Chemical validation of X-ray absorptiometry

US Department of Agriculture Agricultural Research Service
Grand Forks Human Nutrition Research Center
Grand Forks, ND 58202-9034
E-mail: lklevay{at}gfhnrc.ars.usda.gov

Dear Sir:

In a recent issue of the Journal, Koo et al (1) showed clearly that 2 different dual-energy X-ray absorptiometers gave nearly identical results in both piglets and infants when estimates of bone mineral content and other bone measurements were compared. Nearly all points fell on or near the lines in their graphs. Koo et al may have missed a golden opportunity to validate the use of radiology by assessing bone chemistry.

Chemical analysis and radiodensity of bone have been compared only rarely (2). To illustrate, 31 vertebrae were removed from 11 fresh cadavers, scanned by dual-energy X-ray absorptiometry, and converted to ash; radiologic measurements and ash were found to be correlated (3). Both ash and calcium correlated with radiologic measurements in 6 fetal human femurs (4). Limited data from animals also showed a correlation between radiologic measurements and bone ash (5–9).

Ash is both nonspecific and chemically complex. Calcium receives the greatest amount of radiologic attention in this context because of its radiodensity; however, correlations from only 6 excised fetal femurs do not inspire confidence.

Bones are complex organs, and it is important to provide evidence that radiodensity, which correlates well with fracture risk, also correlates with calcium, collagen or other proteins, phosphorus, or even trace elements in bone. For the most part, this evidence is lacking. Trace elements may be important: 2 supplementation trials (10, 11) found benefits of copper in adults with osteoporosis that complemented numerous similar data on osteoporosis in copper-deficient children (2).

I hope that Koo et al removed femurs, radii, and vertebrae from the piglets for chemical analysis so that further studies can be conducted to improve the validity of bone density measurements and to clarify mechanisms of bone pathology.

REFERENCES

1. Koo WW, Hammami M, Hockman EM. Interchangeability of pencil-beam and fan-beam dual-energy X-ray absorptiometry measurements in piglets and infants. Am J Clin Nutr 2003;78:236–40.
2. Klevay LM, Wildman RE. Meat diets and fragile bones: inferences about osteoporosis. J Trace Elem Med Biol 2002;16:149–54.
3. Ho CP, Kim RW, Schaffler MB, Sartoris DJ. Accuracy of dual-energy radiographic absorptiometry of the lumbar spine: cadaver study. Radiology 1990;176:171–3.
4. Braillon PM, Salle BL, Brunet J, Glorieux FH, Delmas PD, Meunier PJ. Dual energy x-ray absorptiometry measurement of bone mineral content in newborns: validation of the technique. Pediatr Res 1992;32:77–80.
5. Kimmel DB, Wronski TJ. Nondestructive measurement of bone mineral in femurs from ovariectomized rats. Calcif Tissue Int 1990;46:101–10.
6. Griffin MG, Kimble R, Hopfer W, Pacifici R. Dual-energy x-ray absorptiometry of the rat: accuracy, precision, and measurement of bone loss. J Bone Miner Res 1993;8:795–800.
7. Hagiwara S, Lane N, Engelke K, Sebastian A, Kimmel DB, Genant HK. Precision and accuracy for rat whole body and femur bone mineral determination with dual X-ray absorptiometry. Bone Miner 1993;22:57–68.
8. Mitlak BH, Schoenfeld D, Neer RM. Accuracy, precision, and utility of spine and whole-skeleton mineral measurements by DXA in rats. J Bone Miner Res 1994;9:119–26.
9. Yamauchi H, Kushida K, Yamazaki K, Inoue T. Assessment of spine bone mineral density in ovariectomized rats using DXA. J Bone Miner Res 1995;10:1033–9.
10. Strause L, Saltman P, Smith KT, Bracker M, Andon MB. Spinal bone loss in postmenopausal women supplemented with calcium and trace minerals. J Nutr 1994;124:1060–4.
11. Eaton-Evans J, McIlrath EM, Jackson WE, McCartney H, Strain JJ. Copper supplementation and the maintenance of bone mineral density in middle-aged women. J Trace Elem Exp Med 1996;9:87–94.