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ORIGINAL RESEARCH COMMUNICATION |
1 From the Department of Gastroenterology (FEA, HS, and EVO), the James Fairfax Institute of Paediatric Nutrition (JRA, KJG, and MAG), the University of Sydney (FEA and SLC), and the Department of Nuclear Medicine (JNB and RBH-G), The Childrens Hospital at Westmead, Westmead, Australia
2 Supported by the National Health and Medical Research Council of Australia, the James Fairfax Institute of Paediatric Nutrition, and Nutricia Australia Pty Ltd.
3 Reprints not available. Address correspondence to EV OLoughlin, Department of Gastroenterology, The Childrens Hospital at Westmead, Locked Bag 4001, Westmead NSW 2145, Australia. E-mail: tedo{at}chw.edu.au.
| ABSTRACT |
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Objective: We aimed to measure and evaluate body protein and to determine whether skinfold-thickness anthropometry and DXA can predict body protein in children with SQCP.
Design: This was a cross-sectional study of 59 children (22 girls, 37 boys) aged 3.919.5 y with SQCP. The children underwent measurements of anthropometric indexes, lean tissue mass by DXA (LTMDXA), and total body protein by neutron activation analysis (TBPNAA). In addition, TBP was estimated from both skinfold-thickness anthropometry (TBPSKIN) and DXA (TBPDXA). The agreement of TBPSKIN and TBPDXA was tested against TBPNAA by using Bland and Altman plot analysis.
Results: Height and weight SD scores (
± SD: 3.1 ± 1.6 and 4.8 ± 5.3, respectively) were significantly lower than reference data in the children with SQCP (P < 0.001). TBPNAA for age and height was low in the children with SQCP (P < 0.001): 56.1 ± 17.3% and 81.5 ± 15.7%, respectively, of the values predicted from control data. TBPSKIN and TBPDXA were both highly correlated with TBPNAA: r = 0.90, P < 0.001, and r = 0.91, P < 0.001, respectively. Despite these significant correlations, agreement analyses showed wide variation of up to 33.3% of the mean for both methods.
Conclusions: Body protein in children with SQCP is significantly reduced for age and height. Skinfold anthropometry and DXA show wide variation in estimation of body protein compared with NAA in this group of children.
Key Words: Cerebral palsy children anthropometry total body nitrogen body protein neutron activation analysis dual-energy X-ray absorptiometry lean tissue mass
| INTRODUCTION |
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Body protein content can be assessed by the measurement of total body nitrogen (TBN) with prompt gamma neutron activation analysis (NAA). The advantage of NAA over other techniques for estimating protein is that it is a direct measure of body nitrogen that is not affected by other factors such as hydration status (14). Indirect methods for estimation of protein involve measurement of fat-free mass (FFM) or lean tissue mass (LTM) from skinfold-thickness anthropometry and dual-energy X-ray absorptiometry (DXA).
Skinfold-thickness anthropometry divides body weight into 2 compartments, fat mass and FFM, whereas DXA provides a three-compartment model of body composition that consists of fat, bone-free LTM, and bone mineral content (BMC). Skinfold-thickness anthropometry is a relatively simple, inexpensive, and portable technique that can be easily performed by trained technicians in the clinic and community setting where sophisticated technology such as direct body protein measurement by NAA and DXA may not be readily available. DXA is becoming more widely available in hospital settings and is therefore an increasingly popular method of assessing body composition. DXA is ideal for children, because it is fast and involves minimal exposure to radiation. However, DXA and skinfold-thickness anthropometry do not directly measure body protein and rely on assumed hydration factors of LTM estimated from healthy children. If hydration is abnormal, this can lead to errors in the estimation of LTM (15). Furthermore, anthropometry and DXA have not been validated for estimating body protein in children with SQCP. Hence, the aims of the present study were 1) to directly measure and evaluate the body protein of children with SQCP by NAA, and 2) to determine whether skinfold-thickness anthropometry and DXA can predict body protein in these children.
| SUBJECTS AND METHODS |
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The control children were from existing DXA (n = 172) and TBN (n = 72) databases and were of similar age (4-19 y) to the children with SQCP (1618). The control children were not significantly different from reference data in height or weight as indicated by their SD scores and were therefore considered to be representative of the healthy population.
Protocol
During an outpatient visit to the hospital, the children had measurements of anthropometry, DXA, and TBN. The CHW Ethics Committee approved the study, and written informed consent was obtained from each parent or caregiver.
Anthropometry
Two trained observers performed all measurements by the use of standardized techniques (19). For the children with SQCP, knee height (±0.1 cm) was measured with an anthropometer (Holtain Ltd, Crosswell, United Kingdom) with the knee and ankle bent to a 90° angle. The distance from the heel to the anterior surface of the thigh over the femoral condyles was measured (20). The knee height measurement was used to estimate height on the basis of the equations of Stevenson 1995 (for children aged
12 y) and Chumlea et al 1994 (for children aged >12 y) (20, 21). For the control children, standing height without shoes was measured by using a Harpenden stadiometer (Holtain Ltd). Weight measurements (±0.1 kg) were made with electronic scales (A&D Mercury Pty Ltd, South Australia) with the children wearing minimal clothing. Because the children with SQCP were unable to stand on the scales, the caregiver stood on the scales while holding the child, and then the caregiver was weighed on his or her own, and the difference between the 2 weights was calculated to give the weight of the child. Weight and height measurements were converted to SD scores and were compared with the Centers for Disease Control and Prevention growth reference data (22). Percentage of ideal weight-for-height was calculated for the children with SQCP by first estimating the childs height-age (the age at which the childs estimated height would be on the 50th percentile) and then dividing the childs actual weight by the weight corresponding to the 50th percentile for the childs height-age. The result was then multiplied by 100 to give a percentage.
Skinfold thicknesses were measured in duplicate on the right side of the body at 4 sites (triceps, biceps, subscapular, and suprailiac) with a Harpenden caliper (British Indicators Ltd, St Albans, Hertfordshire, United Kingdom). An estimate of percentage body fat was calculated from the skinfold thicknesses by using the equations of Brook (23) for prepubertal children and those of Durnin and Rahaman (24) for pubertal children. Fat-free mass (FFMSKIN) was derived by subtracting FM from body mass. Midupper arm circumference was measured with a flexible steel tape at the midpoint between the acromion process and the head of the radius on the right arm. An estimate of upper arm muscle area was derived from the midupper arm circumference and triceps skinfold-thickness measurement. The upper arm muscle area was converted to a SD score derived from the Frisancho reference data (25).
Knee height was not measured in the first 17 subjects enrolled in the study. Therefore, data on height, height SD scores, and percentage of ideal weight-for-height are unavailable for 8 girls and 9 boys with SQCP.
Total body protein from neutron activation analysis
Total body protein was measured by the method of prompt gamma NAA as previously described by this unit (14, 16). The basis of this technique is that the subject is bilaterally irradiated with neutrons from 2 252Cf sources while lying supine. 14N is converted to 15N with the emission of a 10.8-MeV gamma ray that is specific for nitrogen. TBN is calculated by measuring the integral under the nitrogen peak centered on the 10.8 MeV of the gamma ray spectrum. By measuring TBN, total body protein (TBPNAA) can then be determined by using the following relation: mass of protein = 6.25 x mass of nitrogen (14, 17). The total exposure time is
15 min and the effective dose equivalent delivered during a scan is <0.15 mSv (quality factor = 20 for fast neutrons). The technique has a precision and accuracy of 1.45.4% and 97101.5%, respectively, in child-sized phantoms (14). The TBPNAA of the children with SQCP was compared with the TBPNAA predicted from the control data for age, height, and weight.
Eight (1 girl, 7 boys) children with SQCP were excluded from the analysis of TBPNAA predicted for age, height, and weight because their ages fell outside the range for which the predictive equations were developed from control data (girls, 5.014.4 y; boys, 4.015.9 y). There were no significant differences between girls and boys in either the SQCP group or the control group, and therefore the data were analyzed with the sexes combined.
Lean tissue mass from DXA
Total-body LTM was assessed by DXA (LTMDXA). All DXA measurements were performed and analyzed by trained staff in the Department of Nuclear Medicine at CHW with a commercial X-ray bone densitometer (DPX Lunar Radiation Corp, Madison, WI). Whole-body scans were performed by using the recommended scan modes for weight. Software version 4.7 was used to determine body-composition parameters (Lunar Radiation Corp). The total scan time was
10 min with a total radiation dose of 0.2 µSv. Most of the children with SQCP were lightly sedated with 0.35 mg midazolam/kg given orally 30 min before the scan to reduce movement during the test. The precision of the technique to measure LTM in children as assessed at CHW is 0.82% (18).
The LTMDXA results were significantly different between the sexes for both the children with SQCP and the control children; therefore, the data for girls and boys were analyzed separately. Three children with SQCP (1 girl, 2 boys) had metallic implants and were removed from the analysis.
