NMN and TMG: Why Some People Stack Them (And Who Actually Needs To)

Search for NMN and TMG and you will meet a rule that sounds finished: take trimethylglycine with every dose of nicotinamide mononucleotide, or you will drain your methyl groups. The rule is newer than the evidence. NMN can raise NAD+, and the methylation worry is a real pathway, but that is not a proven side effect at the doses most people take.

This is for someone deciding whether to add TMG (betaine) to an NMN routine. It is educational, not personal medical advice. Both are food supplements, not licensed medicines. If a clinician already follows your homocysteine, that advice outranks a stack chart.

What the methyl-drain claim actually says

NMN is a precursor cells use to make NAD+. NAD+ does not sit in storage. When enzymes consume it, a main leftover is nicotinamide (NAM).

Extra NAM is cleared by nicotinamide N-methyltransferase (NNMT). That enzyme moves a methyl group from S-adenosylmethionine (SAM) onto NAM. One product, a step later, is homocysteine. Homocysteine can be turned back into methionine, and methionine can be recharged to SAM. One remethylation route needs folate, vitamin B12, and healthy MTHFR activity. The other uses betaine-homocysteine methyltransferase (BHMT) and does not need folate or B12. TMG is the methyl donor for that second route.

The claim, stated fairly: if NMN increases NAD+ turnover, more NAM may need methylation, SAM demand may rise, and methyl groups used for creatine, phospholipids, and some DNA methylation could be squeezed. TMG is proposed as a top-up so that squeeze never starts. That is a reason to check markers, not proof that a typical NMN dose leaves people methyl-deficient.

Why TMG became the default add-on

TMG won for practical reasons, not because a head-to-head trial crowned it. Spinach, beets, wheat, and shellfish already contain betaine. Capsules are cheap, and the pathway above gives TMG a specific job. It is the most direct over-the-counter match for the reaction people worry about.

Forums then turned a plausible add-on into a requirement. A common script says to match NMN milligram for milligram with TMG, or to take 500 to 1,000 mg of TMG no matter the NMN dose. Those ratios are habits. They are not doses from an NMN-plus-TMG trial, because that trial has not been published.

What human evidence does and does not show

The cleanest human data are nicotinamide riboside (NR) studies that measured one-carbon metabolites, not NMN studies.

Conze, Brenner, and Kruger (Scientific Reports, 2019) randomised healthy adults with overweight to placebo or to 100 mg, 300 mg, or 1,000 mg of NR a day for eight weeks. NAD+ and methylated nicotinamide metabolites both rose, which is what you expect if NNMT is working. NR did not dysregulate one-carbon metabolism and did not disturb plasma homocysteine. Other short NR studies show the same split, and they last weeks, not years.

Plain nicotinamide is a different case. A single oral load has been reported to raise plasma homocysteine within hours and to lower betaine (Sun and colleagues, Hypertension Research, 2012). Free nicotinamide hits NNMT all at once. NMN and NR are routed toward NAD+ first, so they are not the same as a nicotinamide tablet.

Published NMN trials, mostly 250 mg to about 1,250 mg a day for up to about two months, have not reported a consistent homocysteine rise. They were not built as methylation studies, so "nobody flagged it" is weaker than a direct SAM measurement. Years of use sit outside that follow-up.

There is no randomised trial of NMN plus TMG versus NMN alone on SAM, homocysteine, DNA methylation, or how people feel. There is no established methyl-drain syndrome at 250 to 500 mg, and no proof that 500 mg of TMG prevents a problem that has not been shown.

On its own, betaine does lower fasting homocysteine at multi-gram intakes. Trials using about 1.5 to 6 grams a day, including work by Olthof and colleagues, found dose-related drops. Those are not the 500 mg stack doses. Some higher-dose betaine studies also found a small rise in LDL cholesterol. More is not automatically safer.

Who might actually want TMG

A few situations make the add-on more reasonable. None of them is a diagnosis, and none replaces a blood test.

A high NMN intake. At 1,000 mg or more a day, methylated NAM metabolites clearly rise in the NR literature. Those trials still did not show a homocysteine problem, but demand is higher than it is at one 500 mg capsule.

A diet low in choline and betaine. Eggs, fish, and soy supply choline. Beets, spinach, and wheat supply betaine. If those foods are rare in your week, the BHMT donor is already low. Food can cover that. So can a capsule.

A known methylation concern, handled carefully. Common MTHFR variants can slow the folate-dependent route. TMG does not fix a variant. It feeds a parallel route that does not need that enzyme. If a clinician has already said your homocysteine is high, ask them before you add either product. Do not start TMG because a gene-report app printed a red badge. Homocysteine, folate, and B12 are ordinary labs. A variant without a result is not a plan.

Who can skip it

Skip the second bottle if you take 250 to 500 mg of NMN, eat a varied diet, and have no history of high homocysteine. There is no clinical signal that methyl groups are running out in that situation. Also skip TMG if it upsets your stomach, if you have been told to watch LDL, or if you are pregnant, breastfeeding, under 18, in active cancer treatment, or living with meaningful liver or kidney disease. Those groups were not in the precursor trials. Skip it too when the only reason is that everyone online stacks them. NMN does not need TMG to raise NAD+.

Doses people actually use

These are habits, not prescriptions. TMG in this stack is commonly 500 mg or 1,000 mg a day, taken in the morning with the NMN capsule. A 1:1 match with the NMN dose is a forum convention, not a proven methyl replacement.

Homocysteine trials generally used 1.5 to 6 grams. A 500 to 1,000 mg capsule is modest next to that, enough for a small top-up and not a treatment for high homocysteine. If homocysteine is the actual problem, folate, B12, diet, and any prescription belong with a clinician. Take TMG with food if it bothers your stomach. The same morning is enough. Change one product at a time. Avoid a proprietary methyl blend that hides the TMG milligrams.

A plain recommendation

Do not add TMG by default. At 250 to 500 mg of NMN, with eggs, greens, beets, or whole grains still in the diet, the methylation theory is not a strong enough reason to buy another supplement. Nobody has shown that TMG improves what NMN does.

If your dose is high, your diet is low in choline and betaine, or homocysteine is already high, 500 to 1,000 mg of TMG a day is a reasonable optional experiment. It is not a prescription, and it does not replace folate, B12, or medical care. Stop if it disagrees with you. Ask a clinician first if you are pregnant, being treated for cancer, or already under care for a methylation issue. Settle the NMN dose and the product quality before you patch a pathway the trials have not shown is broken.

Related reading

• NMN dosage: how much people actually take
• NMN supplement side effects: what the trials reported
• NMN stack guide: which pairings have a reason
• Is NMN safe long term?