Bone mineral content, not bone mineral density, is the correct bone measure for growth studies

¹ Creighton University 601 North 30th Street, Suite 4841 Omaha, NE 68131 E-mail: rheaney{at}creighton.edu

Dear Sir:

In a recent issue of the Journal, Lehtonen-Veromaa et al (1) reported important findings with respect to the apparent effect of basal vitamin D status on the attainment of peak bone mass in peripubertal girls living at a latitude at which solar vitamin D synthesis in the skin is minimal and in a country (Finland) in which vitamin D fortification of milk is so low as to be nearly negligible. The importance of their findings, although statistically significant, is minimized by an unfortunate choice of outcome variable, ie, areal bone mineral density (BMD). This is precisely the wrong measurement during growth, because it factors out most of the component of bone accumulation that is associated with change in bone size. What is important in a growth experiment is bone mass (measured as bone mineral content, BMC), not bone density. The authors’ title captures that truism, even if their data do not.

The positive correlation between 25-hydroxyvitamin D [25(OH)D] and BMD gain, depicted in Figure 1 in the article (1), could have been due to the fact that vertebral size was expanding more in the girls with low 25(OH)D values than in the girls with higher 25(OH)D values. With a larger denominator, the BMD value would have increased less. I doubt that that is the case, but without the critical data there is no way to tell.

BMD should never be used in a growth study (2, 3). There is no mechanical reason why true density should change appreciably with growth, and Matkovic et al (4) showed that, in fact, it did not. Bone mineral apparent density is an even less appropriate measure under these circumstances, because it represents an empirical method of attempting to adjust for differences in the third dimension that BMD does capture.

My guess is that the reported change in BMD in these girls was almost surely less than the actual change in bone mass, and that, had the authors used BMC as their outcome variable, the association with vitamin D would have been even stronger than the association they report. What Lehtonen-Veromaa et al should provide are the BMC and area values (and their corresponding 3-y changes). Only these are capable of capturing the variable in the title of their paper (bone mass). One hopes that they will make these important data available to the readers of the Journal.

REFERENCES

1. Lehtonen-Veromaa MKM, Möttönen TT, Nuotio IO, Irjala KMA, Leino AE, Viikari JSA. Vitamin D and attainment of peak bone mass among peripubertal Finnish girls: a 3-y prospective study. Am J Clin Nutr 2002;76:1446–53.
2. Prentice A, Parsons TJ, Cole TJ. Uncritical use of bone mineral density in absorptiometry may lead to size-related artifacts in the identification of bone mineral determinants. Am J Clin Nutr 1994;60:837–42.
3. Heaney RP. Design considerations for clinical investigations of osteoporosis. In: Marcus R, Kelsey J, Feldman D, eds. Osteoporosis. 2nd ed, vol 2. San Diego: Academic Press, 2001:513–32.
4. Matkovic V, Jelic T, Wardlaw GM, et al. Timing of peak bone mass in Caucasian females and its implication for the prevention of osteoporosis. J Clin Invest 1994;93:799–808.