SERUM UNDERCARBOXYLATED OSTEOCALCIN IS A MARKER OF THE RISK OF HIP FRACTURE IN ELDERLY WOMEN

It has been previously shown that the level of circulating undercarbox ylated osteocalcin (ucOC) is elevated in elderly women in comparison w ith young, healthy, premenopausal ones. To understand the mechanism of the increase in the ucOC in the elderly and to assess its potential c onsequences on bone fragility, we have measured ucOC in the sera of 19 5 elderly institutionalized women 70-101 yr of age. In 45 women (23%) serum ucOC was above the upper limit of the normal range for young wom en. The level of ucOC was negatively correlated with 25OHD (r = -0.32, P < 0.001) even after excluding the effect of age, parathyroid hormon e (PTH), and creatinine by partial correlation (r = -0.24, P < 0.002). During an 18-mo follow-up, 15 women sustained a hip fracture and thei r baseline ucOC level was higher (P < 0.01) in women who subsequently sustained hip fracture than in the nonfracture group contrasting with no significant differences for serum calcium, phosphate, alkaline phos phatase, creatinine, PTH, 25OHD, and total and carboxylated OC. The ri sk of hip fracture was increased in women with elevated ucOC (relative ratio 5.9, 99.9% Cl 1.5-22.7, P < 0.001). During 1 yr of calcium/vita min D2 treatment, ucOC decreased (P < 0.05), especially in those with the initially increased values (from 2.22+/-0.35 to 1.41+/-0.29 ng/ ml , P < 0.005) contrasting with an increase in the placebo group (P < 0. 05). In conclusion, the increase in ucOC in the elderly reflects not o nly some degree of vitamin K deficiency but also their poor vitamin D status, suggesting that vitamin D may be important, either directly or indirectly through its effect on bone turnover, for achieving a norma l gamma-carboxylation of OC. The ucOC, but not conventional calcium me tabolism parameters, predicts the subsequent risk of hip fracture, sug gesting that serum ucOC reflects some changes in bone matrix associate d with increased fragility.