TMG, short for trimethylglycine, is a supplement used primarily to lower homocysteine levels in the blood, support liver health, and improve body composition during resistance training. Also sold under the name “betaine anhydrous,” TMG works as a methyl donor, meaning it hands off a piece of its chemical structure to help your body run critical metabolic processes. Most people encounter it either as a heart-health supplement or as an ingredient in sports nutrition products.
How TMG Works in Your Body
TMG’s core job is donating a methyl group to homocysteine, a potentially harmful amino acid byproduct that accumulates in your blood. That reaction converts homocysteine into methionine, a useful amino acid your body needs. Methionine then gets recycled into SAMe (S-adenosylmethionine), a molecule involved in hundreds of reactions throughout your body, from repairing DNA to producing neurotransmitters to processing fats in the liver.
This conversion happens mainly in the liver and kidneys, through an enzyme called BHMT. By keeping homocysteine levels in check and maintaining healthy SAMe levels, TMG supports what researchers call your “methylation cycle,” a behind-the-scenes process that influences everything from cardiovascular function to cellular repair.
Lowering Homocysteine
High homocysteine is associated with increased risk of heart disease, stroke, and blood clots. TMG’s best-documented use is bringing those levels down. A meta-analysis of randomized, placebo-controlled trials found that 4 to 6 grams of TMG per day reduced homocysteine by about 12% in healthy adults, translating to an average drop of 1.23 µmol/L. Individual responses ranged from 5% to 20% reduction depending on the study.
That said, lowering homocysteine with TMG comes with a tradeoff worth knowing about (see the section on cholesterol below). Folate and B12 also lower homocysteine through a different pathway, which is why some people use TMG alongside those vitamins rather than at high doses on its own.
Exercise Performance and Body Composition
TMG has gained a following in the fitness world, where it’s commonly included in pre-workout supplements. The most compelling evidence comes from a six-week study on resistance-trained men. Compared to placebo, the group taking betaine saw meaningful changes across several body composition measures: body fat percentage dropped by 3.2 percentage points, lean body mass increased by about 2.4 kg (roughly 5 pounds), and fat mass decreased by nearly 3 kg. Arm size also increased significantly in the betaine group while remaining flat in the placebo group.
The performance results are less clear-cut. Bench press and squat strength didn’t differ consistently between groups, though there was a trend toward improved vertical jump power with betaine. Some earlier research found that betaine improved work capacity during high-rep sets to fatigue at moderate loads. The overall picture suggests TMG may help more with body composition and muscular endurance than with peak strength. Most exercise studies use 2.5 grams per day, typically split into two doses.
Liver Protection
TMG plays a protective role in the liver by maintaining the balance between SAMe and its byproduct SAH. When that ratio gets disrupted, as it does with heavy alcohol use or high-fat diets, the liver’s ability to process and export fat breaks down, leading to fat accumulation. TMG helps restore that balance by feeding the methylation cycle with fresh methyl groups.
This is why betaine has been studied in the context of both alcohol-related liver damage and non-alcoholic fatty liver. By keeping SAMe levels adequate, TMG supports the liver’s normal fat-processing machinery. The enzyme responsible for this, BHMT, is most active in liver and kidney tissue, which is why these organs benefit most directly from supplementation.
Potential Mood and Brain Benefits
Because TMG boosts production of SAMe, and SAMe is independently sold as a supplement for mood support, some people take TMG as an indirect way to raise SAMe levels. SAMe is involved in producing neurotransmitters like serotonin and dopamine. The logic is straightforward: more methyl groups available means more SAMe gets made, which means more raw material for neurotransmitter production. However, direct clinical trials testing TMG specifically for depression or cognitive function are limited. Most of the evidence for this use is mechanistic rather than proven in human mood studies.
The Cholesterol Tradeoff
This is the part many TMG articles gloss over, but it matters. In controlled trials, 6 grams per day of betaine for six weeks increased LDL (“bad”) cholesterol by about 11% and total cholesterol by about 8% compared to placebo. It also raised triglycerides modestly. Doses lower than 6 grams still trended toward higher LDL, though those changes weren’t statistically significant.
The total-to-HDL cholesterol ratio, a key marker of cardiovascular risk, worsened by about 6%. This creates a genuine tension: TMG lowers homocysteine, which is good for your heart, but raises LDL cholesterol, which isn’t. If you’re considering TMG for cardiovascular reasons, this tradeoff is worth discussing with your doctor, especially if your cholesterol is already elevated. People using lower doses (1 to 3 grams) for exercise performance may face less of this issue, but the data at those doses is thinner.
Food Sources of TMG
You already get some betaine from food. Wheat bran and wheat germ are the richest sources, containing over 1 gram per 100 grams. Baked goods made with wheat flour (bread, crackers, pasta) contribute between 33 and 226 mg per 100 grams. Spinach, beets, and shellfish are also solid sources. Fruits, nuts, poultry, and most meats contribute very little, generally under 6 mg per 100 grams.
Interestingly, some of the biggest contributors to betaine intake in the American diet aren’t the most concentrated sources. Beer, iced tea, and ground beef all contain low amounts of betaine per serving, but people consume them in such large quantities that they add up. A typical Western diet provides somewhere in the range of 100 to 400 mg of betaine per day, well below the supplemental doses used in studies.
Typical Doses
The dose depends on the goal. For exercise and body composition, most studies use 2.5 grams per day, often split into a 1.25-gram dose taken twice daily. For homocysteine reduction, trials have used 4 to 6 grams per day, though the higher end of that range is where the cholesterol-raising effects become significant. Some people start at 500 mg to 1 gram and increase gradually to assess tolerance.
Side effects at typical doses are generally mild. Some users report gastrointestinal discomfort, nausea, or a fishy body odor, particularly at higher doses. TMG is water-soluble and doesn’t accumulate in fat tissue, so it clears the body relatively quickly. The osmolyte function of betaine, meaning it helps cells maintain their water balance under stress, is another reason it’s generally well tolerated: your cells are already accustomed to handling it.