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Growth and riboflavin status of rats fed different levels of protein and riboflavin.

The relationship between riboflavin and protein utilization was studied in 5-week-old male Sprague-Dawley rats, by using a factorial design with three levels of riboflavin (8, 16 and 24 microgram per rat per day) and protein (1.0, 1.6 and 2.2 g casein per rat per day) in a 9-week experiment. With the lowest level of casein, protein intake was growth limiting, and the level of riboflavin intake had no effect on either weight gain or liver nitrogen retention. With the two higher levels of casein, both weight gain and liver nitrogen retention increased with riboflavin intake, but 24 micrograms riboflavin per day was inadequate for maximal utilization of nitrogen from 2.2 g casein. Neither protein nor riboflavin intake affected the concentration of liver nitrogen per gram of fresh tissue. Increasing the protein intake from 1.0 to 1.6 g increased riboflavin retention in the liver, but additional protein had no further effect. Liver and muscle (gastrocnemius) riboflavin concentrations, as micrograms per gram wet tissue, increased with riboflavin intake. At the two higher intakes of riboflavin, tissue riboflavin levels decreased and the erythrocyte glutathione reductase activity coefficients (EGR-AC) increased with protein intake. These findings are consistent with the view that the effect of protein on riboflavin requirement is related to the rate of growth and not to protein intake, per se.