In maintenance hemodialysis (MHD) patients, advanced age is a factor associated with relative hypoparathyroidism and an adynamic bone. However, the underlying mechanism remains elusive. The aim of the present study was to analyze whether the observed spontaneous decrease in protein intake in the elderly favors a better control of serum phosphorus (P) and parathyroid hormone (PTH) levels. A cross-sectional study including 207 MHD patients (mean age, 60 +/- 14; 59% males; time on dialysis, 61 +/- 55 months) dialyzed 3.5 to 4.5 hours 3 times a week using bicarbonate hemodialysis from 6 Spanish hemodialysis centers was performed. In 95 patients, the nutrient intake was recorded over a 5-day period, and average daily ingestion of nutrients was calculated using a computerized diet analysis system. One-way analysis of variance showed that serum phosphorus and intact PTH decreased with age. In addition, patients with serum phosphorus lower than 4 mg/dL as compared with those with serum phosphorus greater than 4 mg/dL were older (68 +/- 9 v 58 +/- 15 years, P < 0.001), had a lower protein (0.86 +/- 0.3 v 1.05 +/- 0.4 g/kg body weight, P < 0.01) and caloric intake (21.9 +/- 7.4 v 25.7 +/- 8.3 kcal/Kg body weight, P < 0.01), and had lower PTH levels (102 +/- 155 v 290 +/- 345 pg/mL, P < 0.001). An inverse and significant correlation was observed between age and protein intake (r = -0.48; P < 0.01), caloric intake (r = -0.37, P < 0.01), serum phosphorus concentration (r = -0.40; P < 0.01), and PTH levels (r = -0.26; P < 0.01). Additionally, a significant positive correlation was found between serum phosphorus and PTH levels (r = 0.40; P < 0.01). The results obtained in the present study suggest that a lower serum phosphorus level due to spontaneous reduction of protein intake might contribute to the relative low PTH levels observed in elderly hemodialysis patients.