Letters
Figure. Number of Weight Percentile Lines Crossed From First to Last Measurement in Children Younger Than 1 Year of Age 30 25
Patients, %
20 15 10 5 0
–7
–6
–5
–4
–3
–2
–1
0
1
2
3
4
5
6
7
Percentile Lines Crossed From First to Last Measurement, No.
Discussion | We assessed the trends in weight percentile change that occur in children during the first year of life. We found that more than 64% of children decreased by 1 percentile line or more at some point in this period and 44% of children were at a lower percentile than where they started. Our results suggest that many children’s growth percentiles decline during the first year of life, and this may be normal. Limitations to this study included retrospective assessment of growth, limiting to children younger than 1 year of age, a population of mostly minority patients on Medicaid, and lack of assessment of other socioeconomic variables (eg, maternal education and household income). Our results may differ slightly when UK growth records are used (when analysis was repeated using UK centiles, 4.9% of visits were different when compared with US centiles). Intense study of the longitudinal nature of normal growth may help us better recognize cases of failure to thrive and limit unnecessary use of resources for what, in many cases, may be a normal phenomenon. William E. Bennett Jr, MD Kristin S. Hendrix, PhD Rachel T. Thompson, MD Aaron E. Carroll, MD, MS Stephen M. Downs, MD, MS
Obtained funding: Downs. Administrative, technical, or material support: Downs. Study supervision: Downs. Conflict of Interest Disclosures: None reported. 1. Centers for Disease Control and Prevention. 2000 CDC Growth Charts for the United States: DHHS Publication. Hyattsville, MD: Public Health Service, Centers for Disease Control and Prevention, National Center for Health Statistics; 2002. 2. Borghi E, de Onis M, Garza C, et al; WHO Multicentre Growth Reference Study Group. Construction of the World Health Organization child growth standards: selection of methods for attained growth curves. Stat Med. 2006;25 (2):247-265. 3. Hamill PVV. NCHS Growth Curves for Children: Birth-18 Years, United States. Vital and Health Statistics: Series 11, Data From the National Health Survey No. 165. Hyattsville, MD: US Dept of Health, Education, and Welfare, Public Health Service; 1977. 4. Mansourian M, Marateb HR, Kelishadi R, et al. First growth curves based on the World Health Organization reference in a nationally representative sample of pediatric population in the Middle East and North Africa (MENA): the CASPIAN-III study. BMC Pediatr. 2012;12:149.
COMMENT & RESPONSE
Early-Life Milk and Late-Life Fracture
Author Affiliations: Section of Pediatric Gastroenterology, Hepatology, and Nutrition, Department of Pediatrics, Indiana University School of Medicine, Indianapolis (Bennett); Children’s Health Services Research, Department of Pediatrics, Indiana University School of Medicine, Indianapolis (Bennett, Hendrix, Thompson, Carroll, Downs). Corresponding Author: William E. Bennett Jr, MD, Section of Pediatric Gastroenterology, Hepatology, and Nutrition, Department of Pediatrics, Indiana University School of Medicine, 410 W 10th St, Indianapolis, IN 46202 ([email protected]). Published Online: May 5, 2014. doi:10.1001/jamapediatrics.2014.345. Author Contributions: Dr Bennett had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: All authors. Acquisition, analysis, or interpretation of data: All authors. Drafting of the manuscript: Bennett. Critical revision of the manuscript for important intellectual content: All authors. Statistical analysis: Bennett, Thompson, Carroll, Downs. 682
Positive numbers indicate an increase in percentile lines while negative numbers indicate a decrease in percentile lines.
To the Editor Feskanich et al1 report that early-life milk consumption does not protect against late-life hip fractures in 2 health professions cohorts and, if anything, increased fracture risk in men. In her Editorial, Weaver2 describes a strong mechanistic basis for fracture protection but offers no explanation as to why Feskanich et al seem not to have found it. It is known that teenage milk consumption predicts 2 things, height (found in men by Feskanich et al) and late-life milk consumption. It is also known that the principal causal factors in hip fracture are decreased intrinsic bone strength and the force sustained in a fall (which increases as height increases). There may be an implicit presumption behind this article (and the Editorial) to the effect that bone density (BMD) is the principal factor driving bone strength. Thus, because calcium intake influences BMD, one might have expected increased fracture resistance in high-milk consumers. However, that explanation is probably not correct. Bone-remodeling activity is a far more important fragility factor than BMD3, and bone-remodeling activity is strongly
JAMA Pediatrics July 2014 Volume 168, Number 7
Copyright 2014 American Medical Association. All rights reserved.
