How we grade the evidence
Every guide on The Good Supplements carries one of four grades. They describe the strength of human research behind a claim, not how popular or trendy the supplement is.
Backed by multiple randomized controlled trials and consistent meta-analyses.
Supported by several human trials, though sample sizes or consistency are limited.
Human trials disagree, or benefits show up in some populations but not others.
Mostly animal or preliminary data. Human trials are sparse or use unreliable doses.
We start from trial count and design
Randomized controlled trials outrank observational studies, which outrank animal or in-vitro data. A supplement backed by one small trial is graded differently from one backed by a dozen, even if both technically have 'a study behind them'.
We weigh consistency, not just existence
A single positive trial surrounded by several null results doesn't earn a strong grade. We look at whether independent research groups, using different populations, land on similar conclusions.
We check whether the dose matches
A lot of 'this doesn't work' experiences come from underdosed products. We compare label doses against the doses actually used in the supporting trials, and we say so when they don't match.
We separate deficiency correction from general use
Several supplements, vitamin D and magnesium included, show their strongest effects in people who are actually deficient. We're explicit about that distinction instead of implying universal benefit.