Schneider and Barrett-Connor1 found that elevated urinary levels of cross-linked N-telopeptides of type I collagen (NTX) were associated with reduced bone mineral density (BMD) of the spine and proximal femur in older men and women. The authors concluded that NTX levels could "predict" BMD. Osteoporosis experts have come to a quite different conclusion, ie, that urinary markers of resorption cannot predict individual levels of BMD, however useful these markers may be for group data. The authors went wrong by examining only stratification of groups, not the correlation between NTX levels and BMD. The latter correlation is about −0.1 to −0.2 in normal subjects older than 50 years; ie, less than 5% of the variation in BMD was explained by 1 or even several markers.2 Interestingly, Figures 1 and 2 in Schneider and Barrett-Connor's article show that BMD decreased about 10% from the lowest to the highest quintile of NTX. My colleagues and I previously examined the stratification of spine BMD associated with body weight in 1248 non–estrogen-treated postmenopausal women aged 50 to 79 years3 and found a greater difference (15%) between the highest and lowest quintiles.
Mazess RB. Body Weight Predicts Bone Density Better Than Resorption Markers. Arch Intern Med. 1998;158(3):298-300. doi: