Abstract
Objective
To measure the longitudinal changes in body composition including muscle mass (MM), total body water (TBW), and body fat in term infants and children using the D3Creatine (D3Cr) dilution method combined with 2H2O.
Study design
A total of 36 term infants were enrolled and measured at baseline and after 6 and 12 months. In addition, 30 boys (ages 4-12 years) and 31 girls (ages 4-10 years) were enrolled. A single dose of D3Cr in 20% 2H2O was administered orally using a syringe. Saliva and urine samples were used to determine 2H2O and D3Cr D3creatinine enrichments for determination of TBW (fat-free mass [FFM]) and MM, respectively.
Results
Body weight (BW), MM, and FFM were associated with age and increased longitudinally over 1 year in 21 boys and girls. In infants, MM increased by 256% after 1 year while %MM (MM/BW) remained constant at 26% after 6 months and increased to 36% after 1 year. Positive relationships were found between MM, TBW, BW, and anthropometric values.
Conclusions
D3Cr dilution provides a noninvasive measurement of MM in infants and children and when coupled with the measurement of TBW, a full body composition assessment is possible including MM, FFM, and body fat.
Keywords: fat-free mass, body composition, growth, total body water
Skeletal muscle is the largest reservoir of amino acids and is, arguably, the most accurate index of nutritional status in health and disease. Muscle is a highly plastic tissue demonstrating rapid changes in size and function in response to periods of increased and decreased muscular activity, nutritional status, disease, and growth. As such, muscle mass (MM) is an important biomarker and should be a fundamental element in the assessment of health, nutritional status, and disease. The treatment of any condition in which MM accrual is of central importance, such as in growing infants or when providing nourishment to malnourished children, would benefit from a simple tool to assess changes over time or the results of interventions. At the present time, there are no normative data for MM or MM accretion during growth in infants or children.
The D3Creatine (D3Cr) dilution method is a noninvasive method for assessment of total MM, through measurement of total body creatine pool size, which is proportional to MM.1 D3Cr has the 3 hydrogens on the methyl group (-CH3) replaced with deuterium atoms (D) making it “D3” creatine. Approximately 98% of creatine in the body is found in skeletal muscle,2,3 making measurement of the total body creatine a good marker of total skeletal MM. In this method, a single oral administration of a stable (nonradioactive, cold) isotope of creatine (2 mg of D3Cr for infants, in which the 3 hydrogen atoms of the methyl group are replaced with deuterium atoms) is ingested, enters the circulation, and is actively transported into skeletal muscle and thus diluted in the endogenous creatine pool in skeletal muscle. Creatine is converted into creatinine through an irreversible nonenzymatic reaction, and creatinine is rapidly excreted in urine. This method is different from the 24-hour creatinine excretion method.4 While both use the same rationale that urine creatinine is a reflection of creatine pool size, the D3Cr dilution method only requires a single urine sample rather than the collection of all urine produced in a 24-hour period coupled with a meat-free diet.
In addition to studies in adults, this method has been used to measure MM in neonates,5 boys with Duchenne muscular dystrophy,6,7 and 4-year-old malnourished children in Bangladesh.8 Previously, the measurement of body composition and accrual of MM in 76 premature infants (>26 weeks of gestational age) showed a strong relationship between MM and fat-free mass (FFM) (by total body water [TBW]) and with body weight (BW) and reported rapid 14%/wk. accrual of MM while infants were cared for in a neonatal intensive care unit.5 More recently, a moderate relationship (r = 0.4, P = .003) between D3Cr MM and FFM assessed using air-displacement plethysmography in 67 premature infants was observed.9
The purpose of this study was to measure MM and FFM in term infants as well as in children between the ages of 4 and 12 years. Through the establishment of normative data for MM in growing infants and children, abnormal levels or changes in levels of MM due to disease or malnutrition may be identified. In addition, we also measured 1-year longitudinal changes in body composition in a subset of the infants and children.
