What is TMG and where is it found?
Trimethylglycine (TMG) is the systematic chemical name for betaine, a naturally occurring zwitterion derived from the amino acid glycine with three attached methyl groups. In the body, TMG serves two fundamental functions: it is a methyl group donor and a cellular osmolyte that regulates water balance in cells under osmotic stress.
In the diet, TMG is found in relevant amounts mainly in beets, which can contain up to 600 mg per 100 g fresh weight, as well as in spinach, wheat germ, and quinoa. The average dietary intake in Western populations is estimated at 0.5–2 g daily. The human body can also synthesize TMG endogenously by gradually oxidizing the precursor choline to TMG via the enzyme choline oxidase. However, this synthesis pathway is capacity-limited, so external supply becomes relevant when methylation demand is increased.
The BHMT pathway: TMG as a methyl group donor
Methylation refers to the enzymatic transfer of a methyl group (–CH₃) to a target molecule. This seemingly simple chemical process is one of the most common biochemical reactions in the human body: it regulates gene expression through DNA methylation, controls the synthesis of neurotransmitters like serotonin and dopamine, is necessary for the biosynthesis of creatine and phosphatidylcholine, and activates numerous enzymes. Methyl groups are transferred almost exclusively by S-adenosylmethionine (SAM), the universal methyl group donor. After transfer, S-adenosylhomocysteine (SAH) is formed, which is further broken down into homocysteine.
Homocysteine is a sulfur-containing amino acid intermediate that remains balanced only if it is either remethylated back to methionine or converted to cysteine via the transsulfuration pathway. Two independent enzyme systems exist for remethylation: the folate-dependent pathway via methionine synthase (MS), which requires B12 and 5-methyltetrahydrofolate, and the folate/B12-independent pathway via betaine-homocysteine methyltransferase (BHMT), which uses TMG as a methyl group donor.
In the BHMT pathway, TMG donates one of its three methyl groups to homocysteine, producing methionine and dimethylglycine (DMG). The recovered methionine can be reactivated to SAM, replenishing the methylation pool. BHMT is mainly active in the liver and kidneys, which explains the special importance of TMG for hepatic methylation capacity.
MTHFR variants: when the folate pathway is limited
The enzyme methylenetetrahydrofolate reductase (MTHFR) is a central element in folate metabolism. It catalyzes the conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate (5-MTHF), the active form needed for folate-dependent remethylation of homocysteine to methionine. Two common genetic variants reduce MTHFR enzyme activity: C677T and A1298C.
Carriers of the homozygous C677T variant (TT genotype) have up to 70% reduced MTHFR activity compared to the wild type. This means less 5-MTHF is produced and the folate-dependent remethylation pathway is limited. Compensatorily, the BHMT pathway becomes more important: in the absence of sufficient folate-dependent capacity, TMG takes on a larger share of homocysteine remethylation. People with MTHFR variants and simultaneously suboptimal folate supply therefore have an increased risk of elevated homocysteine levels, and the benefit of TMG supplementation is mechanistically more plausible in this group than in the general population.
It is important to put this into context: an MTHFR variant alone is not a disease condition. The majority of variant carriers have normal homocysteine levels if folate and B12 supply are adequate. Genetic testing and medical assessment are more sensible than a blanket supplementation recommendation based on genotyping.
What clinical studies in humans show
Homocysteine reduction: consistent dose-response relationship
Olthof et al. published a study in 2003 in the Journal of Nutrition that specifically investigated low betaine doses within the range of normal dietary intake. 76 healthy adults received 1.5 g, 3 g, or 6 g of betaine daily or placebo over 6 weeks. Fasting homocysteine levels were reduced in a dose-dependent manner: by 12% (1.5 g/day), 15% (3 g/day), and 20% (6 g/day) compared to the placebo group. Particularly relevant was the finding that betaine also significantly reduced the post-methionine homocysteine increase after a methionine loading test – a marker for acute methylation capacity. [1]
Schwab et al. investigated in 2002 in a controlled study (American Journal of Clinical Nutrition) the effect of 6 g betaine daily over 12 weeks in 42 obese adults. Plasma homocysteine levels decreased significantly. Body weight, body composition, and resting energy expenditure were not affected by betaine. The study thus provides an important control finding: TMG acts selectively on the methylation pathway without metabolic side effects on weight and body fat in healthy overweight individuals. [3]
Lipid Profile: The Most Important Precaution
Olthof et al. analyzed blood lipid data from four controlled betaine studies (n=151 for betaine) in 2005 in PLoS Medicine. The result was clinically relevant: betaine supplementation significantly increased LDL cholesterol and triglycerides. This effect was dose-dependent and was observed even at moderate doses from 1.5 g/day. The authors concluded that the potential cardiovascular benefit from homocysteine reduction could be at least partially offset by the undesirable lipid effect. This means: during long-term TMG supplementation, especially at doses from 3 g/day, monitoring the lipid profile is clinically advisable. [2]
It should be noted that the lipid effects are dose-dependent and were less pronounced in some studies at moderate doses (500 mg to 1.5 g/day) over shorter periods. Nevertheless, lipid monitoring remains a standard safety aspect during chronic intake.
