Riboflavin requirements vary with heredity, growth, environment, age, activity, health, other dietary components and synthesis by the host. Microbial synthesis of riboflavin has been shown to occur in the gastrointestinal tract of a number of animal species and thus affects requirements. Depending on the species, utilization depends on the composition of the diet (Christensen, 1973) and incidence of coprophagy. Young rats fed a riboflavin-free, purified diet with sucrose as the carbohydrate source will cease to grow. However, when sucrose is replaced by starch, sorbitol, or lactose, growth is comparable to that of rats supplied with riboflavin (Fridericia et al., 1927; Haenel et al., 1959). Excretion of riboflavin in urine and feces is also dependent on the carbohydrate in the diet and is suppressed by inclusion of sulfa drugs in the diet (De and Roy, 1951). Antibiotics, such as tetracycline, penicillin and streptomycin, reduce the requirements of several animal species for riboflavin via stimulation of microorganisms that synthesize riboflavin, or inhibition of microorganisms in the gut that compete for riboflavin.
For the dog and cat, the contribution of microbial riboflavin synthesis is not known. However, Gershoff et al. (1959a) fed varying levels of fat and carbohydrates to kittens and concluded that a high-carbohydrate, low-fat diet favored synthesis of riboflavin by intestinal microorganisms as indicated by greater urinary and fecal excretion. The high-carbohydrate diet may also have favored utilization or retention of riboflavin. The high-fat diets, 46% vs. 11% of metabolizable energy (ME) from fat, increased the riboflavin requirement in kittens from 0.15 to 0.20 mg per day (Gershoff et al., 1959a).
