What this page establishes
- A meta-analysis of 11 RCTs of EGCG combined with caffeine over 12-13 weeks reported a mean 1.31 kg greater body weight loss versus control; a separate 15-RCT analysis found 1.38 kg and 1.93 cm waist circumference.
- Green tea extract increased 24-hour energy expenditure by 4% (P<0.01), whereas an equivalent dose of caffeine alone had no effect on energy expenditure or respiratory quotient, indicating the catechins carry the thermogenic effect.
- Acute caffeine intake increases the fat oxidation rate during exercise, which is a substrate-partitioning effect and not evidence of net fat loss.
Quick answers
Do fat burners work?
The best-evidenced ones move about 1-1.4 kg over 12 weeks in trials, on top of a diet. That is real and small, and free-living results run lower than trial results.
Read the full answer
Several popular ingredients move nothing at all.How much caffeine for fat loss?
There is a dose-response relationship in the meta-analysis, but chasing it runs into sleep disruption and tolerance.
Read the full answer
Short sleep cut the fat fraction of weight lost by 55% in the Nedeltcheva crossover, so the sleep cost outweighs the thermogenic gain.Is synephrine a safe ephedrine replacement?
It is not an effective one. A systematic review found no evidence it facilitates weight loss and no effect on body composition, while it tends to raise blood pressure and heart rate.
The best-evidenced product in the entire thermogenic aisle moves about 1.3 kg over twelve weeks, against placebo, in people who were also dieting. That is the ceiling for this category, and most of what is sold in it performs well below the ceiling. Two of the popular ingredients have no demonstrated weight-loss effect at all. Here are the numbers ingredient by ingredient, including where the number is zero.
What thermogenic means
A thermogenic ingredient raises energy expenditure slightly, usually through sympathetic nervous system stimulation. The claim being made is that you burn more calories at rest. The claim not being made, though it is often implied, is that this happens at a scale you could notice against normal day-to-day variation in intake and movement.
Keep the arithmetic in view. A 4% increase in 24-hour energy expenditure on a 2,400 kcal maintenance is under 100 kcal a day. That is one banana. It is real and it is at the edge of what any of these products deliver.
Caffeine
Tabrizi's dose-response meta-analysis of 13 randomised trials in 606 participants found caffeine intake may promote reductions in weight, BMI and body fat, with each doubling of caffeine intake associated with larger reductions. The overall effects are modest, roughly 1-2% of body weight.
There is a trap inside the dose-response finding. Chasing higher doses to chase the curve costs sleep, and short sleep has a larger effect in the opposite direction on body composition. In the Nedeltcheva crossover, restricted sleep did not change total weight lost but cut the fat fraction of that loss by 55%. Trading sleep for a thermogenic effect is a losing trade.
Caffeine also raises fat oxidation during exercise. That is a substrate-partitioning effect, meaning which fuel you use during the session. It is not evidence of net fat loss, and it is routinely presented as though it were.
Green tea catechins and EGCG
This is the best-evidenced entry here. A meta-analysis of 11 randomised trials of EGCG combined with caffeine over 12-13 weeks reported a mean 1.31 kg greater weight loss than control. A separate 15-trial analysis found 1.38 kg and 1.93 cm of waist circumference.
The catechins carry the effect, not the caffeine. Green tea extract increased 24-hour energy expenditure by 4% (P<0.01), while an equivalent dose of caffeine alone produced no change in energy expenditure or respiratory quotient. If you are buying a green tea product for the caffeine, you are buying the wrong half.
1.31 kg over three months, on top of a diet, in trial conditions. That is the honest framing. It is also more than any other ingredient on this page can claim.
Yohimbine
The evidence is weak, and that is the first thing to know. Yohimbine has no meta-analytic weight-loss support comparable to caffeine or catechins, and the available human work is small and often uses topical or combination formulations that do not tell you what oral yohimbine does on its own.
The reliable findings are the side effects: anxiety, raised blood pressure and raised heart rate. An ingredient with well-documented adverse effects and undocumented benefits is an easy call.
Synephrine and bitter orange
The evidence says this one does not work. A systematic review and meta-analysis of Citrus aurantium and p-synephrine found no evidence that synephrine facilitates weight loss, non-significant weight change after prolonged treatment, and no effect on body composition. Synephrine does tend to raise blood pressure and heart rate.
Synephrine is widely sold as the safe successor to ephedrine, which was banned from US supplements in 2004. It inherited the marketing position without inheriting the efficacy. Zero benefit and a cardiovascular signal is the summary.
Capsaicin
The evidence is thin and secondary. An analysis of 10 clinical trials found pre-meal capsaicinoids reduced energy intake at that meal by about 74 kcal on average, with a minimum effective dose around 2 mg and high variability between studies. There is no long-term data.
74 kcal off one meal is roughly 1-3% of daily intake, assuming no compensation later in the day, which has not been tested over time.
L-carnitine
An updated dose-response meta-analysis found L-carnitine reduced weight by 1.13 kg, BMI by 0.36 kg/m2 and fat mass by 1.16 kg. It did not change body fat percentage or waist circumference, and the effects appeared only in overweight or obese subjects who were also making lifestyle changes.
