Temporal variations of the gut microbiome in human health

The colonisation of the human gut microbiome commences at birth and continues to evolve throughout the lifespan. A balanced symbiotic relationship between the host and gut microbiome is essential for maintaining overall health. This two-part Series presents a comprehensive overview of the gut microbiome across temporal and spatial dimensions, considering diurnal, seasonal, and lifespan variations while covering the entire gastrointestinal tract. We also discuss the extrinsic and intrinsic factors that shape the microbial ecosystem and affect host homoeostasis, health, and disease susceptibility. In this first Series paper, we summarise current knowledge on the microbial succession and evolutionary trajectory of the gut microbiome from neonates to adults aged 100 years and older, subsequently focusing on diurnal rhythms and seasonal patterns. We then discuss how these temporal variations in the gut microbiome are determined and how they contribute to beneficial or detrimental health outcomes in the host. Overall, elucidating the multiscale temporal dynamics of the human gut microbiome will open crucial opportunities to expand knowledge of host-microbiome interactions and their biological and clinical implications.