Methods
This study in children and full-term babies was carried out at 3 locations, Oregon Health & Science University, KineMed, Inc, in Emeryville, CA, and University of North Carolina at Chapel Hill, and were approved by the local Institutional Review Boards. Eligible male and female children were 4-12 years old, whereas full-term well infants were within 30 days of birth. Families of infants and children were contacted by pediatricians at the University of North Carolina and Oregon Health & Science University. Families interested in participation were provided detailed explanations of the methods and length of participation. Families of all infants and children consented to participation in a 1-year longitudinal study. The exclusion criteria included diseases that may affect normal growth, cancer, neuromuscular diseases, and any other condition or circumstance that may impact study measurements. Sixty-one children (31 female, 30 male) and 36 term infants were enrolled after consent of their parents. Three assessments were performed in the childrenat baseline, 6 months, and 12 months, which included skeletal MM and FFM as well as, BW, length, and mid-arm circumference (MAC).10 BW, length, and circumferences of head, chest, and waist were measured in the infants. A Gulick tape measure was used to determine circumferences.
Measurement of MM in Children and Infants
Skeletal MM was measured by D3Cr dilution method as described previously.5 A single oral dose of D3Cr (10 mg for children 4-8 years or 15 mg for those >8 years in 2 ml 20% 2H2O for children and 2 mg in 0.5 ml 20% 2H2O for full-term infants was administered using a syringe, weighed before and after dosing, and approximately 72 hours later, a single urine sample was collected. For children, a filter paper stick was saturated by placing it into a cup containing a urine sample and later frozen. For infants, urine was collected from a filter paper strip placed in the diaper.
Deuterated creatinine (D3Crn) enrichment, which is the ratio of labeled to unlabeled creatinine, as well creatine and creatinine concentrations were then determined from urine samples by mass spectrometry as described in detail elsewhere.5,11 The spillage of D3Cr, that is, the extent to which ingested D3Cr “spills” into urine without mixing with whole-body creatine, was estimated from the ratio of Cr/creatinine (Crn) and a correction algorithm and calculation of the whole-body creatine pool size was as described previously.11 No correction for spillage was applied for full-term infants as our previous study in neonates5 showed negligible spillage in infants close to full-term. Total body skeletal MM (kg) for both children and infants was calculated as Cr pool size (g)/4.3(g/kg). Percent MM was calculated as the ratio of MM to BW.
2H2O for TBW
Saliva samples were collected from each subject using SalivaBio infant or children swabs placed orally until saturated (∼30–60 s) at 1-3 hours after administration of the D3Cr in 2H2O dose. Measurement of 2H2O was by isotope ratio isotope ratio spectroscopy, and TBW was derived as previously described.12 For children, FFM was calculated using age and gender appropriate hydration factors.13
Statistical Analysis
Data are Represented as Mean ± SD Split by Different Age Groups
Two-way ANOVA (Graphpad Prism, www.graphpad.com) was performed for all the body composition measurements in age-matched boys and girls to determine age and gender effects. Since the 10-12 years age group was only boys, 1-way ANOVA was performed for these measurements comparing all age groups in boys. Longitudinal measurements in children were analyzed by 1-way repeated measures ANOVA. Post hoc Tukey test was performed to compare the different age groups with corrections for multiple comparisons. For full-term infants, 1-way ANOVA with a mixed effects model was performed for all measurements followed by post hoc Tukey for pair-wise comparisons. Pearson correlation analysis was performed across various pairs of measurements in both children and infants. All comparisons with P < .05 were considered statistically significant.
Results
Cross-Sectional Measurements in Children
The body composition measurements performed in 66 children were separated into different age groups of 4-6 years, 6-8 years, and 8-10 years old for both girls and boys with an additional age group of 10-12 years old boys (Table I). Comparison of the different groups revealed a statistically significant effect of age, but no sex difference for any parameter. The BW and length significantly increased with increasing age in both girls and boys. The MAC was significantly higher only in the oldest age group compared to the younger age groups. The MM in 8-10 years old girls were significantly higher than the younger age groups, whereas 6-8 years and 8-10 years old boys had significantly higher MM than 4-6 years old boys, and 10-12 years old boys had significantly higher MM than all the younger age groups. The FFM estimated from TBW significantly increased with increasing age in both boys and girls, whereas fat mass was significantly increased in 8-10-year-old boys and 10-12-year-old boys compared to the younger boys. Percent MM calculated as a ratio of MM to BW was ∼43% with no significant age or sex difference. FFM and fat mass constituted ∼80% and ∼20% of BW respectively. The ratio of MM to FFM indicates that muscle contributes ∼53% to FFM. Strong and significant cross-sectional relationships (r = 0.74-0.82, P < .0001) were found between MM and age, BW, MAC, and FFM in both boys and girls (Figure).
Table I.
Baseline measurements in girls and boys
| Value | Female |
Male |
|||||
|---|---|---|---|---|---|---|---|
| 4-6 y | 6-8 y | 8-10 y | 4-6 y | 6-8 y | 8-10 y | 10-12 y | |
| Age (y) | 5.2 ± 0.4 (8) | 7.0 ± 0.7 (13) | 8.8 ± 0.6 (10) | 4.8 ± 0.4 (5) | 7.0 ± 0.6 (10) | 8.9 ± 0.5 (10) | 11.3 ± 0.5 (5) |
| Body weight (kg) | 18.9 ± 1.6 (8) | 22.8 ± 4.2 (13)∗ | 28.9 ± 3.3 (10)∗,† | 18.3 ± 1.8 (5) | 25.5 ± 3.3 (10)∗ | 29.6 ± 3.1 (10)∗ | 42 ± 9.1 (5)∗,†,‡ |
| Length (cm) | 110.2 ± 5.1 (8) | 124.4 ± 6.0 (13)∗ | 133.3 ± 5.9 (9)∗,† | 109.2 ± 2.6 (5) | 127.3 ± 4.0 (10)∗ | 135.0 ± 7.4 (10)∗ | 153.0 ± 11.4 (5)∗,†,‡ |
| Mid-arm circumference (cm) | 18.0 ± 1.4 (8) | 17.9 ± 2.2 (13) | 20.4 ± 1.7 (10)∗,† | 17.1 ± 1.2 (5) | 18.9 ± 1.4 (10) | 19.5 ± 1.2 (10) | 22.3 ± 2.9 (5)∗,†,‡ |
| Muscle mass (kg) | 8.2 ± 0.9 (8) | 9.1 ± 2.4 (13) | 12.9 ± 1.5 (10)∗,† | 7.0 ± 1.1 (5) | 11.4 ± 1.6 (10)∗ | 13.4 ± 2.4 (10)∗ | 17.6 ± 3.4 (5)∗,†,‡ |
| Fat-free mass (kg) | 14.6 ± 1.6 (7) | 18.5 ± 4.0 (11)∗ | 23.4 ± 2.6 (9)∗,† | 13.9 ± 1.7 (3) | 23.5 ± 3.6 (9)∗ | 23.1 ± 2.9 (10)∗ | 31.0 ± 4.7 (4)∗,†,‡ |
| Fat mass (kg) | 4.4 ± 1.1 (7) | 4.6 ± 2.7 (11) | 5.6 ± 2.0 (9) | 3.3 ± 0.5 (3) | 2.1 ± 0.8 (9) | 6.5 ± 2.7 (10)† | 8.8 ± 5.3 (4)∗,† |
| Muscle mass index (kg/m2) | 6.8 ± 0.9 (8) | 5.9 ± 1.2 (13) | 7.3 ± 1.4 (9)† | 5.9 ± 0.9 (5) | 7.0 ± 0.9 (10) | 7.4 ± 1.1 (10)∗ | 7.5 ± 0.9 (5) |
| Muscle mass/body weight (%) | 44 ± 7 (8) | 40 ± 5 (13) | 45 ± 6 (10) | 38 ± 5 (5) | 45 ± 5 (10) | 45 ± 5 (10) | 43 ± 6 (5) |
| Fat-free mass/body weight (%) | 77 ± 5 (7) | 80 ± 11 (11) | 81 ± 6 (9) | 81 ± 4 (3) | 92 ± 4 (9) | 78 ± 7 (10)† | 79 ± 8 (4)† |
| Fat mass/body weight (%) | 23 ± 5 (7) | 20 ± 11 (11) | 19 ± 6 (9) | 19 ± 4 (3) | 8 ± 4 (9) | 22 ± 7 (10)† | 21 ± 8 (4)† |
| Muscle mass/fat-free mass (%) | 57 ± 11 (7) | 50 ± 10 (11) | 55 ± 8 (9) | 49 ± 6 (3) | 49 ± 7 (9) | 58 ± 10 (10) | 53 ± 4 (4) |
Mean ± SD (n).
