Effect of Lowering Dietary Calcium Intake on Fractional Whole Body Calcium Retention*

Abstract

Although fractional calcium absorption is known to vary inversely with calcium intake, the extent and timing of individual hormonal and calcium absorption responses to altered calcium intake have not been defined. We measured fractional whole body retention of orally ingested 47Ca, an index of calcium absorption, in nine normal women after they had eaten a 2000-mg calcium diet for 8 weeks and a 300-mg calcium diet for 1, 2, 4, and 8 weeks. After the diet change, serum intact PTH (32.2% increase; P = 0.005), serum 1,25-dihydroxyvitamin D [1,25-(OH)2D; 43.8% increase; P = 0.003], and fractional whole body calcium retention (42.8% increase; P = 0.004) increased within 1 week. Although the PTH and calcium retention responses remained fairly constant throughout the low calcium intake period, serum 1,25-(OH)2D concentrations declined toward baseline after week 1. Thus, the late increase in calcium retention may have resulted from calcium absorption that was independent of 1,25-(OH)2D stimulation.

Effect of Dietary Calcium on Serum BGP (Osteocalcin).

“…On the other hand, in the present study, PTH levels did not differ from the basal levels. Thus, it is likely that the increase in PTH immediately after the reduction of calcium intake is not as large as reported previously [ 15]. Alternatively, a reduction in phosphate intake may attenuate PTH secretion, although the serum inorganic phosphate level was not altered.…”

Bone Turnover Response to Changes in Calcium Intake Is Altered in Girls and Adult Women in Families with Histories of Osteoporosis

“…Dawson-Hughes et al found whole body calcium retention of a radiotracer in adult women increased by 43% during an 8-week period of lowcalcium intake (300 mg/day) in comparison with that observed during the preceding 8-week period in which a highcalcium intake (2000 mg/day) was consumed. (29) Furthermore, the reported differences in calcium retention were apparent within 1 week and were maintained over the 8-week study interval. (29) Females from the control families consumed significantly higher calcium intakes during the high-calcium period in comparison with the osteoporotic families.…”

“…(29) Furthermore, the reported differences in calcium retention were apparent within 1 week and were maintained over the 8-week study interval. (29) Females from the control families consumed significantly higher calcium intakes during the high-calcium period in comparison with the osteoporotic families. These higher calcium intakes were most pronounced in the Ms (503 mg/day) while only minor differences were observed between bone groups in the GMs and GDs (236 and 105 mg/day, respectively).…”

“…Dawson-Hughes et al also observed that despite initial elevations in 1,25-dihydroxyvitamin D levels, these levels returned to baseline following 14 days of a low-calcium diet. (29) No detectable differences in indirect indicators of bone resorption (deoxypyridinoline) or formation (osteocalcin) were apparent between the low-and high-calcium period, despite the marked alterations that were detectable from the isotopic measurements of bone calcium turnover. These measurements may not be as sensitive or as responsive to acute changes in bone calcium turnover in comparison with stable isotopic measurements.…”

Metabolic bone disease in IBD

“…This process is enhanced by the PTH-mediated stimulation of renal 1,25 (OH 2 )D 3 . Calcium absorption may actually be increased up to 50% for brief periods in young patients by a combination of different processes [10,11].…”