Urinary alpha and beta C-telopeptides of collagen I: Clinical implicationsin bone remodeling in patients with anorexia nervosa

Fragments derived from degradation of type I collagen C-telopeptide (CTX) c an be nonisomerized (alpha) or beta-isomerized (beta) depending on the age of bone; i.e., mainly the alpha form is derived from new bone and the beta form from old bone. We have studied 41 female patients with anorexia nervos a (AN), aged 18.5 +/- 2.2 years (range 16-24 years), and with an evolution time between 1.5 and 11 years, and 31 healthy control females (C), with a m ean age of 19 +/- 2.3 years (range 16-24 years). The AN patients showed a s ignificant decrease in bone mass, with a mean Z-score of bone mineral densi ty (BMD) of -3.2 +/- 0.8 (range -0.9 to -5.4). The aim of our study was to determine the levels of urinary alpha- and beta-CTX markers of bone resorpt ion, the alpha/beta ratio (alpha/beta), and the level of bone alkaline phos phatase (bAP), a biochemical marker of bone formation, in order to relate t hem to the degree of osteopenia and the status of bone remodeling. Statisti cal analysis was by the Mann-Whitney test. The degree of osteopenia correla ted with bAP levels (p = 0.0027) but not with the other parameters. Patient s with AN were divided into three groups according to their levels of bAP: high (H), normal (N), low (L). We found that BMD was significantly lower, a nd alpha- and beta-CTX were significantly higher, in groups H and N than in group L. Bone AP correlated significantly with alpha-CTX (p = 0.0042) and alpha/beta (0.0095) in the controls, but not with beta-CTX, while in AN pat ients bAP correlated with beta-CTX (p = 0.0000) and with alpha-CTX (p = 0.0 22) but not with the alpha/beta ratio. The ratio CTX/bAP (resorption/format ion) was similar in AN patients and controls. It is concluded that: (1) pat ients with AN have a high degree of osteopenia which con-elated with bAP le vels; (2) urinary CTX fragments found in AN patients seem to come mainly fr om old bone (beta-CTX) while CTX found in healthy adolescent control female s come from new bone (alpha-CTX). For this reason, alpha-CTX is more suitab le than beta-CTX for measuring bone resorption in controls and beta-CTX is more suitable in patients with AN; (3) the resorption/formation ratio (CTX/ bAP) was similar in AN patients and controls. From points (2) and (3) it is possible to suggest that, although bAP reflects bone formation in control females, this marker does not reflect effective bone mineralization in AN p atients, a similar feature to that of patients with osteomalacia.