Term

Randomised Controlled Trial

The study design that supports a causal claim. Why a supplement trial's controls matter more than its size.

A randomised controlled trial assigns participants to treatment or control by chance, then compares the outcomes. Randomisation is what allows a causal claim: if groups differ only by assignment, a difference in outcome is plausibly caused by the intervention.

The three design features that matter

Randomisation. Removes the possibility that people who choose a supplement differ systematically from those who do not — which is the flaw in every “people who take X report feeling better” observation.

A control group. Needed to separate the intervention from everything else happening at once. The choice of control changes the answer: a trial against nothing, a trial against placebo, and a trial against the standard diet answer three different questions.

Blinding. Prevents expectation from shaping reported outcomes. This matters more for subjective endpoints — bloating, energy, pain — than for a lab value.

Why the control condition changes the headline

A fibre trial comparing psyllium against nothing will report a larger effect than the same fibre compared against ordinary starch. Both are valid RCTs and both are correct; they are not answering the same question. This is the single most common way a real trial number gets over-read.

When comparing two figures for the same ingredient, check the comparator before concluding that one is wrong.

Size is not the same as quality

A 3,000-person trial with no blinding and an absent control is weaker evidence than a 200-person blinded trial against placebo. Sample size improves precision; design determines whether the estimate means anything.

This is why our grading standard caps a row at C when the population or the intervention does not match the reader, regardless of how many people were enrolled.

What an RCT cannot tell you

Efficacy in a trial population is not effectiveness in the general population. Trial participants are screened, monitored, and often healthier than the people who would buy the product. A result in a diagnosed population does not transfer to someone acting preventively, which is the reason most of our glucose rows are graded B rather than A.

Facts checked: 29 October 2026 · VitalRoundup Editorial Team · How we review