Betaine Side Effects: What the Evidence Actually Shows

Betaine, also sold as trimethylglycine or TMG, turns up in pre-workout formulas, "methylation support" products and stand-alone powders. It is generally regarded as one of the better-tolerated supplements on the shelf, but "well tolerated" is not the same as "free of side effects". Our editorial team has gathered what clinical trials and safety assessments say about its downsides, and who should think twice before using it.

What betaine is and why athletes take it

Betaine is a small molecule made of the amino acid glycine carrying three methyl groups. It occurs naturally in beetroot, spinach, wheat bran and shellfish, and the body also produces it from choline. A typical Western diet supplies somewhere in the region of a few hundred milligrams per day, with wholegrain-heavy diets providing more.

In the body betaine has two main jobs. First, it is an osmolyte: it helps cells hold on to water and protects proteins when cells are exposed to heat, dehydration or salt stress. Second, it donates a methyl group to homocysteine, converting it back into methionine in the liver and kidneys. This is why betaine reliably lowers blood homocysteine.

Sports interest comes from small trials suggesting modest improvements in power output, training volume or body composition. A frequently cited study by Cholewa and colleagues in trained men reported favourable changes in body composition and work capacity, although results across studies are inconsistent and effect sizes are small.

It is important to distinguish betaine anhydrous, the form used in sports products, from betaine hydrochloride. Betaine HCl is sold as a digestive aid to increase stomach acidity and has a different side-effect profile, chiefly heartburn and gastric irritation. This article focuses on betaine anhydrous unless stated otherwise.

Common, mostly mild side effects

The complaints reported most often in trials are gastrointestinal. Nausea, loose stools, bloating and a general sense of stomach discomfort appear mainly when people take several grams in a single dose on an empty stomach. Splitting the daily amount and taking it with food usually resolves these symptoms.

Some users notice a fishy body or breath odour. Betaine can be broken down by gut bacteria into trimethylamine, the same compound responsible for the smell in the genetic condition trimethylaminuria. Most people clear trimethylamine efficiently, but individuals with reduced activity of the enzyme FMO3 may find the odour persistent and socially distressing.

Occasional reports describe headache or a mild feeling of restlessness, although these are hard to attribute to betaine itself when it is taken inside a multi-ingredient pre-workout that also contains caffeine and other stimulants. When a product with ten ingredients causes jitteriness, betaine is rarely the main suspect.

In the trials by Schwab and by Olthof, betaine was otherwise well tolerated, with no meaningful changes in liver or kidney markers at the doses tested. Dropout rates due to side effects were low, which is broadly consistent with the safety picture from clinical use in homocystinuria.

Cholesterol: the effect worth knowing about

The most clinically relevant downside of betaine is not a symptom you can feel but a change on a blood test. Several controlled trials found that daily intakes of around 4–6 g raised total and LDL cholesterol. In the Schwab trial, for example, LDL rose in the betaine group compared with placebo over several weeks.

A meta-analysis by Zawieja and colleagues pooled the available randomised trials and concluded that betaine supplementation moderately increases total cholesterol, with the effect most apparent at higher doses. Lower doses of around 1.5 g per day, as used in the Olthof study for homocysteine lowering, appear to have less effect on lipids.

The mechanism is not fully established. One hypothesis is that extra methyl groups influence how the liver packages and secretes lipoproteins. Whatever the explanation, the practical message is that the "heart-friendly" image of betaine, based on homocysteine lowering, does not automatically translate into a better overall cardiovascular profile.

For a young, healthy athlete with normal lipids, a small rise may be of limited importance. For anyone with familial hypercholesterolaemia, existing cardiovascular disease, or who is already experiencing lipid changes from other substances, it is a reason to be cautious and to check a lipid panel.

Rarer problems, interactions and at-risk groups

Betaine anhydrous is a licensed medicine for homocystinuria, a rare inherited disorder. In that setting, much higher doses are used under specialist care, and regulators have warned of rare cases of cerebral oedema associated with very high blood methionine levels in patients with cystathionine beta-synthase deficiency. This is not a concern at supplement doses in healthy people, but it shows that betaine is pharmacologically active.

People with kidney disease should seek medical advice before using betaine, as it is handled by the kidneys and its osmolyte role is closely tied to renal physiology. Likewise, anyone under specialist care for a metabolic disorder should not add betaine without discussing it with their team.

There are no well-documented dangerous interactions with common medicines at supplement doses. However, because betaine interacts with the methylation cycle alongside folate, vitamin B12 and B6, combining several "methylation" products can make it harder to interpret blood results such as homocysteine.

Safety data in pregnancy and breastfeeding are insufficient, so supplementation in these periods is best avoided unless advised by a clinician. In the UK, betaine is sold as a food supplement and is not on the World Anti-Doping Agency Prohibited List, but tested athletes should still choose batch-tested products to reduce contamination risk.

Using betaine sensibly

If you decide to try betaine, a few simple habits reduce the chance of side effects. These recommendations apply to healthy adults using ordinary food supplements; they are not a substitute for medical advice.

• Start low: studies in athletes commonly used around 2.5 g per day, often split into two doses.
• Take it with food to minimise nausea and stomach upset.
• Avoid stacking several products that each contain betaine without adding up the total.
• If you plan to use it for months, consider a baseline and follow-up lipid profile through your GP or a private lab.
• Stop if you develop persistent odour, digestive symptoms or unexpected lab changes.

Food sources remain a sensible alternative. Wholegrains, beetroot and spinach provide betaine together with fibre, folate and other nutrients, and there is no evidence that dietary betaine causes the lipid changes seen with concentrated supplements.

Finally, keep expectations realistic. The performance benefits are modest and inconsistent. For many recreational lifters, the money is better spent on adequate protein, creatine monohydrate and good sleep, which have a far stronger evidence base.

Conclusions of the editorial team

Betaine is a low-risk supplement for most healthy adults. The most common side effects are digestive and usually resolve with lower or divided doses taken with meals.

The effect most people overlook is a moderate rise in LDL and total cholesterol at intakes of several grams per day. Anyone with lipid problems or cardiovascular risk should weigh this against the small potential performance gains.

People with kidney disease, inherited metabolic disorders, or who are pregnant or breastfeeding should consult a doctor before use.

For further reading, see our articles on creatine safety, how homocysteine is measured, and what a lipid panel can tell an athlete.

References

1. Craig SA. Betaine in human nutrition. Am J Clin Nutr. 2004;80(3):539–549.
2. Schwab U, Törrönen A, Toppinen L, et al. Betaine supplementation decreases plasma homocysteine concentrations but does not affect body weight, body composition, or resting energy expenditure in human subjects. Am J Clin Nutr. 2002;76(5):961–967.
3. Olthof MR, van Vliet T, Boelsma E, Verhoef P. Low dose betaine supplementation leads to immediate and long term lowering of plasma homocysteine in healthy men and women. J Nutr. 2003;133(12):4135–4138.
4. Cholewa JM, Wyszczelska-Rokiel M, Glowacki R, et al. Effects of betaine on body composition, performance, and homocysteine thiolactone. J Int Soc Sports Nutr. 2013;10(1):39.
5. Zawieja E, Zawieja B, Chmurzynska A. Betaine supplementation moderately increases total cholesterol levels: a systematic review and meta-analysis. J Diet Suppl. 2021;18(1):105–117.
6. Lever M, Slow S. The clinical significance of betaine, an osmolyte with a key role in methyl group metabolism. Clin Biochem. 2010;43(9):732–744.