Previous studies have shown that the intake of low-quality dietary protein causes a decrease in plasma Zn levels and an uptake by other organs(Reference Pallaro and Slobodianik1, Reference Pallaro, Roux and Slobodianik2). It is known that nutritional disorders affect the immune system and that Cu and Zn play a critical role in its integrity. Moreover, an increase in Cu:Zn is associated with a higher risk of morbidity and mortality in patients who are immunodeficient(Reference Fraker3, Reference Dardenne, Pleau and Nabarm4). In the present work the effect of feeding a cereal-based diet and a recovery diet on serum Cu and Zn levels, Cu:Zn and thymus Zn concentration was investigated in weaning rats.
Wistar rats at weaning (21–23 d) were fed a diet containing 65 g precooked maize protein (M) or casein (Cas)/kg for18 d. Group M was refed with a 200 g casein/kg diet (MC) for 20 d. Age-matched control groups received stock diet (C1 (40 d) and C2 (60 d)). Body weight (BW; g) and body-weight gain (BWG; g/d per 100 g BW) were determined. Serum Cu (μg/ml) and Zn (μg/ml) concentrations were determined by atomic absorption spectrophotometry and Cu:Zn was calculated. Thymuses were removed and weighed (TW; mg) and the Zn content (νg/ g organ) was determined by atomic absorption spectrophotometry.
Serum Zn was decreased in M and Cas groups compare with the age-matched controls (group C1); thus, Cu:Zn was higher in these experimental groups. This ratio, as well as serum Zn level, returned to normal in group MC. Thymus Zn level in group M was higher compared with groups Cas and C1, which had similar values. Thymus Zn was normal in group MC and no significant differences were found between groups MC and C2.
a,b Mean values within columns with unlike superscript letters were significantly different (0.05>P>0.001).
The increase in serum Zn level and the reduction in Cu:Zn during recovery was concomitant with a higher BWG (MC 5.48 (sd 0.66) v. M −0.15 (sd 0.37); P<0.001) and TW (MC 625.4 (sd 157.2) v. M 88.6 (sd 26.2); P<0.001).
These results indicate that serum Zn and Cu:Zn are dependent on the quantity and/or quality of dietary protein. The reduced serum Zn concentration could be caused by a higher uptake of this mineral by the thymus, which was observed to have an increased Zn content. This outcome may represent a compensatory mechanism to overcome the low concentration and/or activity of thymic hormones described in malnourished status, as it is known that Zn could be an immunomodulator of thymocyte proliferation and maturation(Reference Dardenne, Pleau and Nabarm4). These data suggest that it would be not necessary to determine the Cu:Zn, since data for serum Zn and for Cu:Zn lead to the same conclusion.
