Influence of zinc supplementation to diets at graded levels of protein on taste sensitivity, morphological changes of tongue epithelia and serum zinc concentration in growing rats

Objective: We previously reported dietary protein was needed to maintain normal taste function and low serum zinc concentration led due to protein insufficiency [1], but generally zinc has been known to improve taste dysfunction. The present study was designed to determine whether zinc supplementation is effective in maintaining normal taste function at different levels of dietary protein. Methods: Sixty weanling rats were given free access to one of six diets: protein-free, 2.5% and 8% purified egg protein (PEP), and 10 ppm zinc supplementation at each protein level for 28 days. Preference tests between aqueous solution of 1, 2, 5 or 10 mmol/L NaCl vs. deionized water were conducted on days 14, 17, 20 and 23, respectively. Blood was collected for the measurements of serum zinc concentration, and the tongue of each rat was excised for observation by electron microscopic scanning. The number of fungiform papillae with normal taste pores and also the number of rats with abnormal morphological changes on tongue epithelia were recorded. Results: In basal diet groups, the rats fed 8% and 2.5% PEP diets discriminated 2 mmol/L NaCl liquid, while those fed the protein-free diet discriminated only the highest concentration of 10 mmol/L in this experiment. Serum zinc concentration and the ratio of fungiform papillae with normal taste pores increased with increasing dietary protein level. Abnormal morphological changes of tongue epithelia were observed in the protein-free diet group whether or not it contained additional zinc. No significant differences were observed between the basal and zinc supplemented diets at the other protein levels in the other parameters. Conclusion: These results suggest that adequate dietary protein is more important in maintaining normal taste function and zinc supplementation did not improve taste function when dietary protein was suboptimal.