Fat burners: what L-carnitine, green tea, and caffeine do in the body

A fat burner bottle usually says something true: carnitine does indeed transport fatty acids into the mitochondria, and caffeine does indeed increase energy expenditure. The question is what happens between that statement and the number on the scale — and at exactly which step the connection breaks.

Participating in a process and limiting it are different things

Almost all advertising for this category is built on a single substitution. First, a substance is named that honestly participates in fat oxidation. Then, it is inferred that taking the substance will increase the rate of oxidation. The second does not follow from the first.

Any multi-stage process proceeds at the speed of its slowest step. Adding something that is already sufficient is pointless: the bottleneck is not there. A cashier in a store will not work faster if you put ten more carts next to them.

In fat oxidation, the bottleneck is known and is not in transport. The rate is set by the enzyme carnitine palmitoyltransferase 1 on the mitochondrial membrane, and it is inhibited by malonyl-CoA — a substance that is abundant when the body as a whole receives enough energy, and scarce when energy is lacking. Simply put, the key to this step is the body's energy balance, not the number of carriers. Hence all subsequent results.

L-carnitine: a carrier that is already sufficient

The mechanism is described correctly. Long-chain fatty acids cannot pass through the inner mitochondrial membrane on their own; they are attached to carnitine, transported inside, and detached there. Without carnitine, fat oxidation in the cell does indeed stop.

Then the caveats begin, which are not on the bottle. Carnitine is not a vitamin: the body synthesizes it itself, in the liver and kidneys, from lysine and methionine. About 95% of the supply lies in skeletal muscles, and in a healthy person, this supply is full regardless of whether they eat meat or not. They simply do not have a deficiency that needs to be covered.

There is also a physical barrier. Only a small fraction of the ingested dose is absorbed, the rest is broken down by gut bacteria, and muscle concentration hardly increases from intake: for carnitine to enter the muscle, high insulin is needed, that is, taking it along with a large portion of carbohydrates and over the course of weeks. In trials where muscle carnitine was measured directly, standard intake regimens did not raise it. Accordingly, results regarding fat mass in controlled studies are, on average, indistinguishable from a placebo.

At the same time, levocarnitine itself is a substance with real indications. It is prescribed for primary carnitine deficiency, which is associated with a breakdown of the transport protein, and for secondary deficiencies — in particular, in people on hemodialysis, for whom carnitine is lost through the dialyzer. These are doctor's prescriptions based on blood tests, not sports nutrition. The topic of supplements for training is analyzed separately in the article on sports supplements.

Caffeine: the only one with a reproducible effect

Caffeine blocks adenosine receptors — the very ones through which accumulated adenosine informs the brain about fatigue. Consequently, the activity of the sympathetic nervous system increases, the level of catecholamines rises, the breakdown of triglycerides in adipose tissue intensifies, and there are more free fatty acids in the blood. Energy expenditure at rest does indeed increase, but we are talking about a few percent.

Practically, a second effect is more important: caffeine reduces perceived exertion. The same work seems easier, the workout turns out to be longer or more intense — and these extra minutes of movement provide more than an increase in resting metabolism.

The limitations are also specific. Tolerance to the increase in energy expenditure and the vascular effect develops over days and weeks, so "boosting metabolism" with caffeine is a temporary effect. For a healthy adult, the guideline is about 400 mg of caffeine per day and about 200 mg at one time; during pregnancy, the upper limit is lower — about 200 mg per day.

You need to count everything at once, not just cups of coffee:

• Coffee — approximately 60–100 mg per cup, depending on the brewing method and grind.
• Tea — approximately 30–50 mg per cup.
• Energy drink — usually about 80 mg per 250 ml can, but large cans have a significantly higher dose.
• Pre-workout complex — often 150–300 mg per serving, and this is the most underestimated line.
• The fat burner itself — as a rule, caffeine is the main active ingredient there, although it may not be listed first on the label.

Two such cans plus coffee in the morning — and the daily limit is reached by lunch. Hence the heart palpitations, anxiety, and ruined sleep, and lack of sleep the next day increases appetite and works against a calorie deficit. For people with arterial hypertension, arrhythmias, anxiety disorders, as well as during pregnancy, the dose should be discussed with a doctor, not chosen by the label.

Green tea extract: where a cup stops being a cup

Catechins are considered the active ingredients, primarily epigallocatechin-3-gallate (EGCG). The proposed mechanism is the inhibition of the enzyme that breaks down norepinephrine, that is, an indirect prolongation of the same adrenergic signal that caffeine provides. Green tea extracts usually already contain caffeine, so it is difficult to separate one from the other in studies.

The results of human trials are small and inconsistent: some studies show a difference of a fraction of a kilogram over several months, some show nothing, and meta-analyses note a noticeable spread between studies. This is not the scale for which it is worth changing a nutrition plan.

But the risk here is stated directly, and this is the main difference between this topic and the previous ones. The European Food Safety Authority reviewed catechins separately and concluded that a drink made from leaves is not a problem, while concentrated extracts in high daily doses of EGCG are associated with rare but documented cases of liver damage. The threshold at which the signal becomes discernible was estimated at approximately 800 mg of EGCG per day from a supplement.

The difference in dose is fundamental. In a cup of brewed tea, EGCG is in the tens of milligrams; in an extract capsule, it is in the hundreds, and there are usually several capsules in a daily dose. "It's just green tea" is an incorrect description of what is in the bottle. For any liver diseases and when taking medications with hepatic metabolism, such extracts should not be started without a doctor.