P < .05 vs 4-6 y.
P < .05 vs 6-8 y.
P < .05 vs 8-10 y.
Figure.
Baseline muscle mass vs age (A), body weight (B), mid-arm circumference (C), and fat-free mass (D) in girls (red circles) and boys (blue circles).
Longitudinal Measurements in Children
Multiple assessments of MM were performed in a subset of 21 children, with an initial baseline measurement of D3Cr dilution followed by a second measurement 6-10 months later and a third measurement 12-18 months after the initial dose. BW, FFM, and MAC were also measured at these time points. One-way repeated measures ANOVA with post hoc Tukey multiple pair-wise comparisons revealed a significant increase of MM and BW at each consecutive measurement, with each time point significantly different from the others (Table II). There was an 18% increase in MM at the 6-10 months assessment compared to baseline, and another 15% increase at the 12-18 months time point, with an overall 36% increase from baseline at the final measurement. In contrast, %MM and FFM did not show any changes when measured at ∼6-month intervals, whereas MAC was significantly increased at the final time point compared to baseline and 6 months (Table II).
Table II.
Longitudinal measurements in children
| Value | Baseline | 6-10 mo | 12-18 mo |
|---|---|---|---|
| Age (y) | 7.2 ± 1.5 (21) | 7.7 ± 1.5 (21)∗ | 8.3 ± 1.4 (21)∗,† |
| Age range | 4-10 y old | 5-10 y old | 5-11 y old |
| Muscle mass (kg) | 8.8 ± 2.6 (21) | 10.11 ± 3.7 (21)∗ | 11.56 ± 4.4 (21)∗,† |
| Body weight (kg) | 24.1 ± 5.3 (21) | 25 ± 5.4 (21)∗ | 26.7 ± 5.6 (21)∗,† |
| Muscle mass/body weight (%) | 36.6 ± 6.8 (21) | 40.1 ± 9.6 (21) | 42.7 ± 12.5 (21) |
| Fat-free mass (kg) | 20 ± 3.7 (21) | 20.6 ± 5 (21) | 20.2 ± 4.1 (21) |
| Mid-arm circumference | 18.6 ± 2.2 (21) | 18.9 ± 2.1 (21) | 19.6 ± 2.2 (21)∗,† |
Mean ± SD (n).
P < .05 vs baseline.
P < .05 vs 6 mo, 1-way repeated measures ANOVA.
Longitudinal Measurements in Infants
Body composition was measured in 36 term infants within 1 month of birth (baseline) followed by 2 repeated measurements performed at 6-month intervals. As shown in Table III, all the parameters increased with growth. Interestingly, MM as a proportion of BW remained constant between baseline and 6 months but increased after 1 year. Compared to baseline values, we observed a 133% increase in MM after 6 months and a further 73% increase at 12 months, with an overall 256% increase at the final measurement. Significant positive relationships were found between all the measurements as seen in the correlation matrix (Table IV). The relationship between 6 m change in MM vs change in TBW was r = 0.6087 (P < .05), n = 16; for 6 m change in body weight (BW) vs change in TBW: r = 0.6521, (P < .005), n = 17; and for the 12 m change in BW vs change in TBW r = 0.6473, P = .005, n = 17.
Table III.
