Introduction

Legumes are rich in proteins, as well as being good sources of dietary minerals, vitamins, and calories (Salunkhe et al., 1990). One concern with feeding legumes involves the tannin content of different species and varieties. Tannins are found in many feed sources, including cereal grains, such as barley and sorghum, legumes including soybeans and faba beans, and forages (Mehansho et al., 1987; Salunkhe et al., 1990). Tannins are naturally occurring polyphenolic compounds that have the ability to precipitate proteins (Mehansho et al., 1987).

High tannin diets fed to rats and mice cause hypertrophy of salivary glands and increase the secretion of proline-rich proteins (PRPs) into saliva (Mehansho et al., 1983; Mehansho et al, 1985). The PRPs contain up to 45% proline and have very high binding affinities for tannins (Mehansho et al., 1987). The PRPs are thought to play an important role in neutralizing the negative effects of tannins on protein digestion. Tannins consumed in feeds can cause decreases in feed intake, growth rate, feed efficiency, net and metabolizable energy, and protein digestibility (Mehansho et al., 1983; Mehansho et al., 1985; Jansman et al., 1994). The effect of tannins may occur by binding of tannins to dietary proteins and to intestinal enzymes used in digestion (Reed, 1995).

The objective of this preliminary study was to determine the effects of feeding tannic acid on salivary gland size in young barrows.

Materials and Methods

Twelve barrows (ten weeks of age; bodyweight 32.5 ± 3.5 kg) were randomly divided into two groups and fed a standard diet of corn and soybean meal for the 5 days proceeding the trial. Barrows on control diet received the same standard diet for an additional 5 days, while barrows fed tannic acid (TA) had the same diet containing 2% tannic acid (20g/kg feed) for 5 days. Feed and water were provided ad libitum. Barrows in the TA group received an average of 22 grams of tannic acid per day. Barrows were killed the eleventh day and salivary glands collected.

Result

Parotid glands of the TA barrows dramatically increased in weight (P < 0.01; Table 1). In contrast, the submandibular glands were not significantly different between the groups. In studies using rats, the consumption of high tannin sorghums or high tannin faba-bean hulls increased the weight of parotid glands (Jansman et al., 1994). Additionally, rats fed high tannin diets had an increase in the synthesis and secretion of PRPs with a high affinity for binding tannins (Mehansho et al., 1983; Jansman et al., 1994). Although we have not measured PRPs, the enlarged parotid salivary glands might result in higher secretion of PRPs into saliva in the tannic acid fed barrows compared with control barrows. We are in the process of isolating the PRPs from these salivary glands, as well as determining histological changes in salivary tissue that may be associated with the rapid tissue growth induced by feeding tannic acid.

Conclusion

Addition of tannic acid to diets induced increased parotid gland weights by more than 4-fold during 5 days of feeding tannic acid. These preliminary observations need to be studied further to characterize the effects of dietary tannic acid on feed intake and body weight responses of growing barrows. Changes seen in the salivary glands in response to feeding tannic acid need to be related to any changes in daily feed intake or growth of swine. If the dramatically increased salivary weights induced by feeding tannic acid are paralleled by increased production of salivary PRPs, then high levels of PRPs in saliva may aid in overcoming the potentially negative dietary effects of tannic acid. It remains to be determined if the tannin-neutralizing PRPs are also induced by tannic acid in the pig.

Table 1. Weight of right salivary gland¹

¹Grams wet weight.

²Significant difference between Control and TA diets (p<0.01). Values are mean ± standard error of the mean; n = 6 for each diet group.

REFERENCES

Jansman, A. J. M., A. A. Frohlich, and R. R. Marquardt. 1994. Production of proline-rich proteins by the parotid glands of rats is enhanced by feeding diets containing tannins from faba beans (vicia faba L). J. Nutr. 124:249.

Mehansho, H., A. Hagerman, S. Clements, L. Butler, J. C. Rogler , and D. M. Carlson. 1983. Modulation of proline-rich protein biosynthesis in rat parotid gland by sorghums with high tannin levels. Proc. Natl. Acad. Sci. USA 80:3948.

Mehansho, H., J. C. Rogler, L. G. Butler, and D. M. Carlson. 1985. An unusual growth inhibiting effect of tannins on hamsters. Fed. Proc.44:1960

Mehansho, H., L. G. Butler, and D. M. Carlson. 1987. Dietary tannins and salivary proline-rich proteins: Interactions, induction and defense mechanisms. Ann. Rev. Nutr. 7:423

Reed, J. D. 1995. Nutritional toxicology of tannins and related polyphenols in forage legumes. J. Anim. Sci. 73:1516.

Salunkhe, D. K, J. K. Chavan, and S. S. Kadam. 1990. Dietary Tannins: Consequences and Remedies. CKC Press, Florida.