Synephrine, CLA, chitosan, garcinia

The rest of the shelf assortment fits into short paragraphs because the mechanism in each case breaks off at the same point.

Synephrine and "bitter orange". Appeared in formulations as a replacement for ephedrine after it came under restrictions. It acts on the same adrenergic receptors, is usually sold together with caffeine, and it is this combination that raises questions regarding cardiovascular safety: cases of rhythm disturbances and increased blood pressure have been described. A gain in fat mass has not been shown.

CLA (conjugated linoleic acid). In rodents, the effect was pronounced; in humans, it is poorly reproduced. In some studies, long-term intake of high doses noted a deterioration in carbohydrate metabolism and lipid markers — that is, the cost is possible, but the benefit is not confirmed.

Chitosan. It binds fat in the intestinal lumen — this is true, but the amount bound is measured in grams per day, which corresponds to a few dozen kilocalories. Against the background of a daily diet, this is a value within the margin of error for weighing food.

Garcinia cambogia. Hydroxycitric acid inhibits the enzyme that builds fat from carbohydrates. The problem is that in a human on a normal mixed diet, this pathway is secondary: fat in storage is mainly dietary, not synthesized anew. Inhibiting a secondary pathway is again not the right bottleneck. Like green tea concentrates, there are reports of liver damage associated with garcinia.

Why you can write almost anything on the label

In Russia, such products are sold as dietary supplements (BAD), that is, they are registered as food products according to the technical regulation TR TS 021/2011, not as medicines. The manufacturer is obliged to confirm safety and compliance of the composition with what is declared — but not effectiveness. Clinical trials proving fat loss are not required to get on the shelf, and they are usually absent. How this differs from drug registration is analyzed in detail in the article on dietary supplements, vitamins, and medicines.

Separately, there is what is sold outside of pharmacies and stores — through messengers and classifieds, with the promise of a "strong" effect. Here, there are two things you need to know by name.

2,4-dinitrophenol (DNP). This is not a supplement, but an industrial poison. It uncouples the mitochondrial respiratory chain: energy stops being stored and is entirely dissipated as heat. Fat is indeed consumed — along with an uncontrollable rise in body temperature. There is no antidote, no safe dose, and fatal cases have been described, including among young people who took the drug for "cutting." The gap between a "working" dose and a lethal dose is so small that you can only make a mistake once.

Sibutramine. A prescription drug with limited circulation, which has contraindications and blood pressure monitoring. It is regularly found in "herbal" weight loss capsules, where it is not listed on the label at all. In this case, a person takes an unknown dose of a prescription drug without considering their illnesses and other medications — and attributes the effect to herbal tea.

What remains

Fat mass goes away when the body spends more energy than it receives — how this calorie deficit is calculated and maintained is analyzed separately. This is the only mechanism that works in this topic, and everything listed above at best moves the result by a few percent — that is, by an amount that is easily offset by one uncounted portion.

There are three practical conclusions. Caffeine makes sense as a working tool before a workout, not as a fat-loss agent, and with an eye on the daily total from all sources. A healthy person does not need carnitine. Concentrated plant extracts are substances with a dose and side effects, and they should not be treated like tea.

Money spent on a bottle is more reliably invested in understanding your own diet: it almost always turns out that the discrepancy with the plan is caused not by the absence of a fat burner, but by uncounted oil, sauces, and drinks. For diabetes, kidney and liver diseases, during pregnancy, and while on regular medication, any supplements in this group should be discussed with a doctor before the first capsule, not after.

Count it from a photo

Frequently asked questions

Does L-carnitine help burn fat?

In controlled studies, oral L-carnitine supplementation does not, on average, lead to fat mass loss compared to a placebo. While carnitine does transport fatty acids into the mitochondria, a healthy person's muscle stores are already full, and supplementation barely increases this concentration. The rate of fat oxidation is limited by a different step.

How much caffeine is safe per day?

For a healthy adult, the guideline is about 400 mg per day and about 200 mg per single dose; for pregnancy, it is approximately 200 mg per day. You must account for all sources at once: coffee, tea, energy drinks, pre-workout complexes, and the fat burner itself, where caffeine is usually the main active ingredient.

Is green tea extract safe for weight loss?

Brewed tea and a capsule of concentrated extract differ by an order of magnitude in their catechin dose. High daily doses of EGCG from supplements are linked to rare but documented cases of liver damage, and European safety assessments highlight this situation specifically. If you have liver disease or are taking medication, such extracts should not be taken without consulting a doctor.

Why are fat burners sold if they don't work?

In Russia, they are registered as dietary supplements, i.e., as food products. The manufacturer confirms safety and composition, but is not required to prove efficacy to get them on the shelf. The lack of clinical trials does not prevent their sale.

What is DNP and why is it called dangerous?

It is 2,4-dinitrophenol, an industrial substance that uncouples mitochondrial function and converts energy into heat. It does indeed burn fat, but simultaneously causes uncontrollable overheating of the body. There is no antidote, fatalities have been documented, and the margin between an effective dose and a lethal dose is very small.

Read next

• Sports supplements: creatine, glutamine, lecithin — The sports nutrition shelf is arranged so that expensive and useless products stand next to cheap and effective ones.
• How dietary supplements differ from vitamins and medications — In a pharmacy, two packages of vitamin D sit on the same shelf.
• Metabolism: is it true that it can be "boosted" and that it can "break" — One person is promised a metabolism boost through spices and frequent small meals, another explains weight plateaus by claiming their metabolism is broken from dieting.

This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.

Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.