A depletion-repletion folate bioassay based on growth and tissue folate concentrations of rats.

To improve standardization of a folate bioassay, folate-depleted rats were repleted with a folate-free amino acid-based diet supplemented with 29 levels of folic acid. Growth was the main response variable and body tissue folate concentrations were also assessed. Because a positive correlation was observed between low levels of dietary folic acid and growth and little or no correlation was observed between high levels and growth, six regression models with a steep slope for low levels and a shallow or zero slope for high levels of dietary folic acid were evaluated. The model referred to as the "two-phase regression" or "change-point" model best described the relationship. Depleted rats needed 674 +/- 71 nmol folic acid/kg diet to reach their full growth potential. This value is biologically sensible, and this regression model is well established in the statistical literature. The change-point model is highly recommended to characterize the growth response, because growth is a functional response and, in the range of 226 to 680 nmol folic acid/kg, this response is linear, which is an additional advantage. Linear responses are easier to interpret because of complicated issues of interpretation and confidence intervals with nonlinearities. Linear regressions described serum and liver folate responses, whereas exponentials described whole blood and carcass folate responses. Depleted rats needed 5920 and 5780 nmol folic acid/kg diet to maximize their whole blood and carcass folates, respectively.