The supplement industry is one of the most aggressively marketed categories in health and wellness. Products are sold with clinical-sounding language, testimonials from credible-looking sources, and claims that often exceed what the evidence supports by a significant margin. Before looking at any specific supplement, it is worth having a framework for evaluating those claims.

The hierarchy of evidence

Not all evidence is equal. When a supplement brand claims their product "has been shown to" produce a particular benefit, the question is: shown in what kind of study?

Randomised controlled trials (RCTs) are the gold standard. A group of people is randomly assigned to receive either the supplement or a placebo, without knowing which they're receiving. Outcomes are compared. A well-designed RCT with a meaningful population and a relevant outcome measure is the most reliable type of evidence for whether a supplement does what it claims.

Observational studies show associations — they find that people who consume more of X tend to have better Y outcomes — but cannot establish that X causes Y. Many supplement claims are based on observational evidence and presented as if they were causal.

Animal and in-vitro studies show that something happens in a petri dish or a mouse model. This is useful mechanistic evidence but frequently does not translate to the same effect in humans.

Testimonials are not evidence. Individual experiences reflect too many variables to attribute an outcome to any single intervention.

Statistical vs clinical significance

A study can show a statistically significant effect that is clinically meaningless. If a supplement increases some marker by a small amount in a tightly controlled study, but the magnitude of that effect is well within normal daily variation, the finding has limited practical relevance regardless of the p-value.

When evaluating evidence, ask not just "was there an effect?" but "how large was the effect, and does that magnitude matter in a real-world context?"

Individual variation

Population-level averages conceal significant individual variation. A supplement that shows a meaningful average effect in a study contains within that average people who responded strongly, people who didn't respond, and people who responded negatively. The average result may not describe anyone in the study accurately.

This is why individualised approaches — and where possible, blood testing to understand actual status before supplementing — produce better outcomes than blanket recommendations.

The food-first principle

Most nutrients that are worth supplementing can also be obtained from food, and in most cases the food form is better absorbed, comes with cofactors that aid its function, and produces broader nutritional benefits. Supplementation makes sense where:

  • A genuine dietary gap exists that food sources can't practically close
  • A specific nutrient is consistently deficient in a population (as with Vitamin D in low-sun environments)
  • The evidence for supplementation at a specific dose is stronger than for food intake alone

Where a nutrient can be reliably obtained from diet, closing that gap through food is generally preferable to supplementation.

A useful question to ask

Before buying a supplement, ask: "What is the specific, well-powered RCT evidence that this supplement, at this dose, produces this outcome in people like me?"

For many supplements, the honest answer is: the evidence is weak, mixed, or only exists in populations that don't describe you. For a small number of supplements, the evidence is genuinely strong. The following articles in this series cover the ones that sit in the latter category.