Journal of the American College of Nutrition, Vol. 19, No. 1, 38-41 (2000)
Published by the American College of Nutrition
A Noninvasive Measure of Physical Maturity as a Predictor of Bone Mass in Children
Dorothy A. Nelson, PhD and
David A. Barondess, PhD
Department of Internal Medicine, Division of Rheumatology, Wayne State University School of Medicine, Detroit, Michigan
Address reprint requests to: Dorothy A. Nelson, PhD, Department of Internal Medicine, Wayne State University, Rheumatology Division, Hutzel Hospital, 4707 St. Antoine, Detroit, MI 48201
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ABSTRACT
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Objective: The purpose of this study was to describe the accumulation of whole body bone mass in a longitudinal study of prepubertal boys and girls using Rochersquo;s physical maturity index as a measure of developmental age.
Methods: We measured 561 children (39% white, 61% African-American) from a suburban school district, representing an ethnically mixed, middle-class community adjacent to Detroit. Anthropometric measures taken for the present study included recumbent length (cm), stature (cm), weight (kg), whole body bone mineral content (WBBMC in g) and a noninvasive measure of physical maturity (PM%). PM% was calculated from published formulae derived from data from the Fels Longitudinal Study, using recumbent length, weight, midparental stature, age, and age- and gender-specific regression coefficients.
Results: At average age 9.9 (±0.6) years, there were no significant gender differences in stature, recumbent length, weight, or WBBMC in either ethnic group. Average PM for girls was significantly greater than that for boys within each ethnic group. There were no significant ethnic differences in PM in either gender. Stature and WBBMC were significantly different in the two ethnic groups for both boys and girls. Regressions of WBBMC on PM and chronological age indicated that PM explained more of the variance in WBBMC than did age (r2 ranging from 0.28 to 0.75 for PM versus 0.01 to 0.06 for age). In the case of African-American boys, r2 was similar (0.09 for PM and 0.06 for age).
Conclusions: PM is a useful, noninvasive measure of developmental age that is significantly correlated with bone mass in children. Our study also indicates that PM is a better predictor of WBBMC than chronological age. Because PM can be calculated without using invasive and potentially expensive methods, PM may be useful in some clinical as well as research settings.
Key words: physical maturity, bone mass, ethnicity, children
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INTRODUCTION
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There are few large studies of bone mass during childhood and few, if any, that have included non-white ethnic groups and both boys and girls. Such studies are essential for developing strategies to maximize peak bone mass in order to reduce the risk of osteoporosis later in life [1–5]. It is especially important to consider variation in body size and physical maturity at a given age when studying children’s bone growth, since these variables are highly correlated with bone mass measurements [2].
Traditionally, developmental age or physical maturity in children has been determined with the use of standardized hand-wrist radiographs [6] for assessment of bone age or by determining Tanner stages of maturity by the physical examination of secondary sex characteristics [7]. For a number of reasons, however, these methods may not be suitable or feasible for research studies of healthy children [8]. Radiographs can be costly, and they involve radiation, which may not be acceptable to parents of healthy children in whom such radiological evaluation is not clinically necessary. Furthermore, these radiographs must be read by an expert in order to accurately estimate skeletal age, further adding to the cost of a research study. Tanner staging also has several drawbacks in a research setting. It is invasive to the extent that it involves undressing and, therefore, may be regarded as an invasion of privacy for many children. Tanner staging must be done by a trained expert and is, therefore, costly. Additionally, Tanner staging is based on an ordinal scale scoring system, 1 through 5. In contrast, continuous or interval-level data allow stronger inferences to be made about magnitudes of measurements and may, therefore, be more useful to researchers interested in growth and development.
One of the difficulties in studying children is the wide range of variation in the timing and magnitude of growth changes. In fact, it is well known that, when assessing growth, developmental age is more useful than chronological age because of the tremendous individual variability associated with growth at a given age.
In the present study we employ a continuous measure of physical maturity described by Roche et al. that is both noninvasive and inexpensive [8,9] and compare it with the use of chronological age for predicting bone mass. We also relate our data on physical maturity in the African-American and white children in our cohort to the white children studied by Roche et al. in the Fels Longitudinal Study on growth, maturation and body composition [10].
