What peer‑reviewed clinical trials exist for common 'fat‑burner' ingredients like green tea extract, Garcinia Cambogia and CLA?

1. Green tea extract — dozens of small RCTs, a handful with measurable metabolic effects

Randomized, double‑blind, placebo‑controlled trials of green tea extract and standardized Camellia sinensis preparations have been published and include studies showing changes in energy‑metabolism markers or modest effects on weight or substrate oxidation, such as a 60‑day trial in overweight post‑menopausal women reporting statistically significant differences in respiratory quotient and substrate use ^[6], and other trials testing high‑dose extracts in centrally obese women ^[1]; systematic reviews summarize multiple RCTs but stress heterogeneity in formulation, dose and populations studied ^[1].

2. Garcinia cambogia (HCA) — randomized trials with small average weight losses but real safety warnings

Multiple randomized clinical trials of Garcinia/HCA have been pooled in systematic reviews and meta‑analyses that found statistically significant but small mean weight reductions (meta‑analyses incorporating eight trials/≈530 subjects reported mean weight loss ~1.3 kg versus placebo) and nonlinear dose–response relationships, while earlier reviews judged the magnitude clinically uncertain ^{[2] [4] [7]}. Those efficacy signals coexist with pharmacovigilance and case‑series reports: peer‑reviewed clinical literature documents moderate to severe hepatotoxicity linked to Garcinia products (sometimes in combination with green tea), with hospitalizations, transplant and fatal outcomes reported, and a proposed immune‑linked mechanism associated with HLA‑B*35:01 in some cases ^{[8] [9] [5]}.

3. CLA — some RCTs, occasional between‑group differences, long‑term metabolic concerns

Conjugated linoleic acid has been the subject of randomized trials and appears in summaries of nutraceutical RCTs as having at least one study reporting clinically significant between‑group weight losses, but no formulation has produced consistent, clinically meaningful weight loss across trials ^[3]. Broader reviews and consumer‑facing summaries note small average effects and raise concerns about digestive side effects and potential adverse changes in liver fat and insulin sensitivity with longer use, emphasizing the limited and mixed nature of evidence ^{[10] [11]}.

4. What the systematic reviews say — modest averages, inconsistent clinical relevance

Systematic reviews that searched multiple databases and pooled RCTs conclude that some single‑source nutraceuticals (including green tea, Garcinia and CLA among many others) show statistically significant effects in individual trials, but none produce consistent, clinically significant and sustained weight loss; reviewers emphasize small sample sizes, heterogeneous products and dosing, short durations and the need for higher‑quality trials to determine durable benefit ^{[3] [4] [2]}.

5. Safety and real‑world implications — heterogeneity of products and real harms

Publication‑level evidence shows safety signals that cannot be ignored: clinical case series and systematic safety reports link Garcinia‑containing products and concentrated green tea extracts to hepatocellular injury — sometimes severe — and reviewers explicitly compare those injury patterns, urge caution, and note variability between commercial product labels and chemical analyses ^{[8] [9] [5]}. The evidence base for efficacy is not strong enough to dismiss safety concerns, because much of the trial literature uses different extracts, combinations and doses from what consumers buy ^{[8] [3]}.

6. Bottom line for clinicians and consumers — trials exist but they don’t settle the question

Peer‑reviewed randomized clinical trials and multiple meta‑analyses document small, inconsistent effects of green tea extract, Garcinia cambogia and CLA on body weight and metabolic endpoints, while case reports and safety investigations document rare but serious adverse events for Garcinia and some green tea extracts; the balance of evidence calls for skepticism about overblown “fat‑burner” claims and for larger, rigorous trials and clearer safety monitoring before these supplements can be endorsed as effective, safe weight‑loss agents ^{[4] [2] [5]}.