Liver: Betaine Deficiency and MASLD
Sookoian et al. published in 2017 in Liver International a case-control study (n=48 biopsy-confirmed NAFLD patients plus 390 validation participants) that showed circulating betaine levels were inversely correlated with the severity of liver disease. Patients with non-alcoholic steatohepatitis (NASH) had significantly lower betaine levels than patients with simple hepatic steatosis (NAFL), and the correlation was statistically significant with liver inflammation, ballooning degeneration, and fibrosis. The authors described this condition as "betaine insufficiency" as an associated finding in NASH. [5]
The clinical intervention data for liver diseases are less clear. Abdelmalek et al. conducted a randomized, placebo-controlled trial in 2009 in Hepatology (n=35 participants with biopsy-confirmed NASH, 20 g betaine daily over 12 months). The primary outcome showed no significant improvement in ALT, AST, and histology after 12 months with betaine. However, the betaine group less frequently showed worsening of steatosis grade. The authors interpreted the result as an indication that betaine may protect against progression, even if it does not clearly improve existing NASH. The high dose and small study population limit generalizability. [4]
Mukherjee summarized the available evidence in 2020 in World Journal of Gastroenterology and concluded that a reassessment of betaine in clinical trials for NASH and alcoholic liver disease is justified due to its mechanism of action, good tolerability, and low cost. The study situation for liver diseases is overall exploratory and does not justify an independent therapy recommendation. [8]
Athletic Performance
Zawieja et al. published in 2024 a systematic review and meta-analysis including 17 controlled studies with a total of 317 participants. The result for maximal strength (1RM, 3RM, isometric and isokinetic strength) showed a significant effect size of 0.47 (95% CI: 0.04–0.89), with particularly pronounced effects for lower body strength (SMD: 0.49). Muscle endurance and sprint performance showed no consistent effects in the meta-analysis. The authors emphasize the high heterogeneity of the included studies and recommend further research. [6]
A new study by Nieman et al., published in 2025 in Nutrients, provides the first direct metabolomic human data on one-carbon activation by TMG during exercise. In a randomized, placebo-controlled crossover trial, 21 non-elite cyclists received 3 g of betaine daily or placebo for two weeks, followed by a 60 km time trial. The betaine group completed the time trial on average 1.4 minutes faster (112.8 vs. 114.2 min, effect size 0.47, p=0.042). Muscle damage markers, inflammation parameters, and gut permeability did not differ between groups. Untargeted metabolomics showed significant increases in plasma betaine, dimethylglycine, sarcosine, methionine, and S-adenosylhomocysteine after betaine intake – a direct in vivo proof that betaine activates one-carbon metabolism in humans under exercise. [9]
The possible mechanism for performance effects runs through TMG’s osmotic property: TMG regulates cellular hydration under stress and can act as an osmolyte in muscle cells to improve protein stability and enzyme function under physiological stress. An additional mechanism is the lowering of homocysteine with a consequent reduction in homocysteine thiolactone, which can affect insulin signaling and protein synthesis.
TMG, senescence, and biological aging
Zawieja and Chmurzynska published a narrative review in 2025 in Ageing Research Reviews on the connection between betaine and aging. A key finding: Long-term exercise significantly increases plasma betaine levels, while acute exertion has no effect. The increase correlated with a decrease in inflammatory markers. In animal studies with old mice, betaine supplementation reduced the number of senescent cells in the brain, muscle, and heart to levels resembling those of young animals. At the same time, muscle fiber cross-sectional area and bone density improved, and organ atrophy in kidney and muscle was mitigated. These findings from the animal model are mechanistically plausible, but so far there are no controlled human studies that have specifically examined betaine on senescence burden or biological age. [10]
TMG and NAD+ precursors: What the evidence really says
In the longevity community, the practice has spread of taking TMG together with NMN or NR to counteract a supposed loss of methyl groups due to NAD+ metabolism. The underlying theory: When NAD+ is broken down, nicotinamide (NAM) is produced. To excrete excess NAM, the enzyme NNMT (nicotinamide N-methyltransferase) methylates NAM to 1-methylnicotinamide (MeNAM), consuming SAM as a methyl group donor. With increased NAD+ turnover, methylation capacity could theoretically be under pressure, and homocysteine could rise.
