The microbial population of the gut, or gut microbiota is extremely dense, predominantly comprising bacteria. Whilst a core community of gut microbes—approximately one third—is common to most healthy adults, the composition of each gut microbiota varies according to factors including genetics, diet, age, state of health and geographical location. Microbes are considered important—but probably not essential—to digestion. This is primarily because the gut microbiota breaks down substances that the gut alone is unable to digest and is also involved in synthesis of vitamins.