A Metabolism Supplement Guide: What Is On The Shelf And What The Evidence Says

A map of the category, compiled from the trials cited at the foot of this guide.

Read down that last column and the shape of the shelf is clear. The evidence is strongest for the thing everybody already knows about — caffeine raises energy expenditure — and thins steadily as the ingredient becomes more exotic.

That ordering matters because a label's ingredient list usually runs the other way. The novel-sounding ingredient is what the marketing is built on, and the ingredient with the actual evidence is the one nobody boasts about because it is in coffee.

The 1999 respiratory-chamber trial is the study this ingredient rests on. Ten healthy men were given an extract supplying 50 mg of caffeine and 90 mg of EGCG at each of three meals. Twenty-four-hour energy expenditure rose four per cent and the respiratory quotient fell, which is the signature of a shift towards burning fat. Caffeine alone at the same amount did neither, which is why green tea extract is on this shelf rather than being redundant beside a coffee.

Over months the picture shrinks. The Cochrane review of green tea preparations in overweight and obese adults found weight changes small and not clinically important. A 2009 meta-analysis reached a similar conclusion. A twelve-week trial at 856.8 mg of EGCG a day reported about 1.1 kg, with lower ghrelin and higher adiponectin. A 2024 meta-analysis of catechins alongside exercise training found the addition modest.

There is also a safety literature worth knowing about before buying a concentrated extract. The United States Pharmacopeia review of green tea extract hepatotoxicity exists because catechin concentrates have been associated with liver injury, more often when taken on an empty stomach. Taking any such product with food is a free precaution.

Neither story is a reason to avoid a product containing these ingredients. Both are reasons to be sceptical of a page that presents either as settled, and to look at what else is on the label rather than at the ingredient with the best story.

• A trial of the finished product. Almost nothing on this shelf has been tested as sold. The research is about single ingredients at stated doses.
• Pre-market approval. A dietary supplement is not approved before sale. The manufacturer is responsible for safety and for the truth of its label, and regulators act afterwards.
• A permitted disease claim. Every claim on every label here is a structure and function statement, and that is the only category available.
• An effect independent of diet and activity. Every trial cited in this guide ran alongside a diet, an exercise programme or both.
• Guaranteed contents, absent a certificate of analysis. A 2025 meta-analysis of adulteration in weight-loss products and a review of the FDA tainted-supplements database both found undeclared pharmaceutical ingredients in a meaningful share of what they examined.

That last point is the reason a certificate of analysis is worth more than any seal on a bottle. A GMP mark describes how a facility is run; an analysis describes what is in the capsule, and only one of those answers the question a buyer has.

A narrative review of supplements for weight management reaches the conclusion this guide would: there is real chemistry here, the effects are modest, and they are additions to diet and activity rather than replacements for either.

1. Find the stimulant first, and find its amount. If there is caffeine and no figure, you cannot plan the rest of your day around it.
2. Check whether each botanical carries an amount and a standardisation. An extract without a marker percentage is an unknown quantity even when a weight is printed.
3. Compare the amounts you find against the doses in this guide. They are all from the trials' own methods sections.
4. Look for an ingredient whose evidence is animal-only, and discount the story attached to it rather than the ingredient itself.
5. Ask whether a certificate of analysis exists, and who ran it.
6. Then look at the guarantee, because on a shelf where nothing is tested as sold, the ability to change your mind is the concrete thing you are buying.

Those six checks take five minutes and they are the difference between buying a story and buying a product. The longer version, with what a good answer looks like at each step, is in choosing a metabolism supplement.