Fat mass down but body fat percentage unchanged is an internally awkward result, and it is the reason this sits in the weak-evidence tier despite a headline kilogram figure.
The honest summary
Ranked by what the trials actually found, over roughly 12 weeks, against control, in people who were already dieting.
| Ingredient | Published effect | Evidence | Verdict |
|---|---|---|---|
| Green tea catechins with caffeine | 1.31-1.38 kg over 12-13 weeks, 1.93 cm waist | Moderate, multiple meta-analyses | The best of a small category |
| Caffeine alone | Roughly 1-2% body weight, dose-responsive | Moderate | Small, and the dose competes with your sleep |
| L-carnitine | 1.13 kg, no change in body fat percentage | Weak | Inconsistent, only alongside lifestyle change |
| Capsaicin | About 74 kcal off a single meal | Weak, secondary sourcing | Too small and too short-term to count on |
| Yohimbine | No meta-analytic support | Weak | Documented side effects, undocumented benefit |
| Synephrine / bitter orange | No effect on weight or body composition | Reviewed as ineffective | Raises blood pressure and heart rate for nothing |
Who should avoid stimulant fat burners
Beyond the small size of the benefit, some people carry a specific risk from the stimulant load.
- Anyone with hypertension, arrhythmia or known cardiovascular disease.
- Anyone with an anxiety disorder, given the anxiety signal on yohimbine in particular.
- Anyone pregnant or breastfeeding.
- Anyone taking an MAO inhibitor, a stimulant medication, or a drug with a known cardiovascular interaction. Check with a pharmacist.
- Anyone whose sleep is already short, because the trade runs the wrong way.
The evidence, one row per claim
| Claim | Tier | Source |
|---|---|---|
| Tabrizi et al.'s dose-response meta-analysis of 13 RCTs (606 participants) found caffeine intake may promote reductions in weight, BMI and body fat, with each doubling of caffeine intake associated with larger reductions; overall effects are modest, roughly 1-2% of body weight. | emerging | Tabrizi et al., Crit Rev Food Sci Nutr |
| A meta-analysis of 11 RCTs of EGCG combined with caffeine over 12-13 weeks reported a mean 1.31 kg greater body weight loss versus control; a separate 15-RCT analysis found 1.38 kg and 1.93 cm waist circumference. | established | Am J Clin Nutr — green tea catechins with or without caffeine |
| Green tea extract increased 24-hour energy expenditure by 4% (P<0.01), whereas an equivalent dose of caffeine alone had no effect on energy expenditure or respiratory quotient, indicating the catechins carry the thermogenic effect. | established | PMC — green tea catechins and resting metabolic rate systematic review |
| A systematic review and meta-analysis of Citrus aurantium and p-synephrine found no evidence that synephrine facilitates weight loss, non-significant weight change after prolonged treatment, and no effect on body composition, while synephrine tends to raise blood pressure and heart rate. | not supported | PMC — safety and efficacy of bitter orange extracts and p-synephrine |
| An updated dose-response meta-analysis found L-carnitine reduced weight by 1.13 kg, BMI by 0.36 kg/m2 and fat mass by 1.16 kg, but did not change body fat percentage or waist circumference, and showed effects only in overweight/obese subjects combined with lifestyle modification. | emerging | PubMed — L-carnitine weight management meta-analysis |
| An analysis of 10 clinical trials found pre-meal capsaicinoids reduced energy intake at that meal by about 74 kcal on average, with a minimum effective dose around 2 mg and high between-study variability. | weak evidence | Ro clinical summary (secondary; trace to the underlying capsaicinoid meta-analysis before publishing) |
| Acute caffeine intake increases the fat oxidation rate during exercise, which is a substrate-partitioning effect and not evidence of net fat loss. | established | PMC — acute caffeine and fat oxidation during exercise meta-analysis |
| Yohimbine has no meta-analytic weight-loss support comparable to caffeine or catechins; the available human work is small and often in topical or combination formulations. | weak evidence | PubMed — topical lotion containing yohimbe and others |
Questions people ask
Does burning more fat during exercise mean losing more fat?
Should I stack several of these?
Sources
- Tabrizi et al., Crit Rev Food Sci Nutr
- Am J Clin Nutr — green tea catechins with or without caffeine
- PMC — green tea catechins and resting metabolic rate systematic review
- PMC — safety and efficacy of bitter orange extracts and p-synephrine
- PubMed — L-carnitine weight management meta-analysis
- Ro clinical summary (secondary; trace to the underlying capsaicinoid meta-analysis before publishing)
- PMC — acute caffeine and fat oxidation during exercise meta-analysis
- PubMed — topical lotion containing yohimbe and others
Keep reading
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- Fibre Supplements and Glucomannan for Appetite: What the Trials SupportShow both conflicting meta-analyses and explain why the conflict is itself informative, then pivot to food fib
- Creatine: A Lean-Mass Supplement, Not a Fat-Loss SupplementCorrective. The best-evidenced sports supplement in existence has no fat-loss effect, and it is still worth ta
- Sleep and Alcohol: The Two Variables That Change What You LoseNeither changes the scale much. Both change what the scale is made of, which is why people dismiss them.
- Energy Balance: The One Mechanism Behind Every Method on This SiteThe mechanism page. Every other page here is a different way of arriving at the same physiology.