The National Research Council defined the minimum nutritional requirements for rabbits back in the mid 1960's. Much has changed since then in terms of actual nutrients and especially with regards to additives, probiotics, and other non-nutritive components that have seen tremendous progress in just the last few years. House rabbits need far less calories and fat in the diet than production rabbits. Rabbits need foods that contribute to a luxurious fur, but this goal must be weighed against controlling fat intake. Typically, these pellets are low in fiber and higher in fat and protein.
Rabbits are herbivores.
Herbivores have developed evolutionarily to consume plant material as the primarily component of the diet. The digestive tract of herbivores is also specialized. Rabbits are no exception. Their digestive tract includes a significant hindgut, including the cecum, which is the first part of the large intestine, except it is dilated and ends bluntly. The cecum is where most of the fermentation of fiber by microbes occurs.