Magnesium deficiency and bone loss after cardiac transplantation

Magnesium depletion adversely affects many phases of skeletal metabolism an d has been implicated as a risk factor in several forms of osteoporosis. Ma gnesium deficiency has also been reported after cardiac transplantation. To evaluate whether altered magnesium homeostasis could be related to the pat hogenesis of early bone loss after cardiac transplantation, we prospectivel y measured serum and urinary magnesium and evaluated them with respect to b iochemical indices of mineral metabolism and rates of bone loss. The study population included 60 patients (45 men, 15 women) aged 53 +/- 11 years (SD ) with measurements of biochemistries and bone mineral density by dual-ener gy X-ray absorptiometry before and 3 months after transplantation. All rece ived prednisone, cyclosporine A, and azathioprine, plus calcium (1000 mg) a nd vitamin D (400 IU). After transplantation, serum magnesium decreased by 16 +/- 15% (SD) from 2.0 +/- 0.3 mg/dl to 1.6 +/- 0.2 mg/dl (normal 1.8-2.2 mg/dl; p < 0.0001), accompanied by an increase in the fractional excretion of magnesium (7.1 +/- 3.9% to 13.3 +/- 5.6%; p < 0.0017). Forty-three pati ents with low 3-month serum magnesium levels (less than or equal to 1.7 mg/ dl) sustained 41% less bone loss at the lumbar spine (4.0 +/- 4.6% vs. 6.8 +/- 5.3%;p = 0.051) and 58% less bone loss at the femoral neck (3.1 +/- 5.9 % vs. 7.3 +/- 5.6 %; p < 0.01), despite receiving similar doses of predniso ne and cyclosporine A. Of the biochemical parameters measured (including re nal function, serum calcium, phosphorus, vitamin D metabolites, osteocalcin , urinary calcium, and markers of bone resorption), only intact parathyroid hormone (PTH) levels, which were 40% lower in patients with low serum magn esium levels differed significantly (45 +/- 23 pg/ml vs. 74 +/- 60 pg/ml; p = 0.019). Patients with low serum magnesium levels also had evidence of lo wer bone turnover. In summary, a highly significant decline in serum magnes ium concentrations, associated with continued urinary magnesium losses, occ urs after cardiac transplantation. Patients with low serum magnesium levels had significantly lower rates of bone loss, lower serum PTH concentrations , and lower bone turnover. Although the precise mechanisms by which the red uction in serum magnesium tends to protect against bone loss in the early p ost-transplantation period remain to be elucidated, reduced serum PTH and l ower rates of bone turnover are likely to be important factors.