Measurements in full-term babies
| Full-term infants | Baseline | 6 mo | 12 mo |
|---|---|---|---|
| Weight (kg) | 3.5 ± 0.7 (35) | 7.7 ± 1.1 (36)∗ | 9.2 ± 0.9 (28)∗,† |
| Length (cm) | 50.1 ± 5.3 (35) | 67.4 ± 4.7 (35)∗ | 72.5 ± 3.8 (28)∗,† |
| Head circumference (cm) | 34.7 ± 2.0 (35) | 43.3 ± 1.6 (36)∗ | 46.2 ± 2.1 (28)∗,† |
| Waist circumference (cm) | 33.4 ± 2.9 (35) | 44.4 ± 2.6 (36)∗ | 46.1 ± 3.1 (28)∗ |
| Chest circumference (cm) | 33.3 ± 3.6 (35) | 45.0 ± 3.3 (36)∗ | 46.8 ± 3.5 (28)∗,† |
| Muscle mass (kg) | 0.9 ± 0.4 (34) | 2.1 ± 0.8 (33)∗ | 3.2 ± 0.9 (24)∗,† |
| Muscle mass index (kg/m2) | 3.6 ± 1.3 (31) | 4.6 ± 1.7 (32) | 6.1 ± 1.8 (24)∗,† |
| Muscle mass/body weight (%) | 26.3 ± 9.1 (34) | 26.9 ± 9.8 (33) | 34.6 ± 9.8 (24)∗,† |
| Total body water (kg) | 3.1 ± 0.5 (24) | 4.8 ± 1.2 (26)∗ | 6.4 ± 1.4 (24)∗,† |
| Total body water/body weight (%) | 81.1 ± 11.1 (24) | 60.6 ± 12.9 (26)∗ | 70.2 ± 13.2 (24)∗,† |
| Muscle mass/total body water (%) | 31.2 ± 11.4 (21) | 41.7 ± 12.2 (22)∗ | 52.2 ± 19 (20)∗ |
Mean ± SD (n).
P < .05 vs baseline.
P < .05 vs 6 mo, 1-way ANOVA mixed-effects model.
Table IV.
Correlation matrix showing cross-sectional relationships between body composition variables in full-term infants for all time points
| Value | Body weight (kg) | Muscle mass (kg) | Total body water | Length (cm) | Head circumference (cm) | Chest circumference (cm) |
|---|---|---|---|---|---|---|
| Muscle mass (kg) | r = 0.7766 P < .0001 n = 91 |
|||||
| Total body water (kg) | r = 0.8038 P < .0001 n = 74 |
r = 0.6722 P < .0001 n = 65 |
||||
| Length (cm) | r = 0.7492 P < .0001 n = 98 |
r = 0.6275 P < .0001 n = 88 |
r = 0.6738 P < .0001 n = 74 |
|||
| Head circumference (cm) | r = 0.8863 P < .0001 n = 98 |
r = 0.7147 P < .0001 n = 88 |
r = 0.6835 P < .0001 n = 74 |
r = 0.7040 P < .0001 n = 98 |
||
| Chest circumference (cm) | r = 0.9296 P < .0001 n = 98 |
r = 0.6830 P < .0001 n = 88 |
r = 0.7746 P < .0001 n = 74 |
r = 0.7364 P < .0001 n = 98 |
r = 0.8648 P < .0001 n = 98 |
|
| Waist circumference (cm) | r = 0.8945 P < .0001 n = 98 |
r = 0.6513 P < .0001 n = 88 |
r = 0.7002 P < .0001 n = 74 |
r = 0.6694 P < .0001 n = 98 |
r = 0.8357 P < .0001 n = 98 |
r = 0.9403 P < .0001 n = 98 |
Discussion
In the present study, we measured body composition in 66 children and 36 term infants. In term infants, MM comprised an average of 31.2 ± 11.4% of TBW (as a surrogate for FFM) and 26.9 ± 9.1% of BW. For the infants, TBW will be used rather than FFM as the content of water in FFM changes with growth and we do not assume that a conversion of TBW to FFM is well established. The relative content of MM to BW remained constant for 6 months and increased to 34% after 1 year of growth. The relative amount of MM in TBW increased from 31.2 ± 11.4% at birth to 41.7 ± 12.2% at 6 months and 52.2 ± 19% after 1 year of growth. The relative amount of MM to BW in the infants in the present study was very similar to that reported in preterm infants5 of varying gestational ages and suggests a constant content of muscle as a percentage to total body mass in infants. MM (kg) increased by 133% after 6 months and 256% after 1 year of growth.5 The % content of TBW in term infants was also similar to that of preterm infants but changed during the first year of growth as total fat and % fat increased. These data demonstrate a rapid increase in total MM during the first year of growth, and because the relative amount of MM in TBW also increased, TBW or other assessments of FFM cannot be used as surrogate assessments of MM. Increases in MM during growth in premature and term infants are likely an indicator of healthy growth. Establishment of changes in MM during growth in healthy infants may therefore provide a noninvasive indicator of nutritional and health status. Ooi et al has suggested that pediatric sarcopenia may have promise for evaluation of malnutrition in infants and children.14 However, previous suggested assessments of pediatric sarcopenia are largely measurements of FFM rather than MM. In the present study, we demonstrated that MM is not a constant relative component of TBW during the first year of growth, increasing from 31.2 ± 11.4% at birth to 52.2 ± 19% after 1 year of growth, representing strong evidence that FFM is not an appropriate surrogate measure of MM in infants and children. In these growing infants, total MM increased by an average 281% while TBW increased by 48%. Changes in FFM during the initial years of growth have been associated with assessments of cognitive development and quality of growth.15,16 Skeletal muscle has direct connections to the central nervous system via myelinated motor