Every fat burner on the market leads with thermogenic ingredients. The category is enormous, the claims are bold, and the quality of evidence ranges from strong to essentially non-existent. This guide covers the most common thermogenic compounds you'll encounter, graded by the rigour of the science behind them.
Caffeine — The Benchmark
Caffeine is the most studied thermogenic compound in existence and the standard against which others are often compared. Its effects are well-characterised, dose-dependent, and reproducible across populations.
At 3–6mg/kg of bodyweight, caffeine increases resting metabolic rate by 3–11% and enhances fat oxidation during moderate exercise. It also provides performance benefits that indirectly support calorie expenditure through improved training output.
- Effective dose: 100–400mg depending on bodyweight and tolerance
- Best sources: Green coffee bean extract (slower release), anhydrous caffeine (fast onset)
- Key limitation: Tolerance develops within 1–2 weeks of consistent daily use; cycling off periodically preserves efficacy
- Verdict: The most reliably effective thermogenic available — but it's not magic, it's a tool
EGCG (Green Tea Extract) — Synergistic With Caffeine
Epigallocatechin gallate (EGCG) is the primary bioactive catechin in green tea. Its thermogenic effects are modest when taken alone, but they stack meaningfully with caffeine — the combination produces a synergistic increase in fat oxidation that is greater than either compound alone.
Multiple meta-analyses confirm that green tea preparations (containing both EGCG and caffeine) produce small but statistically significant reductions in body weight compared to placebo — typically 1–3kg over 12 weeks. Decaffeinated versions are considerably less effective.
- Effective dose: 400–500mg EGCG equivalent
- Quality indicator: Look for standardised extracts with a declared EGCG content, not simply "green tea extract"
- Verdict: A well-justified inclusion in any thermogenic formula when combined with caffeine
Capsaicin / Cayenne Pepper — Real Effects, Practical Limitations
Capsaicin — the compound responsible for the heat in chilli peppers — is a genuine thermogenic. It activates TRPV1 receptors in the body, triggering a transient increase in adrenaline release and heat production. Studies consistently demonstrate a short-term increase in metabolic rate of around 4–5% and enhanced fat oxidation.
The practical limitation is tolerance. The thermogenic response to capsaicin habituates quickly — typically within a week of regular intake. This makes it more effective as an intermittent tool than a daily supplement.
- Effective dose: 2–6mg capsaicin equivalent; often listed as cayenne pepper extract standardised to capsaicin
- Alternative delivery: Capsimax and Capsugel are commercial forms designed to reduce GI irritation
- Verdict: Effective short-term; rotate use to avoid tolerance. A good addition to a formula that's already rotating rather than taken daily indefinitely
Grains of Paradise — Emerging Evidence
Grains of paradise (Aframomum melegueta) is a West African spice related to ginger, and one of the more interesting newer entries in the thermogenic category. It contains 6-paradol, which activates brown adipose tissue — a metabolically active form of fat that burns energy to generate heat.
A small number of human trials (including a well-designed double-blind study) show statistically significant increases in energy expenditure and reductions in visceral fat compared to placebo. The sample sizes are small but the methodology is sound.
- Effective dose: 40mg standardised extract
- Verdict: Genuinely interesting mechanism; limited but promising evidence. Worth watching — more human data is needed before a firm recommendation
Yohimbine — Effective but Needs Caution
Yohimbine works by blocking alpha-2 adrenergic receptors, which normally suppress fat breakdown. By blocking these receptors, it allows adrenaline to exert a stronger lipolytic effect — particularly in stubborn fat areas that are rich in alpha-2 receptors.
The evidence for yohimbine's fat-loss effects is reasonable, particularly in lean or athletic populations. However, its side effect profile is among the most notable of any common thermogenic: anxiety, elevated heart rate, blood pressure spikes, and headaches are frequently reported, especially at higher doses.
- Effective dose: 0.2mg/kg bodyweight, taken fasted
- Not suitable for: Anyone with anxiety disorders, hypertension, heart conditions, or who is taking MAOIs or stimulant medications
- Verdict: Works, but the risk-benefit ratio is less favourable than caffeine or EGCG for most people. Only worth considering if you've already optimised the safer options
What Doesn't Have Meaningful Evidence
Raspberry Ketones
Raspberry ketones became extremely popular after appearing on a major US television programme. The science, however, has not kept pace with the marketing. The mechanistic evidence is from cell and animal studies. There are no high-quality human clinical trials demonstrating meaningful thermogenic or fat-loss effects. Current dosing in supplements is likely far below anything that could plausibly produce an effect.
Synephrine / Bitter Orange (Citrus aurantium)
Synephrine is structurally similar to ephedrine, the now-banned stimulant. It was widely adopted as a substitute after ephedra was removed from the market. Thermogenic effects exist, but so do documented cardiovascular risks including elevated blood pressure and heart rate — and case reports of serious adverse events.
The risk-benefit ratio does not support its use given the alternatives available. We flag any product that includes synephrine or bitter orange extract prominently as a primary thermogenic — the risk is not justified.
CLA (as a Thermogenic)
CLA is sometimes marketed as thermogenic, but the mechanism is more accurately described as anti-lipogenic (it may reduce fat deposition) rather than thermogenic. Effect sizes in humans are small and inconsistent. Its calorie-burning effect is not meaningfully comparable to caffeine or EGCG.
How Thermogenics Work Together
The most effective thermogenic formulas combine ingredients with different mechanisms, which can produce additive or synergistic effects:
- Caffeine + EGCG: The most evidence-backed combination in the category. The synergy is well-documented.
- Caffeine + capsaicin: Additive thermogenic effects via different pathways (CNS stimulation vs. TRPV1 activation).
- Adding grains of paradise: Brown adipose tissue activation complements rather than duplicates the above two mechanisms.
What doesn't help is stacking five or six thermogenic ingredients at sub-clinical doses. This is a label marketing strategy, not a formulation strategy — the total effect will be less than one ingredient at a proper dose.
Bottom Line
For thermogenic support, a product built around caffeine (100–200mg), standardised green tea extract (400–500mg EGCG), and either capsaicin or grains of paradise will cover the strongest ground. Everything else is secondary.
When evaluating a product, check that the thermogenic ingredients are individually dosed on the label — not hidden in a proprietary blend. A properly dosed formula of three ingredients will always outperform an underdosed ten-ingredient blend.