Total body protein from DXA and anthropometry
Total body protein was calculated from the DXA measurement (TBPDXA) and also from skinfold-thickness anthropometry (TBPSKIN) according to the method of Fuller et al (12) as follows:
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Statistics
The data were analyzed by using SPSS (version 11.5.1; SPSS Inc, Chicago, IL). A two-factor analysis of variance test was used to investigate differences in body composition between the SQCP and control subjects by using sex and group (SQCP or control) as fixed factors. Two-tailed one-sample t tests were used to compare mean differences between subjects and reference data. Pearsons product-moment correlations (r) were calculated to investigate relations between anthropometric measures, LTMDXA, and TBPNAA. A paired t test was used to test for differences between TBP measured by NAA, DXA, and skinfold-thickness anthropometry. Bland and Altman plots were used to test the agreement between the methods of measuring body protein (NAA versus DXA, and NAA versus skinfold-thickness anthropometry) in both children with SQCP and control children (28). A univariate analysis of covariance (ANCOVA) was used to statistically analyze and compare the Bland and Altman plots with "difference between methods" as the dependent variable, "average of methods" as the covariate, and group (SQCP or control) as fixed factors. The significance level was set at P < 0.05.
| RESULTS |
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Height, weight, and FFMSKIN were significantly positively correlated with age in both the children with SQCP and the control group (data not shown). However, height SD scores, weight SD scores, upper arm muscle area SD scores, and subscapular skinfold-thickness SD scores in the children with SQCP were all significantly negatively correlated with age (data not shown).
Total body protein from neutron activation analysis
Fifty-three (21 girls, 32 boys) children with SQCP underwent measurements of TBPNAA. The mean TBPNAA of the children with SQCP was 3.0 ± 1.2 kg, whereas that of the controls was 6.4 ± 2.9 kg (43 girls, 29 boys). The TBPNAA results for the children with SQCP, expressed as a percentage of predicted values from control data for age, height, and weight, respectively, were significantly low: 56.1 ± 17.3% (P < 0.001; n = 45), 81.5 ± 15.7% (P < 0.001; n = 34), and 83.9 ± 17.2% (P < 0.001; n = 45).
TBPNAA was significantly positively correlated with age in both the children with SQCP (r = 0.74, P < 0.001) and the control group (r = 0.89, P < 0.001). However, the percentage TBPNAA predicted for age from the control data was significantly negatively correlated with age (r = 0.58, P < 0.001; n = 45) in the children with SQCP (Figure 1
). In the children with SQCP, TBPNAA was highly correlated with FFMSKIN (r = 0.86, P < 0.001), weight (r = 0.80, P < 0.001), height (r = 0.75, P < 0.001), upper arm muscle area (r = 0.69, P < 0.001), and midupper arm circumference (r = 0.56, P < 0.001). There were no significant correlations between TBPNAA and measures of body fatness (data not shown).
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Total body protein from DXA and skinfold-thickness anthropometry
Complete data sets for measures of NAA, DXA, and skinfold-thickness anthropometry were available for 52 (21 girls, 31 boys) children with SQCP and 28 (17 girls, 11 boys) controls. There were no significant group-by-sex interactions. Mean TBPDXA and TBPNAA did not differ significantly in the children with SQCP (3.1 ± 1.2 kg and 3.0 ± 1.2 kg, respectively; P = 0.12), but did differ significantly in the control group (6.2 ± 3.1 and 7.2 ± 3.6 kg, respectively; P < 0.001). Mean TBPSKIN was significantly different from TBPNAA in the children with SQCP (3.4 ± 1.1 and 3.0 ± 1.2 kg, respectively; P < 0.001) and in the controls (6.1 ± 2.8 and 7.2 ± 3.6 kg, respectively; P < 0.001). There was a strong positive correlation between TBPNAA and TBPDXA, and also between TBPNAA and TBPSKIN, in both the children with SQCP (r = 0.91, P < 0.001, and r = 0.90, P < 0.001, respectively) and the control group (r = 0.99, P < 0.001, and r = 0.99, P < 0.001, respectively).
