<p>Fruits and vegetables are rich in compounds with health-promoting effects, such as vitamins. Nevertheless, no previous research has simultaneously considered the factors affecting vitamins’ bioavailability, namely the actual amount absorbed and the health effect, and thus the health benefits that can be claimed. Therefore, in this study, we conducted a critical appraisal by presenting the theory and body of evidence on the bioavailability and legislation of vitamins C, B2, and B9. After a thorough review of the evidence, we observed that bioavailability is a complex concept for the three vitamins analysed. The characterisation of such vitamins in regulations and by general knowledge seems oversimplified. Also, measurement protocols should be detailed and focus on the bioactive forms in humans. The way vitamins are characterised and measured has implications for the use of nutrition and health claims. A better understanding of what each vitamin compound comprises is needed for decision-makers and users, as such insights would help harmonise what is measured in the laboratory, regulated by EU law, and taken up in our cells.</p> Graphical Abstract <p></p>