Minimum thiamin requirement of weanling Sprague-Dawley outbred rats.

To determine the minimum thiamin requirement for maximal growth, two trials were conducted using male weanling Sprague-Dawley rats fed graded doses of thiamin from thiamin mononitrate as a component of a chemically defined diet. This diet included 16% amino acids, 72% sucrose and cornstarch and 5% soybean oil. Total weight gain and food intake were recorded over 2- (trial 1) or 3- (trial 2) wk periods. In trial 1, graded levels of thiamin were fed at 0, 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 mg thiamin/kg diet, and growth rate reached a plateau in rats fed 0.50 mg thiamin/kg. In trial 2, lower doses of thiamin were fed (0, 0.25, 0.50, 0.75, 1.0, 4.0 and 5.0 mg/kg) to determine the minimum requirement for maximal growth. Using broken-line least-squares analysis, weight gain reached a plateau (6.8 g/d) at a thiamin concentration of 0.55 +/- 0.07 mg/kg. No differences (P > 0.05) in weight gain, food intake or gain:food ratio were observed at thiamin levels at or above 0.5 mg/kg, but food intake was substantially lower (P < 0.05) in rats fed 0 and 0.25 mg thiamin/kg (9.9 and 13.4 g/d, respectively) than in rats fed higher doses of thiamin (16.1 g/d). Hepatic transketolase, a measure of enzymatic thiamin status, increased with dietary thiamin in rats fed diets containing 0-5.0 mg/kg thiamin. However, an inflection point occurred at 0.53 mg thiamin/kg, with the slope being eight times greater below than above the inflection point. The data suggest that the thiamin requirement for maximal growth of weanling rats fed a chemically defined diet is approximately 0.55 mg thiamin/kg, which is substantially below the current National Research Council estimated requirement of 3.1 mg thiamin/kg diet.