units and deposition of lean tissues such as muscle and brain require energy and protein, determination of MM during the initial years of growth may provide an indirect biomarker of cognitive development and quality of growth. Body Mass Index and trajectory of changes in body mass index at birth and with growth have been associated with risk of metabolic diseases in later life.17 However, it is not clear if this association is related to body fatness, lean mass, MM or % of MM. Using a dose of D3Cr in 2H2O in population study of growing infants and children, it is possible to obtain each of these components of body composition, noninvasively, to better understand how changes in % MM may influence disease risk in later life.
In children, total MM increased with advancing age in both boys and girls while %MM remained constant at about 45%, despite age related increases in total body MM and FFM.
As we previously reported in neonates, use of the D3Cr dilution method and TBW provides a noninvasive measurement of body composition that requires only collection of saliva and urine and includes MM, FFM, and fat mass in infants and children. While there is no “gold standard” for these assessments in both infants and children, the MM by D3Cr dilution was strongly associated with FFM by TBW and BW. The cross-sectional relationships between BW and MM were strong in infants and children of all ages. Accrual of MM was also closely associated with BW and FFM. Similar to our previous observation in premature infants,5 infants in the present study did not exhibit D3Cr dose spillage, and therefore no spillage correction algorithm was applied to the calculation of MM. In children, we observed a low urine Cr/Crn ratio indicating a low amount of spillage.
Current methods for the quantification of MM (magnetic resonance imaging, dual x-ray absorptiometry, computerized tomography), are indirect or lack precision or rely on expensive equipment, that is, not available in the field. The measurement of body composition in children is even more problematic. The most common methods used for children include anthropometry,18 bioelectric impedance,19 whole-body plethysmography, or dual x-ray absorptiometry.20 Each of these methods involves assumptions based on the data gathered in adults, assumes that subjects are adequately hydrated and that FFM has a constant relative amount of water. None of these assumptions are valid in infants, sick, or malnourished children. As a result, body composition measures in infants and children can be fundamentally inaccurate and misleading regarding effects of malnutrition, disease, and even what represents normal growth for skeletal MM. Wells et al stated that “the lack of a gold standard for measurements of body composition in children makes evaluation of simple methods difficult” and “the need to improve bedside methods remains a priority, as does the need to revise values for the reference child, particularly with regard to FFM composition”.21 However, despite the inaccuracy of these measurements, relative change in body composition has proven to be a powerful predictor of morbidity and mortality among malnourished children.22 The development of a simple, rapid, accurate, noninvasive, and inexpensive method to quantify skeletal MM would be a major advance in efforts to optimize nutrition in the most rapidly growing humans as well as dramatically advance the detection and treatment of muscle depletion in infants and children. Ooi et al described the need to measure MM in children to establish criteria for determining pediatric sarcopenia.14 The methods they describe are not, however, measures of MM. They indicated that the measurement of TBW using deuterium oxide dilution is the “gold standard” for determining FFM but this method is not an assessment of MM. In the present study, we employed deuterium oxide dilution to measure FFM and showed that FFM is about 80% of BW in boys and girls and has a strong relationship between MM, but MM is only about 50% of FFM. This result in the present study is similar to a previous report in 9 boys aged 8-17 years.6 MAC is frequently used as an assessment of MM and nutritional status in children.23 We observed a significant but moderate relationship (r2 = 0.551) between MM and MAC in children measured at all time points (Figure) demonstrating that approximately 55% of the variability in MM was explained using MAC. Our data show that neither MAC nor FFM is an accurate surrogate assessment of MM in boys or girls. While MAC and FFM are significantly associated with MM, they are unlikely to provide accurate evidence of longitudinal changes