Downloaded From: http://archpedi.jamanetwork.com/ by a Purdue University User on 05/19/2015
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inversely related to current calcium intake (as Wastney and colleagues have demonstrated4). Thus, while dairy intake would be predicted to affect BMD, BMD is probably not the basis for whatever fracture protection dairy might putatively afford. Instead, reduction of excessive bone remodeling is the more important protective mechanism. Here is where the problem lies. Feskanich et al adjusted for current calcium intake in their statistical analysis. Though they do not tell us how they did that, such adjustment, by effectively removing variation in late-life calcium intake, would eliminate the major mechanism by which teenage dairy intake might plausibly influence late-life fracture. No wonder their finding was null. It is precisely latelife calcium (and dairy) intake that would be predicted to affect remodeling (hence fracture risk). With the adjustment for current calcium intake, the study begs the question it sought to answer. It would be useful to know the risk gradient for hip fracture on actual late-life calcium (and specifically dairy) intake or, alternatively, the risk gradient without the adjustment for current calcium intake. Robert P. Heaney, MD
sion of this limitation minimizes the significance of this problem. Correlations between documented consumption and recalled intake are weak. This is one of several problems that are a serious pitfall for nutritional research.2 Feskanich et al characterize greater height as an independent risk factor for hip fracture and they consider this a possible intermediate variable in their analysis. A critical appraisal of published cohort and case-control studies pertaining to height and fracture risk (2 studies referenced by the authors and multiple studies referenced in articles by Hemenway et al3 and Compston et al4) reveals mixed findings from these studies. Not all studies confirmed an association and positive effects were uniformly small. The relationship between height and fracture location (hip, spine, or limb) also differed in some analyses. The effect of sex noted by Feskanich et al conflicts with findings from some of these studies. Virtually all of these studies (including the study by Feskanich et al) suffer from confounding variables and bias. Weak and inconsistent associations, taken from observational epidemiologic studies, are likely to be false and attributable to various sources of bias.2,5 Because of method limitations in their study design and data analysis, the conclusions of Feskanich et al cannot be considered valid.
Author Affiliation: Creighton University, Omaha, Nebraska. Corresponding Author: Robert P. Heaney, MD, Creighton University, 2500 California Plaza, Omaha, NE 68178 ([email protected]).
Robert H. Howland, MD
Conflict of Interest Disclosures: None reported.
Author Affiliation: University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania.
1. Feskanich D, Bischoff-Ferrari HA, Frazier AL, Willett WC. Milk consumption during teenage years and risk of hip fractures in older adults. JAMA Pediatr. 2014;168(1):54-60. 2. Weaver CM. Milk consumption and bone health. JAMA Pediatr. 2014;168(1): 12-13. 3. Heaney RP. Is the paradigm shifting? Bone. 2003;33(4):457-465. 4. Wastney ME, Martín BR, Peacock M, et al. Changes in calcium kinetics in adolescent girls induced by high calcium intake. J Clin Endocrinol Metab. 2000; 85(12):4470-4475.
