Long-term anticonvulsant therapy and intestinal calcium absorption

Twelve patients on anticonvulsant therapy were studied to determine whether or not the drugs induced alterations in gastrointestinal absorption of calcium, response to parathyroid hormone (PTH), or serum 25-hydroxy vitamin D (25-OHD) concentrations. Fractional calcium absorption (FCaA) was determined by giving 45Ca intravenously and orally. The short-term response to PTH was assessed by giving 200 U of parathyroid extract (PTE) intravenously over 15 min and measuring hourly urine cyclic adenosine monophosphate (cAMP) and tubular reabsorption of phosphate (TRP). Calcemic response to PTH was followed by giving intramuscular injections of PTE, 200 U every 6 hr. FCaA was 30.8 +/- 3.7% lower than the normal of 42.2 +/- 2.5% (P less than 0.025), and baseline 25-OHD levels were 30.5 +/- 3.4 ng/ml (normal 15 to 50 ng/ml). Anticonvulsant drugs did not alter renal response to PTE. There was a rise in urinary cAMP from 3.7 +/- 0.23 to 6.1 +/- 0.47 mumol/gm creatinine (P less than 0.005) with a fall in TRP from 87.8 +/- 1.2% to 78.8 +/- 1.6% (P less than 0.005). Serum calcium rose from 9.4 +/- 0.1 to 11.1 +/- 0.3 mg/dl (P less than 0.005). We conclude that FCaA is low in patients receiving anticonvulsant drugs, even when serum 25-OHD levels and the response of bone and kidney to PTH remain normal.