Serum zinc in aging germ-free and conventional rats

Zinc nutritional status appears to decline with age in humans and rodents. Since germ-free rats outlive their conventional counterparts in better health, serum Zn levels were determined in male germ-free and conventional Lobund Wistar rats in samples originating from the Lobund Aging Study. Starting at 5 months of age, germ-free rats showed significantly higher serum Zn levels than did their conventional counterparts. In conventional rats sacrificed up to 30 months of age in apparently good health, serum Zn levels showed no effect of age, while a slight but significant increase with age was observed in the germ-free rats. In healthy germ-free adults (6-24 months of age), serum Zn concentrations were approximately 25% higher than those in comparable conventional animals. In conventional rats 18-30 months of age (average, 24.5 months), sacrificed because of an obvious moribund condition, serum Zn levels were significantly lower than those in rats of the same age range (average, 24.9 months) that were obviously healthy. Results suggest that the often observed higher absorptive capacity of the germ-free gut might have contributed to higher serum Zn levels, and that a decline in serum Zn concentration with age may be a consequence, rather than a causative factor, of declining health.