To the Editor In their data analysis from 2 observational cohort studies, Feskanich et al1 found that greater milk consumption during adolescence was not significantly associated with a lower risk of hip fracture in older women, but was weakly and significantly associated with a greater risk of hip fracture in older men. They also reported that the weak positive association in men was partially mediated by greater height. Their data analysis is limited by not including many important confounding variables that are associated with increased fracture risk. These variables include the risk of falls (which itself is multifactorial, but can be quantified), family history of fracture or osteoporosis, concurrent medical disorders associated with osteoporosis or fractures, carbonated beverage consumption (past and present), and the use of certain drugs (eg, proton pump inhibitors, anticonvulsants, thyroid hormone). Patients with a history of hip fracture were excluded from the analysis, but history of any fracture (data not reported by the authors) is associated with an increased risk of future fractures; this should be considered a confounding variable. The investigators acknowledge the possibility of error in retrospective reporting of milk consumption, but their discus-
Corresponding Author: Robert H. Howland, MD, University of Pittsburgh Medical Center, 3811 O'Hara St, Pittsburgh, PA 15213 ([email protected]). Conflict of Interest Disclosures: None reported. 1. Feskanich D, Bischoff-Ferrari HA, Frazier AL, Willett WC. Milk consumption during teenage years and risk of hip fractures in older adults. JAMA Pediatr. 2014;168(1):54-60. 2. Ioannidis JPA. Implausible results in human nutrition research. BMJ. 2013;347: f6698. 3. Hemenway D, Azrael DR, Rimm EB, Feskanich D, Willett WC. Risk factors for hip fracture in US men aged 40 through 75 years. Am J Public Health. 1994;84 (11):1843-1845. 4. Compston JE, Flahive J, Hosmer DW, et al; GLOW Investigators. Relationship of weight, height, and body mass index with fracture risk at different sites in postmenopausal women: the Global Longitudinal Study of Osteoporosis in Women (GLOW). J Bone Miner Res. 2014;29(2):487-493. 5. Grimes DA, Schulz KF. False alarms and pseudo-epidemics: the limitations of observational epidemiology. Obstet Gynecol. 2012;120(4):920-927.
In Reply Dr Heaney questioned our adjustment for calcium intake during adult years, which was done because our primary question was the independent contribution of milk intake during adolescence to future risk of hip fracture. However, repeating the analysis without adjustment for adult milk consumption and supplemental calcium intake made little difference. In men, the relative risk associated with each additional glass per day of adolescent milk intake changed from 1.09 (95% CI, 1.01-1.17) to 1.08 (95% CI, 1.00-1.16) and results for women remained null. The lack of confounding by adult intake was not surprising, given that adult milk and calcium intakes are not strongly associated with risk of hip fracture in our cohorts, meta-analyses of prospective studies, or randomized trials of calcium supplements without vitamin D.1,2
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Letters
Dr Howland concluded that our results are invalid owing to method limitations, including confounding by adult risk factors for hip fractures that were not in our statistical models. However, these factors would only confound our results if they were also associated with adolescent milk consumption, which is unlikely. As shown in the Table in our article, adult behaviors related to fracture risk including smoking, physical activity, and use of various medications were not associated with adolescent milk intake and adjusting for these factors had no effect on our findings. Adolescent milk intake was associated with adult milk intake and adjusting for this also had no appreciable effect. Dr Howland was critical of our statement that height is a known risk factor for hip fracture (confirmed by Dr Heaney) and therefore it could not be an intermediate variable. He referenced a relatively small study (309 hip fractures) by Compston et al3 with a nonsignificant association; however, the 9 other studies cited in that study all reported significant positive associations between height and hip fracture incidence. This association is not weak, as an 8-inch difference in height translates to approximately a doubling of risk of hip fracture. Lastly, Dr Howland concluded that the error in retrospective reporting of milk consumption during teenage years by older adults rendered our results invalid. Although some aspects of diet in the distant past may be poorly recalled, whether milk is infrequently consumed or consumed multiple times a day is more likely to be remembered. The validity of reported adolescent milk intake in our study was directly supported by the positive association with attained height, which is consistent with our earlier prospective findings on current milk intake among adolescents.4 Random error in exposure assessment tends to attenuate a true association toward the null, yet we observed a positive association between teenage milk consumption and hip fracture in men. In conclusion, observational epidemiologic studies like ours can contribute useful information, and often the only available direct information, on topics of great public health importance. Specifically, our findings suggest a partial explanation for the long-standing enigma that hip fracture rates are
684
highest in populations with the greatest milk consumption and that further research in this area is needed. Diane Feskanich, ScD Walter C. Willett, MD, DrPH Author Affiliations: Channing Division of Network Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts (Feskanich, Willett); Department of Nutrition, Harvard School of Public Health, Boston, Massachusetts (Willett). Corresponding Author: Diane Feskanich, ScD, Channing Division of Network Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, 181 Longwood Ave, Boston, MA 021115 ([email protected]). Conflict of Interest Disclosures: None reported. 1. Bischoff-Ferrari HA, Dawson-Hughes B, Baron JA, et al. Milk intake and risk of hip fracture in men and women: a meta-analysis of prospective cohort studies. J Bone Miner Res. 2011;26(4):833-839. 2. Bischoff-Ferrari HA, Dawson-Hughes B, Baron JA, et al. Calcium intake and hip fracture risk in men and women: a meta-analysis of prospective cohort studies and randomized controlled trials. Am J Clin Nutr. 2007;86(6):1780-1790. 3. Compston JE, Flahive J, Hosmer DW, et al; GLOW Investigators. Relationship of weight, height, and body mass index with fracture risk at different sites in postmenopausal women: the Global Longitudinal Study of Osteoporosis in Women (GLOW). J Bone Miner Res. 2014;29(2):487-493. 4. Berkey CS, Colditz GA, Rockett HRH, Frazier AL, Willett WC. Dairy consumption and female height growth: prospective cohort study. Cancer Epidemiol Biomarkers Prev. 2009;18(6):1881-1887.
