The Metabolism Supplement Guide: What Each Ingredient Was Tested At

The single most useful habit a reader can develop in this category is to treat an ingredient name as a question rather than an answer. Green tea extract is not one thing: it is a family of preparations that differ by an order of magnitude in catechin content, and the trials that found something used amounts at the top of that range.

That is why every section below leads with a number. A formula containing an ingredient at a tenth of its trial amount is not a weaker version of the trial; it is an untested preparation that borrows the trial's reputation. And a formula that prints no amount at all cannot be placed on that scale in either direction, which is a different problem and a more common one.

Work on supplement databases built from label information makes the general point: a label is a claim about a product rather than a measurement of one, and everything downstream inherits that limit.

Chlorogenic acid is the compound green coffee bean extract is standardised for, and unroasted beans are used because roasting destroys much of it. The proposal is that it slows intestinal glucose absorption, flattening the rise in blood sugar after a meal.

A 2023 systematic review and meta-analysis pooled three randomised trials, 103 people in total, of extract carrying at least 500 mg of chlorogenic acid a day, and found a mean reduction of 1.3 kg with no heterogeneity and no evidence of publication bias. A separate eight-week trial used 400 mg a day of the extract alongside an energy-restricted diet.

This ingredient also carries the category's clearest lesson about publicity. The 2012 crossover study that made green coffee famous reported 8 kg of loss in 16 people at 700 and 1,050 mg, was promoted heavily on daytime television, and was retracted by its journal in 2014. PubMed still returns it with the publication type Retracted Publication. Almost every dramatic claim made for this ingredient traces back to it.

What survives is smaller and worth having. A review of the blood-pressure claim and a meta-analysis of cardiovascular risk markers both report measurable effects on risk factors. The mouse study the story began with remains a mouse study.

The practical reading: 500 mg a day of chlorogenic acid is the figure to hold any green coffee product against, and a product that does not state its chlorogenic acid content cannot be held against it.

Hydroxycitric acid, from the rind of Garcinia cambogia, competitively inhibits ATP-citrate lyase, the enzyme that starts the conversion of carbohydrate into fat. It is also widely sold for appetite suppression.

The best-known trial ran twelve weeks in 135 overweight adults, everyone on a high-fibre, low-energy diet, with 1,500 mg of hydroxycitric acid a day against placebo. Both arms lost weight. Neither weight nor fat mass separated the two to statistical significance.

A 2020 dose-response meta-analysis of eight trials in 530 people found a pooled reduction of 1.34 kg, which is a genuine positive on a modest scale. The main safety overview of the standardised extract was written by authors with a commercial interest, which is worth knowing when weighing its conclusions.

Garcinia is also the ingredient most often named in this category's liver literature. A systematic review of herb-induced liver injury and a review of supplement-related liver injury both discuss weight-loss products containing it, generally at amounts far above a once-daily capsule.

The practical reading: 1,500 mg a day is the figure the literature is attached to, and the largest single trial at that amount found nothing.

Taken as a whole the picture is consistent and it is modest. The pooled effects in this category cluster around one to one and a half kilograms over trials lasting weeks to months, in people who were also changing something else. A 2021 systematic review of dietary supplements and alternative therapies for weight loss reached the same conclusion across the field.

That is not nothing and it is not what the category advertises. A product promising a transformation is promising something no ingredient on this list has produced in a controlled trial, and the one study that did report a transformation was retracted.

The second thing the whole picture shows is where the risk sits. Emergency department surveillance across the United States found weight-loss products over-represented among supplement-related visits, driven largely by stimulant effects in younger adults. The risk in this category is concentrated in the ingredient that also has the best evidence, which is an uncomfortable combination and the reason a stated caffeine content matters so much.

The eight-point checklist for judging an individual bottle is in the companion guide: choosing a metabolism supplement.