This hypothesis is biochemically plausible but not clinically proven. The most important counterargument comes from the NR-SAFE study by Berven et al., published in 2023 in Nature Communications: 20 Parkinson's patients received 3,000 mg of NR daily for 4 weeks. The authors found no evidence of methyl group depletion or a clinically relevant increase in homocysteine. An older, separately published analysis from the same research group in Bergen confirmed this finding: NR supplementation had no effect on DNA methylation homeostasis.
The practical conclusion for people without elevated homocysteine, without clinically relevant MTHFR variant, and with sufficient folate and B12 supply: prophylactic TMG intake with NMN or NR is currently not supported by controlled human data. For people with elevated homocysteine or MTHFR variant, TMG may be useful for independent reasons, regardless of NAD+ supplementation.
What the study situation has not yet proven
TMG’s homocysteine-lowering effect is one of the most robust findings in supplement research and well replicated. The clinical consequence is less clear: lowering homocysteine has not consistently led to reduced cardiovascular events in several large prospective studies and meta-analyses. Homocysteine may be a biomarker of methylation stress without being the primary causal factor for cardiovascular disease.
Body composition and weight are not improved by TMG in available human studies. Liver effects in MASLD are robust in animal models, inconsistent in human studies, and without clear positive signal in high-quality RCTs. Cognitive endpoints and neurological effects have not been studied in large controlled trials. The mechanism via improved methylation remains a plausible but unconfirmed link.
State of evidence
| Endpoint | Evidence status | Comment |
|---|---|---|
| Homocysteine reduction | 🟢 Human studies | Consistently replicated, dose-dependent, about 12–20% reduction at 1.5–6 g/day. Most robust finding in the TMG literature. |
| Cardiovascular events | 🔴 Human studies | Homocysteine reduction did not consistently lead to event reduction in prospective studies. Surrogate markers without confirmed endpoint benefit. |
| Methylation support | 🔵 Mechanistic | BHMT pathway biochemically established. Clinical benefit of supplementation (except in hyperhomocysteinemia) not proven in large RCTs. |
| Body composition | 🔴 Human studies | No effect on body weight or body composition in controlled human studies. |
| Liver health (MASLD) | 🟡 Pilot RCTs | Association data: low betaine levels in NASH. RCT data mixed; no clear therapeutic effect in high-quality studies. |
| Athletic performance / strength + endurance | 🟡 Human studies | Meta-analysis 2024 (17 studies): significant effect size for maximal strength (0.47). RCT 2025 (Nieman): 60 km time trial −1.4 min with metabolomics evidence of one-carbon activation. |
| Protection against NAD+ methyl loss | 🔵 Theoretical | Mechanistically plausible (NNMT pathway). NR-SAFE study 2023 showed no methyl group depletion at 3,000 mg NR. No clinical proof. |
🟢 Well-supported human studies · 🟡 Exploratory evidence / pilot RCTs · 🔵 Mechanistic / animal model · 🔴 Not supported
Who is TMG useful for?
TMG is primarily relevant when there is a specific methylation need: elevated homocysteine levels (above 10–12 µmol/l), proven MTHFR variant (especially TT genotype at the C677T polymorphism) combined with borderline folate levels, or medically confirmed hyperhomocysteinemia due to enzymatic defects, where betaine is used as a medication under supervision.
For general longevity supplementation without elevated homocysteine, the standalone evidence for TMG is limited. People who actively exercise and aim for maximal strength effects can consider the data from the meta-analysis – the evidence is exploratory but more consistent than many other sports supplements.
The common recommendation to always combine TMG with NMN or NR is not currently supported by controlled human data. Those taking NMN or NR without elevated homocysteine do not need TMG prophylactically. Those unsure can have homocysteine measured during a regular blood test.
Dosage and practical notes
For lowering homocysteine, clinical studies have used 1.5–6 g daily, typically divided into two doses. Higher doses produce stronger effects but carry the risk of increased LDL and triglycerides. In the context of methylation support without proven hyperhomocysteinemia, 500 mg to 2 g daily is used in practice – however, specific RCT data for this indication are lacking.
TMG has a slightly sweet, mild taste and is available in powder and capsule form. It is well tolerated; occasional gastrointestinal discomfort at higher doses is known. Medical use for genetic homocystinuria (CBS deficiency) is at 6–20 g daily under medical supervision and regular lab monitoring, as these patients are at risk of hypermethioninemia. At these doses, TMG is prescription-only.