CORRECTION Incorrect Footnote in Table: In the Original Investigation titled “Electronic Cigarettes and Conventional Cigarette Use Among US Adolescents: A Cross-sectional Study” published online March 6, 2014, in JAMA Pediatrics (doi:10.1001 /jamapediatrics.2013.5488), an incorrect footnote appeared in Table 2. In the row “Ever cigarette smokers (n = 1832),” footnote b should have been used. Error in Phthalate Metabolite Name: In the article titled “Environmental Phthalate Exposure and Preterm Birth” published in the January 2014 issue of JAMA Pediatrics (2014;168[1]:61-67. doi:10.1001/jamapediatrics.2013.3699), incorrect information appeared. The phthalate metabolite for benzyl butyl phthalate appeared as “mono-benzyl-butyl phthalate.” It should have appeared as “monobenzyl phthalate” in the top of the right column on page 63, in the top of the left column on page 64, in row 6 in the second column of Table 2, and in row 6 in the first column of Table 3. This article was corrected online.
JAMA Pediatrics July 2014 Volume 168, Number 7
Copyright 2014 American Medical Association. All rights reserved.
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Figure. Number of Weight Percentile Lines Crossed From First to Last Measurement in Children Younger Than 1 Year of Age 30 25
Patients, %
20 15 10 5 0
–7
–6
–5
–4
–3
–2
–1
0
1
2
3
4
5
6
7
Percentile Lines Crossed From First to Last Measurement, No.
Discussion | We assessed the trends in weight percentile change that occur in children during the first year of life. We found that more than 64% of children decreased by 1 percentile line or more at some point in this period and 44% of children were at a lower percentile than where they started. Our results suggest that many children’s growth percentiles decline during the first year of life, and this may be normal. Limitations to this study included retrospective assessment of growth, limiting to children younger than 1 year of age, a population of mostly minority patients on Medicaid, and lack of assessment of other socioeconomic variables (eg, maternal education and household income). Our results may differ slightly when UK growth records are used (when analysis was repeated using UK centiles, 4.9% of visits were different when compared with US centiles). Intense study of the longitudinal nature of normal growth may help us better recognize cases of failure to thrive and limit unnecessary use of resources for what, in many cases, may be a normal phenomenon. William E. Bennett Jr, MD Kristin S. Hendrix, PhD Rachel T. Thompson, MD Aaron E. Carroll, MD, MS Stephen M. Downs, MD, MS
Obtained funding: Downs. Administrative, technical, or material support: Downs. Study supervision: Downs. Conflict of Interest Disclosures: None reported. 1. Centers for Disease Control and Prevention. 2000 CDC Growth Charts for the United States: DHHS Publication. Hyattsville, MD: Public Health Service, Centers for Disease Control and Prevention, National Center for Health Statistics; 2002. 2. Borghi E, de Onis M, Garza C, et al; WHO Multicentre Growth Reference Study Group. Construction of the World Health Organization child growth standards: selection of methods for attained growth curves. Stat Med. 2006;25 (2):247-265. 3. Hamill PVV. NCHS Growth Curves for Children: Birth-18 Years, United States. Vital and Health Statistics: Series 11, Data From the National Health Survey No. 165. Hyattsville, MD: US Dept of Health, Education, and Welfare, Public Health Service; 1977. 4. Mansourian M, Marateb HR, Kelishadi R, et al. First growth curves based on the World Health Organization reference in a nationally representative sample of pediatric population in the Middle East and North Africa (MENA): the CASPIAN-III study. BMC Pediatr. 2012;12:149.
