The human large intestine is home to trillions of bacteria that have a major impact on the health of their host. For example, these microbes are able to extract nutrients from dietary complex carbohydrates that are not susceptible to human digestive enzymes. These bacteria also interact with the immune system of their host, resulting in a positive, and in some circumstances, negative impact on human health. Understanding how these microorganisms use different nutrients will underpin the use of dietary strategies designed to maximize the positive impact this microbial ecosystem can have on human health.