As a category, no. The review on the topic concludes that from the usual list only caffeine and green tea have data supporting an effect on fat metabolism, and that for many others the evidence is lacking. And what the supported part delivers is small: green tea is around a kilo in the meta-analysis and practically zero in the Cochrane review.

Which fat burner ingredient has evidence?

Caffeine, by far, and mainly for its effect on performance at 3-6 mg per kilo according to the ISSN position stand. Buying caffeine on its own is cheaper and comes with the dose written on the tub.

Are fat burners safe?

It is the category with the longest history of adulteration. In an analysis of supplements the FDA had recalled for containing drugs, most of those tested again still contained them. Here batch testing and manufacturer traceability matter more than in any other category.

What is a proprietary blend and why does it matter?

It is when the label gives a total in milligrams for a group of ingredients without saying how much of each is inside. It prevents comparing the dose with the studies and allows almost none of the expensive ingredient to be used. On this site it costs points, and in this category it does the most damage.

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Fat burners · Weak evidence

What fat burners are for and what the evidence says

The short answer. A fat burner is not an ingredient, it is a blend, and the only honest way to judge it is to open it up and look piece by piece. The review on the topic sums it up: from the usual list, caffeine and green tea have data supporting an effect on fat metabolism, and for the rest the evidence is missing or merely promising. The other half of this page is about safety: in an analysis of supplements the FDA had recalled for containing drugs, two thirds of those reanalysed still contained them.

Reviews find no consistent effect. It is listed because it sells, not because it works. No figure on this page is attributed to a specific product: the effect belongs to the ingredient and every number carries its source next to it.

01 What it does and how much

Each row is a measured effect, with the figure the study gives and the source number. Where the evidence gives no figure, the column says so instead of making up a percentage.

Effects of fat burner with their source
Effect	How much	Detail
Ingredients with supporting data	caffeine and green tea	They are the two the review identifies as supported for increasing fat metabolism. Of the rest of the usual list, including carnitine, CLA, forskolin, chromium, seaweed and fucoxanthin, it says evidence is lacking even if some look promising. [1]
What green tea contributes to weight	-1.31 kg	The meta-analysis figure, which in the Cochrane review drops to practically zero. It helps size up what the best-supported part of a formula can deliver. [3]
What caffeine contributes	3-6 mg/kg	The dose at which caffeine improves performance according to the ISSN position stand. It is a proven effect on performance and perceived effort, and that is why many fat burners feel like they work: they contain caffeine. [2]
Undeclared drugs	2 in 3	Of the supplements recalled by the FDA for containing drugs, most of those reanalysed still contained them. That is why in this category batch testing and traceability are not a detail. [4]
Number of ingredients in the formula	not an advantage	The review describes the ingredient list as industry-driven and growing without a parallel increase in scientific support. More names on the label does not mean more effect. [1]

02 How to take it

Dose. There is no fat burner dose: there are doses of each ingredient. The only way to assess one is to add up caffeine and EGCG and compare them with their own pages.
When. If it contains caffeine, well away from bedtime. It is the part you actually feel and the part that can ruin your sleep.
How long before you notice it. Caffeine works in the same session. The rest of the formula has no demonstrated effect to wait for.

Details that change the result

A proprietary blend, the kind that gives a total without a breakdown, makes it impossible to know what dose you are taking. On this site that costs points, and this is where it is most justified.
If the tub contains caffeine and you also drink coffee, the total is easy to underestimate.
Be careful about stacking green tea extract from several sources: the 800 mg of EGCG a day threshold at which EFSA sees liver damage is reached by adding up.
Synephrine and bitter orange derivatives often appear in these formulas. They are not among the ingredients the review supports.

03 When it makes sense and when it does not

Worth it if

If what you are after is caffeine before training, buy caffeine, which costs a fraction and has the dose written on it.

Not worth it if

Expecting it to replace a calorie deficit, the only thing that makes you lose fat.
With proprietary blends without a dose breakdown, where there is no way to know what you are taking.
With blood pressure, heart, anxiety or sleep problems, given the stimulant content.
Without published batch testing, given the data on undeclared drugs in products already recalled.

04 Sources

Every figure above comes from here. They are position stands from scientific societies, meta-analyses and peer-reviewed reviews: if a claim cannot hang from one of these lines, it is not on the page. References are cited in their original form.

Jeukendrup AE, Randell R. Fat burners: nutrition supplements that increase fat metabolism. Obes Rev. 2011;12(10):841-851. De los suplementos mas populares (cafeina, carnitina, te verde, CLA, forskolina, cromo, algas y fucoxantina), la cafeina y el te verde son los que cuentan con datos que respaldan sus propiedades sobre el metabolismo de las grasas; para muchos otros falta evidencia. doi:10.1111/j.1467-789X.2011.00908.x
Guest NS et al. ISSN position stand: caffeine and exercise performance. J Int Soc Sports Nutr. 2021;18:1. Dosis de 3-6 mg/kg de peso corporal. doi:10.1186/s12970-020-00383-4
Hursel R, Viechtbauer W, Westerterp-Plantenga MS. The effects of green tea on weight loss and weight maintenance: a meta-analysis. Int J Obes. 2009;33(9):956-961. Diferencia de -1,31 kg frente a control, descrita por los autores como un efecto pequeno. doi:10.1038/ijo.2009.135
Cohen PA, Maller G, DeSouza R, Neal-Kababick J. Presence of banned drugs in dietary supplements following FDA recalls. JAMA. 2014;312(16):1691-1693. De los complementos retirados por la FDA por contener farmacos, la mayoria de los reanalizados seguian conteniendo farmacos prohibidos. doi:10.1001/jama.2014.10308

This is not medical advice. It is what some studies say about an ingredient, not a protocol for you. With an illness, medication or pregnancy involved, the decision belongs to a healthcare professional who knows you.

Questions about fat burner

Answered with the data on this page

Do fat burners work?: As a category, no. The review on the topic concludes that from the usual list only caffeine and green tea have data supporting an effect on fat metabolism, and that for many others the evidence is lacking. And what the supported part delivers is small: green tea is around a kilo in the meta-analysis and practically zero in the Cochrane review.
Which fat burner ingredient has evidence?: Caffeine, by far, and mainly for its effect on performance at 3-6 mg per kilo according to the ISSN position stand. Buying caffeine on its own is cheaper and comes with the dose written on the tub.
Are fat burners safe?: It is the category with the longest history of adulteration. In an analysis of supplements the FDA had recalled for containing drugs, most of those tested again still contained them. Here batch testing and manufacturer traceability matter more than in any other category.
What is a proprietary blend and why does it matter?: It is when the label gives a total in milligrams for a group of ingredients without saying how much of each is inside. It prevents comparing the dose with the studies and allows almost none of the expensive ingredient to be used. On this site it costs points, and in this category it does the most damage.

And now, which to buy

Prices, kept apart from the evidence

This page explains what fat burner does and deliberately names no brand. The comparison is the other half: 34 products from 5 stores from €0.139 per capsule, sorted by price and by how verifiable their certification is.

See the fat burners comparison

Other guides

What each supplement does, with its sources