COMMENT & RESPONSE
Early-Life Milk and Late-Life Fracture
Author Affiliations: Section of Pediatric Gastroenterology, Hepatology, and Nutrition, Department of Pediatrics, Indiana University School of Medicine, Indianapolis (Bennett); Children’s Health Services Research, Department of Pediatrics, Indiana University School of Medicine, Indianapolis (Bennett, Hendrix, Thompson, Carroll, Downs). Corresponding Author: William E. Bennett Jr, MD, Section of Pediatric Gastroenterology, Hepatology, and Nutrition, Department of Pediatrics, Indiana University School of Medicine, 410 W 10th St, Indianapolis, IN 46202 ([email protected]). Published Online: May 5, 2014. doi:10.1001/jamapediatrics.2014.345. Author Contributions: Dr Bennett had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: All authors. Acquisition, analysis, or interpretation of data: All authors. Drafting of the manuscript: Bennett. Critical revision of the manuscript for important intellectual content: All authors. Statistical analysis: Bennett, Thompson, Carroll, Downs. 682
Positive numbers indicate an increase in percentile lines while negative numbers indicate a decrease in percentile lines.
To the Editor Feskanich et al1 report that early-life milk consumption does not protect against late-life hip fractures in 2 health professions cohorts and, if anything, increased fracture risk in men. In her Editorial, Weaver2 describes a strong mechanistic basis for fracture protection but offers no explanation as to why Feskanich et al seem not to have found it. It is known that teenage milk consumption predicts 2 things, height (found in men by Feskanich et al) and late-life milk consumption. It is also known that the principal causal factors in hip fracture are decreased intrinsic bone strength and the force sustained in a fall (which increases as height increases). There may be an implicit presumption behind this article (and the Editorial) to the effect that bone density (BMD) is the principal factor driving bone strength. Thus, because calcium intake influences BMD, one might have expected increased fracture resistance in high-milk consumers. However, that explanation is probably not correct. Bone-remodeling activity is a far more important fragility factor than BMD3, and bone-remodeling activity is strongly
JAMA Pediatrics July 2014 Volume 168, Number 7
Copyright 2014 American Medical Association. All rights reserved.
Downloaded From: http://archpedi.jamanetwork.com/ by a Purdue University User on 05/19/2015
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Letters
inversely related to current calcium intake (as Wastney and colleagues have demonstrated4). Thus, while dairy intake would be predicted to affect BMD, BMD is probably not the basis for whatever fracture protection dairy might putatively afford. Instead, reduction of excessive bone remodeling is the more important protective mechanism. Here is where the problem lies. Feskanich et al adjusted for current calcium intake in their statistical analysis. Though they do not tell us how they did that, such adjustment, by effectively removing variation in late-life calcium intake, would eliminate the major mechanism by which teenage dairy intake might plausibly influence late-life fracture. No wonder their finding was null. It is precisely latelife calcium (and dairy) intake that would be predicted to affect remodeling (hence fracture risk). With the adjustment for current calcium intake, the study begs the question it sought to answer. It would be useful to know the risk gradient for hip fracture on actual late-life calcium (and specifically dairy) intake or, alternatively, the risk gradient without the adjustment for current calcium intake. Robert P. Heaney, MD
sion of this limitation minimizes the significance of this problem. Correlations between documented consumption and recalled intake are weak. This is one of several problems that are a serious pitfall for nutritional research.2 Feskanich et al characterize greater height as an independent risk factor for hip fracture and they consider this a possible intermediate variable in their analysis. A critical appraisal of published cohort and case-control studies pertaining to height and fracture risk (2 studies referenced by the authors and multiple studies referenced in articles by Hemenway et al3 and Compston et al4) reveals mixed findings from these studies. Not all studies confirmed an association and positive effects were uniformly small. The relationship between height and fracture location (hip, spine, or limb) also differed in some analyses. The effect of sex noted by Feskanich et al conflicts with findings from some of these studies. Virtually all of these studies (including the study by Feskanich et al) suffer from confounding variables and bias. Weak and inconsistent associations, taken from observational epidemiologic studies, are likely to be false and attributable to various sources of bias.2,5 Because of method limitations in their study design and data analysis, the conclusions of Feskanich et al cannot be considered valid.
