Letters to the Editor
Bone density in premenopausal women Dear Sir: The possible role of calcium intake in the prevention and/or management of bone loss has engendered considerable debate in recent years. Although results of several studies in postmenopausal women were published, there is a relative paucity ofdata from younger women. Thus, I read with great interest the results of Mazess and Barden's 2-y prospective study of bone density in premenopausal women (l). This impressively large and carefully conducted study drew a number of conclusions, one of which was that there was no association of a woman's calcium intake with her initial bone mineral density or with changes in bone mineral density over 2 y. To have confidence in this conclusion, one would need to be satisfied that the methods used to quantitate both bone mineral density and calcium intake were valid and reliable for individuals. Measurement of bone mineral density by dual-photon absorptiometry appears to meet this criterion, as documented by a precision error of ~2-3'1'0 (1). In other words, one can be confident that the value measured for an individual's bone mineral density on one occasion is very close to that which would be measured at another time. However, I question whether the method used by Mazess and Barden to assess calcium intake meets the criteria of validity and reliability. If it does not, the certainty of their conclusion can be questioned. The method they used to estimate calcium intake consisted of the collection from subjects of six l-d food-intake records over the 2-y study period by using precoded Nutrient Adequacy Recording System (NARS) forms. Use of these precoded forms accurately quantitates intake on a particular day as compared with a longhand dietary record for the same day (2); however, it appears doubtful that 6 d of records accurately estimate an individual's true mean calcium intake. Basiotis et al (3), using data from the year-long Beltsville dietary-intake study (4), found that an average of 88 d was required to estimate an individual woman's calcium intake within 10% of her true mean intake 95% of the time. If one assumes that the coefficient of variation for calcium intake is similar for the groups of women studied by Basiotis et al and by Mazess and Barden, the equation developed by the former can be used to calculate that 6 d of records would result in an estimate accurate within ~38'1'0 ofthe true mean 95% of the time (see Appendix). For example, a woman classified in the lowest quartile for calcium intake, whose average estimated calcium intake was 510 mg/d, would have a true mean intake anywhere between 316 and 704 mgfd (510 mg ± 38%). Similarly, a woman classified in the upper quartile for calcium intake, with an average estimated intake of 1047 mg/d, would have a true mean intake between 649 and 1445 mg (1047 mg ± 38%). It
thus appearS possible that subjects could be misclassified by their calcium intake and this could jeopardize the detection of relationships between calcium intake and bone density, particularly if the relationships are subtle. In summary, the dietary methodology used by Mazess and Barden does not appear to quantitate individual calcium intakes with a high degree ofaccuracy. This may have reduced the possibility of detecting relationships between calcium intake and bone mineral density, should such relationships exist. Susan I Barr University of British Columbia School of Family & Nutritional Sciences 2205 East Mall Vancouver, BC
References I. Mazess RB, Barden HS. Bone density in premenopausal women: effects of age, dietary intake, physical activity, smoking, and birthcontrol pills. Am J Clin Nutr 1991;53:132-42. 2. Johnson NE, Sempos CT, Elmer PJ, Allington JK, Matthews ME. Development of a dietary intake monitoring system for nursing homes. J Am Diet Assoc 1982;80:549-57. 3. Basiotis PP, Welsh SO, Cronin FJ, Kelsay JL, Mertz W. Number of days of food intake records required to estimate individual and group nutrient intakes with defined confidence. J Nutr 1987;117: 1638-41. 4. Lakshmanan FL, Rao RB, Church JP. Calcium and phosphorus intakes, balances, and blood levels of adults consuming self-selected diets. Am J Clin Nutr 1984;40:1368-79.
