No influence of aging on the circadian rhythm of erythropoietin in healthy subjects.

The diurnal rhythm in the circulating serum levels of erythropoietin (Epo) was investigated in a group of 20 clinically healthy subjects aged 30-55 years and in a group of 20 healthy subjects aged 55-75 years. Venous blood samples were drawn during the span of a whole day every 4 h, starting from midnight, for the determination of serum Epo levels by radioimmunoassay. Statistical analysis was carried out by means of the 'cosinor' method. Both groups presented a significant (p < 0.05) circadian rhythm in serum Epo levels, with maximum in the afternoon. The younger subjects had significantly (p < 0.05) higher mean daily levels and higher diurnal variations of serum Epo than older subjects; no difference (p > 0.05) was found between the groups regarding the peaks of the rhythms. These data confirm the presence of a circadian rhythm in serum EPO levels and suggest that the aging process does not influence the physiological diurnal fluctuations but modifies the mean daily levels and the amplitude of the diurnal variations, such as it occurs for many other variables. This behavior is an index of aging, but does not seem to have clinical implications.