Author Affiliation: Creighton University, Omaha, Nebraska. Corresponding Author: Robert P. Heaney, MD, Creighton University, 2500 California Plaza, Omaha, NE 68178 ([email protected]).
Robert H. Howland, MD
Conflict of Interest Disclosures: None reported.
Author Affiliation: University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania.
1. Feskanich D, Bischoff-Ferrari HA, Frazier AL, Willett WC. Milk consumption during teenage years and risk of hip fractures in older adults. JAMA Pediatr. 2014;168(1):54-60. 2. Weaver CM. Milk consumption and bone health. JAMA Pediatr. 2014;168(1): 12-13. 3. Heaney RP. Is the paradigm shifting? Bone. 2003;33(4):457-465. 4. Wastney ME, Martín BR, Peacock M, et al. Changes in calcium kinetics in adolescent girls induced by high calcium intake. J Clin Endocrinol Metab. 2000; 85(12):4470-4475.
To the Editor In their data analysis from 2 observational cohort studies, Feskanich et al1 found that greater milk consumption during adolescence was not significantly associated with a lower risk of hip fracture in older women, but was weakly and significantly associated with a greater risk of hip fracture in older men. They also reported that the weak positive association in men was partially mediated by greater height. Their data analysis is limited by not including many important confounding variables that are associated with increased fracture risk. These variables include the risk of falls (which itself is multifactorial, but can be quantified), family history of fracture or osteoporosis, concurrent medical disorders associated with osteoporosis or fractures, carbonated beverage consumption (past and present), and the use of certain drugs (eg, proton pump inhibitors, anticonvulsants, thyroid hormone). Patients with a history of hip fracture were excluded from the analysis, but history of any fracture (data not reported by the authors) is associated with an increased risk of future fractures; this should be considered a confounding variable. The investigators acknowledge the possibility of error in retrospective reporting of milk consumption, but their discus-
Corresponding Author: Robert H. Howland, MD, University of Pittsburgh Medical Center, 3811 O'Hara St, Pittsburgh, PA 15213 ([email protected]). Conflict of Interest Disclosures: None reported. 1. Feskanich D, Bischoff-Ferrari HA, Frazier AL, Willett WC. Milk consumption during teenage years and risk of hip fractures in older adults. JAMA Pediatr. 2014;168(1):54-60. 2. Ioannidis JPA. Implausible results in human nutrition research. BMJ. 2013;347: f6698. 3. Hemenway D, Azrael DR, Rimm EB, Feskanich D, Willett WC. Risk factors for hip fracture in US men aged 40 through 75 years. Am J Public Health. 1994;84 (11):1843-1845. 4. Compston JE, Flahive J, Hosmer DW, et al; GLOW Investigators. Relationship of weight, height, and body mass index with fracture risk at different sites in postmenopausal women: the Global Longitudinal Study of Osteoporosis in Women (GLOW). J Bone Miner Res. 2014;29(2):487-493. 5. Grimes DA, Schulz KF. False alarms and pseudo-epidemics: the limitations of observational epidemiology. Obstet Gynecol. 2012;120(4):920-927.
In Reply Dr Heaney questioned our adjustment for calcium intake during adult years, which was done because our primary question was the independent contribution of milk intake during adolescence to future risk of hip fracture. However, repeating the analysis without adjustment for adult milk consumption and supplemental calcium intake made little difference. In men, the relative risk associated with each additional glass per day of adolescent milk intake changed from 1.09 (95% CI, 1.01-1.17) to 1.08 (95% CI, 1.00-1.16) and results for women remained null. The lack of confounding by adult intake was not surprising, given that adult milk and calcium intakes are not strongly associated with risk of hip fracture in our cohorts, meta-analyses of prospective studies, or randomized trials of calcium supplements without vitamin D.1,2
jamapediatrics.com
JAMA Pediatrics July 2014 Volume 168, Number 7
Copyright 2014 American Medical Association. All rights reserved.