APPENDIX Calculation of accuracy for Mazess and Barden data Equation 1 [from Basiotis et al (1 )]: X
=
(Z.f(day-to-day variability)22/(A)2(true average intake? (1)
where X is the number ofdays to estimate true average individual intake with defined level of statistical confidence, Z. is the value ofthe standard normal cumulative distribution function at hivel of statistical significance desired (= 1.96 at 0.05 level of significance), day-to-day variability is intraindividual standard deviation, and A is the level of accuracy (eg, 10% = 0.1). By using the mean number of days (X = 88) Basiotis et al found to be required to estimate women's calcium intake within 10% of their true mean intake 95% of the time, equation 1 can
Am J Clin Nutr 1991 ;54:169-74. Printed in USA. © 1991 American Society for Clinical Nutrition 169
Downloaded from https://academic.oup.com/ajcn/article-abstract/54/1/169/4690928 by McMaster University Library, Collections - Serials Processing user on 10 February 2019
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Bone density in premenopausal women Dear Sir: The possible role of calcium intake in the prevention and/or management of bone loss has engendered considerable debate in recent years. Although results of several studies in postmenopausal women were published, there is a relative paucity ofdata from younger women. Thus, I read with great interest the results of Mazess and Barden's 2-y prospective study of bone density in premenopausal women (l). This impressively large and carefully conducted study drew a number of conclusions, one of which was that there was no association of a woman's calcium intake with her initial bone mineral density or with changes in bone mineral density over 2 y. To have confidence in this conclusion, one would need to be satisfied that the methods used to quantitate both bone mineral density and calcium intake were valid and reliable for individuals. Measurement of bone mineral density by dual-photon absorptiometry appears to meet this criterion, as documented by a precision error of ~2-3'1'0 (1). In other words, one can be confident that the value measured for an individual's bone mineral density on one occasion is very close to that which would be measured at another time. However, I question whether the method used by Mazess and Barden to assess calcium intake meets the criteria of validity and reliability. If it does not, the certainty of their conclusion can be questioned. The method they used to estimate calcium intake consisted of the collection from subjects of six l-d food-intake records over the 2-y study period by using precoded Nutrient Adequacy Recording System (NARS) forms. Use of these precoded forms accurately quantitates intake on a particular day as compared with a longhand dietary record for the same day (2); however, it appears doubtful that 6 d of records accurately estimate an individual's true mean calcium intake. Basiotis et al (3), using data from the year-long Beltsville dietary-intake study (4), found that an average of 88 d was required to estimate an individual woman's calcium intake within 10% of her true mean intake 95% of the time. If one assumes that the coefficient of variation for calcium intake is similar for the groups of women studied by Basiotis et al and by Mazess and Barden, the equation developed by the former can be used to calculate that 6 d of records would result in an estimate accurate within ~38'1'0 ofthe true mean 95% of the time (see Appendix). For example, a woman classified in the lowest quartile for calcium intake, whose average estimated calcium intake was 510 mg/d, would have a true mean intake anywhere between 316 and 704 mgfd (510 mg ± 38%). Similarly, a woman classified in the upper quartile for calcium intake, with an average estimated intake of 1047 mg/d, would have a true mean intake between 649 and 1445 mg (1047 mg ± 38%). It
thus appearS possible that subjects could be misclassified by their calcium intake and this could jeopardize the detection of relationships between calcium intake and bone density, particularly if the relationships are subtle. In summary, the dietary methodology used by Mazess and Barden does not appear to quantitate individual calcium intakes with a high degree ofaccuracy. This may have reduced the possibility of detecting relationships between calcium intake and bone mineral density, should such relationships exist. Susan I Barr University of British Columbia School of Family & Nutritional Sciences 2205 East Mall Vancouver, BC
References I. Mazess RB, Barden HS. Bone density in premenopausal women: effects of age, dietary intake, physical activity, smoking, and birthcontrol pills. Am J Clin Nutr 1991;53:132-42. 2. Johnson NE, Sempos CT, Elmer PJ, Allington JK, Matthews ME. Development of a dietary intake monitoring system for nursing homes. J Am Diet Assoc 1982;80:549-57. 3. Basiotis PP, Welsh SO, Cronin FJ, Kelsay JL, Mertz W. Number of days of food intake records required to estimate individual and group nutrient intakes with defined confidence. J Nutr 1987;117: 1638-41. 4. Lakshmanan FL, Rao RB, Church JP. Calcium and phosphorus intakes, balances, and blood levels of adults consuming self-selected diets. Am J Clin Nutr 1984;40:1368-79.
APPENDIX Calculation of accuracy for Mazess and Barden data Equation 1 [from Basiotis et al (1 )]: X
=
(Z.f(day-to-day variability)22/(A)2(true average intake? (1)
where X is the number ofdays to estimate true average individual intake with defined level of statistical confidence, Z. is the value ofthe standard normal cumulative distribution function at hivel of statistical significance desired (= 1.96 at 0.05 level of significance), day-to-day variability is intraindividual standard deviation, and A is the level of accuracy (eg, 10% = 0.1). By using the mean number of days (X = 88) Basiotis et al found to be required to estimate women's calcium intake within 10% of their true mean intake 95% of the time, equation 1 can
Am J Clin Nutr 1991 ;54:169-74. Printed in USA. © 1991 American Society for Clinical Nutrition 169
Downloaded from https://academic.oup.com/ajcn/article-abstract/54/1/169/4690928 by McMaster University Library, Collections - Serials Processing user on 10 February 2019
§2_------=----__:=_;;::___