in MM. We have previously demonstrated in older men and women that MM and %MM but not FFM are strongly associated with strength and functional status24 and with health-related outcomes.25,26 Because the D3Cr dilution method is a more direct assessment of MM, for the first time, it is now possible to determine how MM rate of accrual may vary with normal growth in infants and children and its relationship with health-related outcomes in these populations. However, until the D3Cr dilution method becomes available to clinicians, the use of multifrequency bioelectrical impedance may become an appropriate substitute if an algorithm is developed from a comparison of the 2 methods in children. Such a comparison was recently made in adult athletes.27
The present study has limitations. The total sample size for children was small and stratifying the data by sex and age resulted in samples that likely did not represent each of the general populations described. In particular, we were unable to describe prepubertal and puberty-associated sex differences in MM and % MM. This should be a priority in describing body composition using MM measurement for future research. Because no functional measurements were made, it was not possible to evaluate the relationship between MM and strength or functional status. Although the protocol called for longitudinal data to be collected on all the children enrolled, only 22 completed the full year evaluation. As a result, this study is not sufficiently powered to establish longitudinal changes quantitatively in children, stratified by age and sex. Nevertheless, we did observe increases in MM in all the children who participated in the longitudinal component of this study.
The calculation of MM relies on the assumption that the concentration of creatine is set at 4.3 g/kg wet weight of muscle, as the value of creatine pool size (in grams), that is, estimated by the enrichment of D3Creatinine is divided by 4.3 to obtain total MM in kg. The 4.3 g/kg wet weight is derived from rodent and humans28,29 and is based on an average estimated muscle fiber composition of 50% type I and II fibers. We note that type II, fast, fibers have a higher amount of creatine than type I, slow, fibers. Since D3Cr MM (calculated using the set 4.3-g creatine/kg of wet weight muscle) is a monotonic transformation of the creatine pool size, effect estimates for standardized variables would be identical for either creatine pool size or D3Cr MM (analyzed with or without adjustment for body size).
Conclusion
Here we describe a completely noninvasive measurement of body composition that includes MM, FFM, and fat mass in infants and children. This determination only requires ingestion of D3Cr dissolved in a small amount of heavy water (2H2O) followed by saliva and urine collections. These data should serve as the basis for the assessment of MM data in larger cohorts of growing infants and children. In this way, normative data for MM and %MM can be collected and provide critical assessments of how changes in MM are associated with healthy growth and health-related outcomes in these populations.
Data Statement
Data sharing statement available at www.jpeds.com.
CRediT authorship contribution statement
Mahalakshmi Shankaran: Writing – review & editing, Writing – original draft, Supervision, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Brian Scottoline: Writing – review & editing, Supervision, Project administration, Methodology, Investigation, Conceptualization. Faryal Imam: Writing – review & editing, Supervision, Methodology, Investigation. Kelly Garton: Writing – review & editing, Validation, Methodology, Investigation. Jacob Lohr: Writing – review & editing, Supervision, Project administration, Methodology, Investigation. Edna Nyangau: Writing – review & editing, Visualization, Methodology, Investigation. Hussein Mohammed: Writing – review & editing, Validation, Methodology, Investigation. Marc Hellerstein: Writing – review & editing, Writing – original draft, Supervision, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. William J. Evans: Writing – review & editing, Writing – original draft, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
Declaration of Competing Interest
All phases of this study were supported by the Bill and Melinda Gates Foundation. W.J.E., M.H., and M.S. were employed by Kinemed, Inc, for a period during the conduct of this project. W.J.E. and M.H. work with MyoCorps, Inc. M.S. is a consultant for MyoCorps, Inc. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Supplementary Data
References
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