Bland and Altman plots were constructed to test agreement between TBPNAA and TBPDXA and TBPNAA and TBPSKIN (Figure 2
and Figure 3
). For the children with SQCP, there was a mean difference between TBPDXA and TBPNAA of 0.1 ± 0.5 kg. As shown in Figure 2A
, the 95% range of agreement was 0.9 to 1.1 kg (2 SDs to +2 SDs). For the average child in this study, this equated to a variation of 1.0 kg, or 33.3%. Conversely, for the control children, TBPDXA underestimated body protein by a mean of 1.0 ± 0.6 kg with a variation of ±15.4%. Statistical analysis (univariate ANCOVA) showed that the pattern of the Bland and Altman plots was significantly different between the children with SQCP and the controls (P = 0.02).
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| DISCUSSION |
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The low TBP for height and age in our study could be a result of reduced muscle mass due to malnutrition coupled with muscle atrophy from the underlying neurologic impairment. The relative contribution of either malnutrition or neurologic impairment could not be determined in this study. A longitudinal, prospective nutritional rehabilitation study is required to resolve this issue. A few studies, mostly retrospective studies with small numbers, have shown weight gain and an increase in fat stores after gastrostomy insertion in children with cerebral palsy (2934). However, none of these studies measured body protein.
The children with SQCP in the present study had major deficits in height and weight. However, their percentage of ideal weight-for-height was not significantly different from 100%, which implies that although they were stunted they were not wasted. Yet, the more detailed nutritional assessment including skinfold thicknesses and TBP found that they had depletions of both body fat and protein compartments for age and height that were consistent with wasting. These findings suggest that the commonly used Waterlow classification system of weight-for-height as an indicator of malnutrition with wasting is inappropriate for this group of children (35). Why has this apparent incongruity occurred? One possible explanation is that the use of knee height to estimate height could be inaccurate; this method has not been cross-validated in children with SQCP (20). It is also possible that malnourished children with SQCP have abnormally high TBW, which has been reported in malnutrition (36). Increased TBW could result in falsely elevated body weight and lean body mass (which would therefore result in increased body protein when calculated from DXA and skinfold-thickness anthropometry) but would not affect the measurement of TBP by NAA.
Others have measured TBW in children with cerebral palsy by using isotope-dilution methods and have reported various results (9, 11, 37). One study found a higher TBW (37), another found a lower TBW (11), and another found that TBW did not differ significantly from that in the control population (9). However, each of these studies had small numbers of subjects with varying severities of cerebral palsy and who were relatively well-nourished with equivalent or higher percentages of body fat compared with the healthy controls.
The second aim of the present study was to assess the validity of anthropometry and DXA to predict body protein in children with SQCP. Significant correlations were found between anthropometric measures and TBPNAA and between DXA and TBPNAA. Despite these significant correlations, however, agreement between the methods showed that both skinfold-thickness anthropometry and DXA produced wide variation in the estimation of TBP in children with SQCP.
The method used in the present study to estimate body protein from anthropometry and DXA was based on 2 assumptions. First, that the hydration of the LTM was estimated; and second, that the ratio of nonosseous mineral to bone mineral was 0.18:0.82, such that the mass of nonosseous mineral was 0.23 x BMC. The applicability of these assumptions to our population is questionable because it is unknown whether malnourished children with SQCP have normal body water, and, furthermore, most children with SQCP are osteopenic (1). Studies including measures of both TBW and TBP are required in children with SQCP to determine specific hydration factors to use with indirect measures of body protein.
In conclusion, the body protein of children with SQCP is significantly reduced for both age and height. A longitudinal prospective nutritional rehabilitation study is required to determine whether body protein can be improved with weight gain. In addition, the significant correlations between TBPNAA and anthropometric measures and DXA found in this study suggest that these measures can be used as indicators of body protein in children with SQCP. However, agreement analysis showed wide variation in some individuals, which indicates that the use of skinfold-thickness anthropometry and DXA to derive body protein should be treated with caution.
| ACKNOWLEDGMENTS |
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FEA coordinated the study as part of her PhD and was involved in the preparation of the study, ethics submissions, organization of subject testing, collection and analysis of data, and writing of the manuscript. SLC recruited, tested, and analyzed the data for the first 17 subjects enrolled in the study as part of her Master of Nutrition and Dietetics degree. JRA, KJG, MAG, HS, and EVO were involved in the conception of the study, ethics submissions, subject recruitment and testing, analysis of the data, and writing of the manuscript. KJG was involved in the conception and development of the NAA technique at CHW and holds the current radiation license for this instrument. JRA and MAG performed the TBN measurements. JNB and RBH-G were responsible for the collection and development of the DXA control database, the DXA subject testing and data analysis, and review of the manuscript. None of the authors had a financial conflict of interest.
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