How we grade supplements

Every supplement guide on this site carries a grade from A to D. The grade describes the strength of the human evidence for the effect being claimed — not how popular the ingredient is, and not how much we like it.

The scale

A Strong evidence

Multiple well-conducted human trials agree, and at least one meta-analysis or position stand from a recognised body supports the effect. The effect size is large enough to notice. Creatine and caffeine sit here.

B Moderate evidence

Human trials lean positive and are replicated, but the effect is small, the window of usefulness is narrow, or the literature is thinner than an A grade requires. Beta-alanine and omega-3 sit here.

C Mixed or conditional

The evidence is inconsistent, mostly preliminary, or the supplement works only for a specific subgroup. Vitamin D is graded C because it is an A for deficient people and close to a D for everyone else.

D No credible evidence

No meaningful human evidence for the marketed claim, or trials that show nothing. A D grade is not a gap in the research — it is a result, and we publish those too.

What counts as evidence

In descending order of weight: position stands and meta-analyses from recognised bodies, randomised controlled trials in humans, and controlled trials in the relevant population — trained people, where the claim concerns training.

What does not move a grade: animal studies, in vitro work, mechanistic plausibility, and manufacturer-funded trials that have not been replicated independently. All of those can be interesting. None of them establishes that a thing works in a person.

Every citation on a supplement page links to the study on PubMed so you can read the abstract yourself rather than take our summary on trust.

Effect size matters as much as significance

A statistically significant result is not automatically a useful one. A supplement can reliably produce an effect too small for you to detect, and the honest way to report that is to say so rather than to state that it "works".

Where an effect is real but small, we say small — as onomega-3 andbeta-alanine. Where an effect only applies to a subgroup, we say which subgroup, as onvitamin D.

Conflicts of interest

This site carries advertising and uses affiliate links, both disclosed on theadvertising disclosure page.

Grades are set before any commercial consideration and are never adjusted for it. A supplement that pays well and does nothing gets a D — which is the only reason a grading scale is worth publishing at all. If we ever cannot grade something independently, we will not cover it.

Grades change

Evidence accumulates and sometimes reverses. Where a grade changes, the page records when and why rather than quietly editing the letter.

Health content on this site is reviewed by a named medical reviewer before publication — see the editorial policy for how that works.