Current information on the effect of diet on the secretion of mucus and the recovery of mucin in ileal digesta is summarized. A general description of mucus structure and its degradation in the small and large intestine is provided. As the protective lining of the entire gastrointestinal tract, mucus gels are exposed to all chemical and physical forces of digestion. Most important among these is the proteolytic breakdown of mucus gels into component mucin subunits and their subsequent release into the intestinal lumen. Erosion of mucus gels is countered by synthesis and secretion from the underlying epithelium. Diets can influence this process, both indirectly by their effects on digestive processes most importantly with respect to the amount and distribution of proteolytic enzymes in the intestinal lumen, and directly by the physical forces which they exert on the gastrointestinal mucosa. Adaptive changes in goblet cell activity have been noted in response to different diets. Once in the intestinal lumen, little further degradation of mucus occurs prior to the large intestine. Once in the large intestine mucin is fermented by resident microbial populations. The recovery of undegraded mucin in ileal digesta has important implications for nutritional studies: firstly because it may represent a considerable loss of endogenous amino acids and carbohydrates and secondly because it may provide insight into the effects of diets on the digestive tract itself.