Diet is a major determinant of the gastrointestinal microbiome composition and function, yet our understanding of how it impacts the efficacy of prebiotics and probiotics is limited. Here we examine current evidence of dietary influence on prebiotic and probiotic efficacy in human studies, including potential mechanisms. We propose that habitual diet be included as a variable in prebiotic and probiotic intervention studies. This recommendation is based on the potential mechanisms via which diet can affect study outcomes, either directly or through the gut microbiome. We consider the challenges and opportunities of dietary assessment in this context. Lastly, we provide recommendations for the design, conduct and reporting of human clinical trials of prebiotics and probiotics (and other biotic interventions) to account for any effect of diet and nutrition.