Gamma-butyrobetaine (GBB)

What it is

Carnitine is the transporter that moves long-chain fatty acids into the mitochondria, where they are burned. You take in part of it with meat and dairy products; the body makes the rest itself, from building blocks of the amino acid lysine. The last intermediate step is called gamma-butyrobetaine. The enzyme GBB hydroxylase, genetically BBOX1, attaches a hydroxyl group, and the carnitine is complete.

In humans, this step also takes place in the kidney. A 1980 tracer study with 3 healthy men found 2 to 8 times more labeled carnitine than labeled GBB in the urine, and human kidney tissue showed measurable enzyme activity, whereas rat kidneys did not. Rare cases show how important the step is: in 2025 a research group described 3 patients from 2 families with a defective BBOX1 gene. They had too little carnitine, too much GBB and symptoms affecting the muscles and nervous system.

In commercial products, GBB usually does not appear as the pure substance but as an ester, for example as the ethyl ester, in fat-burner blends. Chemically that is not the same thing, and more on this shortly.

How it is supposed to work

The idea is simple: supplying the last precursor bypasses all earlier bottlenecks and yields more of the body’s own carnitine. More carnitine is supposed to bring more fat into the mitochondria and so stoke fat burning. Unlike swallowed carnitine, GBB is only converted into carnitine in the tissue, which some see as an advantage.

Animal data support the first part. In rats, a single dose of GBB raised carnitine in all tissues just as much as the same amount of L-carnitine. In mice with an inherited defect of the carnitine transporter, plasma carnitine rose to roughly twice the control value after GBB, and free fatty acids in the liver fell to normal levels. In healthy mice, by contrast, hardly anything changed.

In the scene, one effect is described above all: heavy sweating, especially before training. These are statements by users and retailers, not study findings. A possible explanation comes from a 2004 laboratory study. The methyl ester of GBB binds to muscarinic acetylcholine receptors, that is, to the same switches that also control the sweat glands, and lowered blood pressure in rats. GBB itself did not do this. Whether the ethyl ester common in products behaves the same way in humans has not been tested by anyone. Sweating would then be more a side effect of the ester than a sign of fat burning.

The connection to meldonium

Meldonium, also called Mildronate, was developed in Latvia in 1974 and was described early on as a structural analog of GBB. It does exactly the opposite: it inhibits GBB hydroxylase and the carnitine transporter OCTN2 and thus lowers carnitine. In healthy people, plasma carnitine fell by 18 percent over 4 weeks under meldonium, while GBB roughly doubled. In rats, GBB in the heart rose sevenfold under meldonium, and the protective effect on the heart was linked to the rise in GBB.

Meldonium is approved as a medicine in some EU states such as Latvia, Lithuania and Poland, but not in the US. At the 2015 European Games in Baku, 66 of 762 urine samples were positive; since 2016 it has been on the World Anti-Doping Agency’s prohibited list. This matters for GBB because the two substances are often mentioned in the same breath but are legally separate.

What the studies show

Feeding study, 1989

Healthy adults received, in addition to a low-carnitine diet, either lysine plus methionine, trimethyllysine or GBB over 10 days. Carnitine was measured in urine, blood and muscle. GBB increased carnitine production the most. There was no placebo group and no blinding, and only carnitine balance was measured, not fat burning and not performance.

Gut bacteria and TMAO, 2014

A research group at the Cleveland Clinic showed that gut bacteria first form GBB from dietary carnitine, about 1,000 times faster than the next product, TMA. In mice, GBB was the main product of bacterial carnitine breakdown, was further processed into TMA and TMAO, and accelerated arterial calcification as a feed additive. These are animal data, but they concern exactly the route that swallowed GBB can take.

Vascular patients, 2016 and 2025

Two observational studies measured GBB in the blood of vascular patients. In 264 patients with calcification of the carotid artery, a higher GBB level was associated with cardiovascular death, adjusted hazard ratio 3.3. In 395 patients with circulatory disorders of the legs, the hazard ratio for severe events in the legs was 1.93. This does not prove causation and concerns the body’s own GBB, not supplements. The 2016 authors, however, explicitly called for safety studies on supplementation.

Where the data stop

There is no randomized, placebo-controlled study of GBB in humans, including on fat loss, energy expenditure or performance. No study with GBB as the active substance is registered at ClinicalTrials.gov. The only human study on intake dates from 1989, is uncontrolled, lasted 10 days and measures only carnitine levels. Whether more carnitine lets healthy, well-supplied people burn more fat at all has not been studied for GBB.

It is even thinner for the esters found in fat burners. There is a 2004 rat study on the methyl ester and a 1961 paper on the ethyl and methyl esters, but no data in humans. Reliable figures on side effects in humans are missing entirely.

Status, approval and legal

In the EU, isolated GBB is approved neither as a medicine nor as a novel food. There is no entry in the European Commission’s Novel Food Catalogue, nor in the list of authorized novel foods. Under the Novel Food Regulation 2015/2283, a substance is considered novel if it was not consumed to a significant degree in the EU before May 15, 1997; such consumption is not documented for isolated GBB. The Novel Food Catalogue, however, is not legally binding, and we found no official classification or individual decision. A novel food status has therefore not been determined; whether GBB may be marketed as a dietary supplement remains open. GBB is not listed by name on the 2026 WADA prohibited list. Meldonium is listed there under S4.4.3 and is prohibited at all times.

Safety

The safety of GBB supplements has not been studied systematically. Two points deserve attention. First, the TMAO axis: GBB accelerated arterial calcification in mice, and high blood levels in vascular patients go along with a worse prognosis. Anyone with cardiovascular disease should take this seriously. Second, the esters: the methyl ester acts like acetylcholine in rats and lowers blood pressure, which fits the sweating described. With low blood pressure, heart rhythm disorders or medicines that act on the heart, caution is the sensible stance. There are no data for pregnancy, breastfeeding and children. Official upper limits from EFSA, BfR or NIH do not exist.

Sources

• Rebouche et al., J Nutr 1989
• Rebouche & Engel, J Biol Chem 1980
• Li et al., NPJ Genom Med 2025
• Koeth et al., Cell Metab 2014
• Skagen et al., Atherosclerosis 2016
• Chen et al., J Am Heart Assoc 2025
• Dambrova et al., Naunyn Schmiedebergs Arch Pharmacol 2004
• Liepinsh et al., J Pharm Pharmacol 2011
• WADA, Prohibited List 2026
• Regulation (EU) 2015/2283 on novel foods