitoring.

Avoid use during pregnancy, lactation, or in the presence of severe renal or hepatic disease without clinician guidance.

Practical Dosing

For general homocysteine-oriented use, 500 to 3,000 mg/day is the typical wellness range. Higher doses (up to 6 g/day) have been used in clinical trials. Take consistently with food. Review homocysteine levels at 4 to 8 weeks if that is your primary goal. Continue only if the trajectory is favorable and tolerability is acceptable.

The Bottom Line

TMG reliably lowers homocysteine, and that makes it a legitimate tool for people with elevated levels who are already optimizing folate and B12. Beyond that biomarker, its practical benefits are limited. Liver disease outcomes have been disappointing, exercise performance claims lack credible support, and the clinical significance of homocysteine lowering itself remains an open question. If you choose to use TMG, set a clear biochemical target and verify your response with lab work.