Trimethylglycine sits in an odd spot. It’s sold to two completely different audiences — methylation-focused people who want their homocysteine down, and lifters who read that it improves power output — with almost no overlap in the research behind those two pitches.
It also has something rare in the supplement world: a clean, replicated, dose-dependent effect on a measurable biomarker. That’s worth taking seriously. It’s also worth understanding exactly what that biomarker does and doesn’t mean, and what the same doses do to a different biomarker that gets far less airtime.
What Betaine Is
Betaine — trimethylglycine, or TMG — is a glycine molecule carrying three methyl groups. Your body makes it by oxidizing choline, and you get it from food: beets (the source of the name), spinach, wheat bran and germ, quinoa, and shellfish. A typical mixed diet supplies somewhere in the range of 100-400 mg/day, with whole-grain-heavy diets landing higher.
It does two structurally distinct jobs, which is why the two marketing stories exist:
- Methyl donor. Betaine hands a methyl group to homocysteine via the enzyme BHMT, converting it back into methionine. This is a backup route to the more familiar folate- and B12-dependent pathway — see our methylation guide for how the two fit together.
- Osmolyte. Betaine helps cells maintain fluid balance and protein stability under osmotic stress. This is the proposed basis for the athletic performance claims, though the mechanism-to-outcome link there remains speculative.
The Homocysteine Evidence
This is the strongest part of the file, and it’s genuinely solid.
Controlled trials have consistently shown that supplemental betaine lowers plasma homocysteine in a dose-dependent way, in healthy people and in those with elevated levels. Effects appear within weeks. Doses around 1.5 g/day produce measurable reductions; 3 g/day produces larger ones; 6 g/day larger still. European regulators have accepted a health claim that 1.5 g/day of betaine contributes to normal homocysteine metabolism — one of relatively few supplement claims to clear that bar.
Now the part that matters more than the effect itself.
Homocysteine is a biomarker, not an outcome. It is associated with cardiovascular risk in observational data, which made lowering it look like an obvious intervention. But large trials that successfully lowered homocysteine using folic acid and B vitamins largely failed to reduce heart attacks or cardiovascular deaths. That’s one of the more instructive disappointments in nutrition research: the marker moved, the outcomes didn’t.
So the honest framing is narrow. Betaine normalizes a lab value. It does not treat, prevent, or cure cardiovascular disease, and nobody should take it believing otherwise. Where it’s genuinely reasonable is when someone has a documented elevated homocysteine — often alongside folate or B12 status issues, sometimes with a genetic variant affecting folate metabolism — and is working with a clinician on it. In that context betaine is a legitimate tool, usually alongside folate and B12 rather than instead of them, since the pathways are complementary. Our methylated vs. regular B vitamins comparison covers the adjacent debate.
There is also a prescription form of betaine used in a rare inherited metabolic disorder. That is a supervised medical treatment for a diagnosed genetic condition and tells you nothing about what an over-the-counter capsule does for a healthy person.
The Performance Evidence
Here the picture gets muddier, which is the opposite of how it’s usually sold.
Trials in resistance-trained men have typically used 2.5 g/day for two to six weeks and measured strength, power, training volume, and body composition. Results split:
- Several studies reported improvements in bench press throughput, total training volume, or power output versus placebo, along with small favorable shifts in body composition.
- Several others found no meaningful difference on primary strength or power measures.
- Pooled analyses tend to conclude that if there’s an effect, it is small, more consistent for muscular endurance and power than for maximal strength, and complicated by small sample sizes and short durations.
Proposed mechanisms include cellular hydration, a possible influence on creatine synthesis (betaine’s methyl groups feed into that pathway), and effects on anabolic signaling. None of these have been convincingly tied to the performance results, and studies looking for changes in circulating hormones have generally come up empty.
A fair read: betaine is a plausible minor addition for a trained lifter who already has the fundamentals covered, not a foundational supplement. If you’re choosing where to spend, creatine has an evidence base of a different order entirely, and beta-alanine has better-characterized effects for high-intensity endurance work. Our athletic performance stack puts them in order.
The Liver and Metabolic Research
Betaine’s role in fat metabolism in the liver has driven interest in fatty liver conditions, and small human trials have examined it there. Results have been preliminary and inconsistent — some reporting improvements in liver enzyme or imaging measures, others not — and the trials are small enough that no firm conclusion is available. Fatty liver disease is a medical diagnosis requiring medical management; betaine is not a treatment for it, and anyone in that situation should be working with a hepatologist or primary care provider rather than self-experimenting.
The Caveat Nobody Advertises
Here’s the finding that belongs on the label and isn’t: betaine supplementation has increased total and LDL cholesterol in controlled trials, particularly at doses of 4-6 g/day, with some signal at lower doses in certain populations. Studies in both healthy adults and people with elevated cholesterol have reported this, and it’s been replicated well enough to take seriously.
That creates an uncomfortable irony. The main reason people take betaine is to improve a cardiovascular risk marker — and at higher doses it may worsen a different, better-validated one. LDL has far stronger evidence linking it to actual cardiovascular events than homocysteine does.
Practical implication: stay at the lower end, 1.5-3 g/day rather than 6 g. And if you’re taking betaine for methylation reasons and you have a lipid panel on file, check it again after a few months. This is exactly the kind of thing that a before-and-after blood test catches and a subjective impression never will — see our blood tests before supplementing guide.
Sensible Dosing
- Homocysteine support: 1.5-3 g/day, taken with a meal. Effects on the marker appear within a few weeks; recheck labs rather than guessing.
- Performance: 2.5 g/day, commonly split into two 1.25 g doses, taken consistently rather than pre-workout — the proposed mechanisms are accumulation-based, not acute.
- Form: anhydrous betaine powder or capsules. Powder is cheaper at these doses; the taste is mildly sweet and salty. “Betaine HCl” is a different product used for stomach acid support at much lower doses and should not be substituted here.
- With food reduces the GI complaints that are the most common reason people quit.
- Give it eight weeks and judge on measurements — a homocysteine panel, or training logs — not on how you feel.
Safety and Interactions
- GI effects — nausea, bloating, diarrhea — are the most common issue, dose-related.
- Fishy body odor happens to some people. Betaine and choline can be converted by gut bacteria to trimethylamine; individuals who clear it poorly develop the odor. It’s harmless but socially unwelcome, and it resolves on stopping.
- TMAO. That same pathway produces TMAO, which has been associated with cardiovascular risk in observational research. Whether supplemental betaine meaningfully raises it, and whether that matters, remains genuinely unsettled. Another reason not to megadose.
- Kidney disease — impaired clearance is a reason to involve a nephrologist before supplementing.
- Pregnancy and nursing — no meaningful safety data at supplemental doses. Skip it. See our pregnancy supplement safety guide.
- Lipid-lowering therapy — worth mentioning to your prescriber given the LDL findings.
Bottom Line
TMG does one thing well and cheaply: it lowers homocysteine, reliably, at 1.5-3 g/day. That’s a marker worth normalizing under clinical guidance, but lowering it has not been shown to change cardiovascular outcomes, so don’t buy it as heart insurance. The performance case is real but small and inconsistent. And the LDL increases seen at higher doses are the single best argument for keeping the dose modest and checking a lipid panel if you stay on it. The full profile is on our TMG supplement page.
This article is educational and not medical advice. Talk with a healthcare provider before starting any supplement, especially if you are pregnant, nursing, taking medication, or managing a health condition.