Downloaded From: http://archpedi.jamanetwork.com/ by a Purdue University User on 05/19/2015
683
Letters
Dr Howland concluded that our results are invalid owing to method limitations, including confounding by adult risk factors for hip fractures that were not in our statistical models. However, these factors would only confound our results if they were also associated with adolescent milk consumption, which is unlikely. As shown in the Table in our article, adult behaviors related to fracture risk including smoking, physical activity, and use of various medications were not associated with adolescent milk intake and adjusting for these factors had no effect on our findings. Adolescent milk intake was associated with adult milk intake and adjusting for this also had no appreciable effect. Dr Howland was critical of our statement that height is a known risk factor for hip fracture (confirmed by Dr Heaney) and therefore it could not be an intermediate variable. He referenced a relatively small study (309 hip fractures) by Compston et al3 with a nonsignificant association; however, the 9 other studies cited in that study all reported significant positive associations between height and hip fracture incidence. This association is not weak, as an 8-inch difference in height translates to approximately a doubling of risk of hip fracture. Lastly, Dr Howland concluded that the error in retrospective reporting of milk consumption during teenage years by older adults rendered our results invalid. Although some aspects of diet in the distant past may be poorly recalled, whether milk is infrequently consumed or consumed multiple times a day is more likely to be remembered. The validity of reported adolescent milk intake in our study was directly supported by the positive association with attained height, which is consistent with our earlier prospective findings on current milk intake among adolescents.4 Random error in exposure assessment tends to attenuate a true association toward the null, yet we observed a positive association between teenage milk consumption and hip fracture in men. In conclusion, observational epidemiologic studies like ours can contribute useful information, and often the only available direct information, on topics of great public health importance. Specifically, our findings suggest a partial explanation for the long-standing enigma that hip fracture rates are
684
highest in populations with the greatest milk consumption and that further research in this area is needed. Diane Feskanich, ScD Walter C. Willett, MD, DrPH Author Affiliations: Channing Division of Network Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts (Feskanich, Willett); Department of Nutrition, Harvard School of Public Health, Boston, Massachusetts (Willett). Corresponding Author: Diane Feskanich, ScD, Channing Division of Network Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, 181 Longwood Ave, Boston, MA 021115 ([email protected]). Conflict of Interest Disclosures: None reported. 1. Bischoff-Ferrari HA, Dawson-Hughes B, Baron JA, et al. Milk intake and risk of hip fracture in men and women: a meta-analysis of prospective cohort studies. J Bone Miner Res. 2011;26(4):833-839. 2. Bischoff-Ferrari HA, Dawson-Hughes B, Baron JA, et al. Calcium intake and hip fracture risk in men and women: a meta-analysis of prospective cohort studies and randomized controlled trials. Am J Clin Nutr. 2007;86(6):1780-1790. 3. Compston JE, Flahive J, Hosmer DW, et al; GLOW Investigators. Relationship of weight, height, and body mass index with fracture risk at different sites in postmenopausal women: the Global Longitudinal Study of Osteoporosis in Women (GLOW). J Bone Miner Res. 2014;29(2):487-493. 4. Berkey CS, Colditz GA, Rockett HRH, Frazier AL, Willett WC. Dairy consumption and female height growth: prospective cohort study. Cancer Epidemiol Biomarkers Prev. 2009;18(6):1881-1887.
CORRECTION Incorrect Footnote in Table: In the Original Investigation titled “Electronic Cigarettes and Conventional Cigarette Use Among US Adolescents: A Cross-sectional Study” published online March 6, 2014, in JAMA Pediatrics (doi:10.1001 /jamapediatrics.2013.5488), an incorrect footnote appeared in Table 2. In the row “Ever cigarette smokers (n = 1832),” footnote b should have been used. Error in Phthalate Metabolite Name: In the article titled “Environmental Phthalate Exposure and Preterm Birth” published in the January 2014 issue of JAMA Pediatrics (2014;168[1]:61-67. doi:10.1001/jamapediatrics.2013.3699), incorrect information appeared. The phthalate metabolite for benzyl butyl phthalate appeared as “mono-benzyl-butyl phthalate.” It should have appeared as “monobenzyl phthalate” in the top of the right column on page 63, in the top of the left column on page 64, in row 6 in the second column of Table 2, and in row 6 in the first column of Table 3. This article was corrected online.
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Copyright 2014 American Medical Association. All rights reserved.
Downloaded From: http://archpedi.jamanetwork.com/ by a Purdue University User on 05/19/2015
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