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SUBJECTS
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For the present study, we measured 561 children (39% white, 61% African-American) enrolled in the 4th grade within the Southfield, Michigan, school district [11]. This district represents a suburban, ethnically mixed, middle-class community adjacent to Detroit. According to US census data [12], Southfield’s population is approximately 57% African-American, 39% white and 4% "other" in composition. Prospective subjects contacted for the present study self-identified themselves as 37% white, 56% African-American and 7% "other." Because of sample size considerations, we include only white and African-American children in this analysis. Based on a comparison of participants and refusers, we determined that there was no recruitment bias by school (n=10), ethnic group, age, gender or level of parental education (p>0.25). The total sample size for this study, broken down by ethnicity, age and gender is presented in Table 1.
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METHODS
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Anthropometric measures taken for the present study included stature (cm) and recumbent length (cm), using a Harpenden Stadiometer and a Supine Measuring Table, respectively. Also, weight (kg) was recorded using a Seca digital scale. Whole body bone mineral content (WBBMC) was measured using an Hologic 1000W bone densitometer (Hologic Corp., Waltham, MA). Physical maturity (PM) was calculated by the formula: PM=PS/PAS, where PS is the child’s present stature and PAS is the child’s predicted adult stature. PAS was calculated according to the method reported by Roche et al. [8] as follows: PAS=(recumbent length)(ßRL) + (weight)(ßw) + (midparental stature)(ßMPS) + (age)(ß0), where ß=the age- and gender-specific regression coefficients, RL is recumbent length, W=weight, MPS is midparental stature, and "0" represents the intercept of the regression equation. Midparental stature is the average of the parents’ heights; when only one parent was actually measured, we used the reported height for the other parent. According to Wainer et al., this approach results in only a small increase in the error of prediction [13].
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RESULTS
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As indicated in Table 1, at average age 9.9 (±0.6) years, within each ethnic group there were no significant gender differences in stature, recumbent length, weight or WBBMC. Girls had a significantly greater PM than boys in both ethnic groups (p<0.001), but there were no significant ethnic differences in PM for either boys or girls (p>0.05). Among girls, African-Americans had a significantly greater stature, recumbent length, weight and WBBMC than whites. African-American boys had a significantly greater WBBMC than white boys.
In Table 2, regression results are presented for males and females of each ethnic group by WBBMC on PM and on chronological age. For white and African-American girls and for white boys, PM explained more of the variance in WBBMC than did age (r2 ranging from 0.28 to 0.75 for PM versus 0.01 to 0.06 for age). Only for African-American boys, where r2 was similar (0.09 for PM and 0.06 for age), did PM appear not to be a better predictor of WBBMC compared to age.
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Table 2. Regression Coefficients for Whole Body Bone Mineral Content (g) on Physical Maturity (%) and Chronological Age (yr)
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Fig. 1 compares PM in the boys and girls from the present study with reference data from the Fels Study (whites only) at age 10. The data from our cohort were not significantly different from the Fels reference data except for African-American boys, who had a lower PM than the Fels cohort of white boys.
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Fig. 1. Comparison of physical maturity (%) from Fels reference data, white, and African-American males and females at age 10. The comparison denoted with an asterisk (*) is significant at p=0.005; all other pairwise ethnic comparisons are not significant (p<0.05).
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DISCUSSION
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Growth and physical maturation are accompanied by alterations in lean body mass, fat distribution and rapid skeletal growth [7,14]. There is a wide range of variability in the timing and rate of these changes among individual children. Therefore, it is essential to utilize a method of assessing physical maturity in studies of growth, such as research on bone mass accumulation in children. The well-validated index of physical maturity described by Roche et al. [8,10] and utilized in this study appears to be a useful, noninvasive measure of developmental age that is significantly correlated with bone mass in children. Our study also indicates that, overall, physical maturity is a better predictor of WBBMC than chronological age. Because the index of physical maturity described in this paper can be calculated without having to resort to invasive and potentially expensive methods, it is useful and practical in research studies of growth in normal children. In conjunction with traditional methods of assessing a child’s developmental stage, the index has potential utility in the clinical environment as well. Adjusting bone mass measurements using the index is one example of this potential use. While most of our data are virtually identical to the published data for the Fels Study, our results indicate that additional studies may be needed to establish reference data for non-white ethnic groups.
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ACKNOWLEDGMENTS
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This study was supported by National Institute of Health Grant #AR41319. We are grateful to Lynne Vigelius-Slagh and Amanda I. Dudley for their invaluable technical help.
Received July 1, 1999.
Accepted October 1, 1999.
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ABSTRACT
INTRODUCTION
