EDITORIAL CORRESPONDENCE

Effects of calcium intake and exercise on bone mineral density

To the Editor: This letter is in response to the article by Rubin et al. on axial

and peripheral bone mineral density in healthy children and ado- lescents.1 There is considerable interest in the role of calcium in-

take and physical activity in promoting bone mineral density (BMD) accretion in children, adolescents, and adults. The study by Rubin et al. addresses differences in axial and appendicular BMD as a function of age and pubertal status in healthy, white 16- to 18- year-old subjects in a cross-sectional fashion. The authors indicated that calcium intake made an additional small, but significant, con- tribution to peripheral BMD. It would have been more useful if the authors had included not just the p value showing that the contri- bution of average daily calcium intake to variance (R 2) of BMD was significant, but the incremental increase in R z when average daily calcium intake was added to the model that included pubertal stage, height, weight, and age. In addition, it is not clear to me why, after entering pubertal stage, height, and weight into the model, age would have affected the prediction of BMD. Could the authors provide the incremental increase in R z in predicting peripheral BMD related to average daily calcium intake?

The authors also reported that after correcting for weight and pubertal stage, they observed an additional small contribution of exercise on axial bone mineral density of the lumbar vertebrae. Again, the p value is included, but the increment in R z to lumbar BMD by the addition of physical activity would be more useful. By including these incremental R 2 changes, the actual contribution of exercise in this age group could be implied more definitively. In the article's published form, some may conclude that the contribution of physical activity to increased lumbar BMD is greater than is actually the case. These authors imply that the contribution is rel- atively small, yet in their discussion they suggest the effect of ac- tivity and calcium intake could be significant. The R 2 values are needed for interpretation of this relation.

Albert C Hergenroeder, AID Associate Professor of Clinical Pediatrics

Chief, Adolescent Medicine and Sports Medicine Texas Children's Hospital

Baylor College of Medicine Houston, Texas 77030

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REFERENCE

1. Rubin K, Schirduan, Gendreau P, Sarfarazi M, Mendola R, Dalsky G. Predictors of axial and peripheral bone mineral den- sity in healthy children and adolescents, with special atten- tion to the role of puberty. J PEDIATR 1993;123:863-70.

Reply

To the Editor." With calcium intake added to the regression model with periph-

eral bone mineral density (BMD) as the dependent variable, the incremental increase in R E is 1.2%. Similarly, with physical activ- ity added to the regression model with axial BMD as the dependent variable, the incremental increase in R E is 2.7%. Although these incremental increases in R 2 are quite small, I do not believe that they represent an artifact of the data but rather that they represent a true additional effect of calcium intake and exercise on BMD. The magnitude of the effect of calcium intake and exercise is likely lim- ited by the cross-sectional design of this study and by the hetero- geneity of the population studied.

However, longitudinal study designs can overcome some of these limitations. Two prospective studies have demonstrated a more convincing effect of calcium intake on BMD by the administration of calcium supplementation in excess of the recommended dietary allowance for calcium: the report by Johnston et al. 1 discussed in our article and a more recent report of calcium supplementation in adolescent girls. 2 Similar prospective studies assessing the role of exercise in BMD in children have not been reported. My sugges- tion that the effect of physical activity and calcium intake on BMD

"may" be significant was not based entirely on the weakly positive results of this study, but on all the currently available data combined with an understanding of the current concepts of bone remodeling.

Karen Rubin, MD Associate Professor, Department of Pediatrics

Division of Diabetes/Endocrinology School of Medicine

University of Connecticut Health Center Farmington, CN 06030-1515

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REFERENCES

1. Johnston CC, Miller JZ, Slemenda CW, et al. Calcium sup- plementation and increases in bone mineral density in children. N Engl J Med 1992;237:82-7.

2. Lloyd T, Andon MB, Rollings N, et al. Calcium supplemen- tation and bone mineral density in adolescent girls. JAMA 1993;270:841-4.

Kegel exercises and squatting behavior

To the Editor: We read with interest the article by Schneider et al. 1 on the use

of Kegel exercises in the treatment of urge incontinence in children.

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