Do salivary proline-rich proteins counteract dietary hydrolyzable tannin in laboratory rats?

We hypothesized that dietary hydrolyzable tannins would not act as digestibility reducing substances but would be excreted in the feces if the tannin were ingested by rats producing salivary proline-rich proteins (PRPs). To test that hypothesis we used two groups of Sprague-Dawley rats: tannin-naïve rats that were secreting basal levels of salivary PRPs and tannin-habituated rats that were secreting elevated levels of PRPs. The animals were fed for 10-18 d on diets containing 3% (w/w) purified hydrolyzable tannin [pentagalloyl glucose (PGG)] that was periodically spiked with chemically synthesized, radiolabeled 1,2,3,4,6-penta-O-galloyl-[U-14C]-D-glucopyranose (1 microCi per gram diet). The PGG-habituated rats excreted three times more of the consumed 14C in their feces than did the PGG-naive rats (11.4% for PGG-habituated rats vs. 3.5% for PGG-naïve rats, P < 0.05). The addition of 3% PGG to the diet of the PGG-naïve rats had no significant effect on apparent dry matter or nitrogen digestibility (P > 0.05). However, dry matter digestibility and nitrogen digestibility were significantly decreased by PGG in the diets of the PGG-habituated rats (7 and 25%, P < 0.001, respectively). Production of PRPs increased the amount of PGG excreted intact in the feces but at the cost of diminishing apparent dry matter and nitrogen digestibility.