Body Potassium Measurements Research Articles

Long-term effect of childhood liver transplantation on body cell mass.

Malnutrition is common in end-stage liver disease, but a correction after transplantation is expected. Body cell mass (BCM) assessment using total body potassium (TBK) measurements is considered the gold standard for assessing nutritional status. The aim of this study was to examine the BCM and, therefore, nutritional status of long-term survivors after childhood liver transplantation. This was a longitudinal nested cohort study of patients undergoing transplantation at <18 years of age and surviving >3 years with ongoing review at our center. TBK measurements were obtained before transplantation and during long-term follow-up. BCM was calculated from TBK and was adjusted for the height raised to power p, which depended on sex (BCM/height(p)). The effects of the age at transplant, linear growth impairment, a diagnosis of biliary atresia, and steroid use were assessed. Thirty-two patients (20 males) participated; 59% had biliary atresia. The median age at transplant was 2.11 years (range = 0.38-10.92 years). Posttransplant testing was performed at a median of 7.23 years (range = 3.28-14.99 years) when they were 10.12 years old (range = 4.56-20.77 years). This cohort attained mean z scores for height, weight, and body mass index of -0.41 ± 1.36, -0.26 ± 1.14, and 0.04 ± 0.99, respectively. BCM/height(p) was reduced before transplantation but was further reduced after transplantation (P < 0.001) despite the normalization of height and weight. Weight recovery, therefore, likely came from increased fat mass and not BCM. Linear growth impairment was associated with a greater reduction in posttransplant BCM/height(p) (P = 0.02). In multivariate analyses, only an older age at transplant predicted reduced posttransplant BCM/height(p) (P = 0.02). The age at transplant, sex, steroid use, and underlying diagnosis did not predict changes in BCM/height(p) after transplantation. In conclusion, weight recovery in long-term survivors of childhood liver transplantation is likely due to increased fat mass because BCM remains reduced. Nutritional compromise persists in long-term survivors of childhood liver transplantation.

Is Weight Normalization After Childhood Liver Transplant Fat?

Introduction: Growth normalization after liver transplant is a desirable long term outcome in children with chronic liver disease. Weight recovery is usually rapid and often normalizes within a year although height recovery can be prolonged. It is unknown whether this recovery occurs as a result of increased body cell mass. Aims: To assess whether there was improvement in the body cell mass of long term survivors after liver transplant in childhood. Methods: Total body potassium (TBK) measurements to estimate metabolically active body cell mass were performed with a shadow shield whole body counter over a scanning bed. Body cell mass (BCM) was calculated from TBK measurements [BCM (kg) = (TBK*9.18)/39.1], andadjusted for height and gender (males=BCM/Ht3; females=BCM/Ht2.5). Measurements were performed before and after liver transplant. Paired t-tests were used to analyse the data.Results: Pre and post transplant measurements were available in 20 patients. There was no difference in the age of transplant or timing of TBK measurement after transplant between the genders. Body cell mass for height was significantly reduced after transplant in long term survivors (t= 3.83, p=0.00). When analysed according to gender, this was significant for boys (t=6.04, p=0.00)but not girls. Summary/Conclusions: Body cell mass was significantly reduced in long term survivors after liver transplant, especially boys. This implies that fat accumulation is the main reason for weight recovery, which may subsequently predisposethese children to obesity.

A high-sensitivity multidetector whole-body counter

A prototype multidetector shadow-shield whole-body counter is presented to be used as a monitor of internal contamination and as a tool in clinical research. The counter is equipped with 16 NaI(Tl) detectors located in the central region of a shielded tunnel surrounding the subject to be measured. The accuracy of the counting efficacy predictions was tested in a group of adults with various body shapes and sizes using X-ray absorptiometry. The precision of the total body potassium measurements allows the use of the counter in clinical follow-up studies.

A new method of body habitus correction for total body potassium measurements

This paper describes an accurate and time-efficient method for the determination of total body potassium via a combination of measurements in the Birmingham whole body counter and the use of the Monte Carlo n-particle (MCNP) simulation code. In developing this method, MCNP has also been used to derive values for some components of the total measurement uncertainty which are difficult to quantify experimentally. A method is proposed for MCNP-assessed body habitus corrections based on a simple generic anthropomorphic model, scaled for individual height and weight. The use of this model increases patient comfort by reducing the need for comprehensive anthropomorphic measurements. The analysis shows that the total uncertainty in potassium weight determination by this whole body counting methodology for water-filled phantoms with a known amount of potassium is 2.7% (SD). The uncertainty in the method of body habitus correction (applicable also to phantom-based methods) is 1.5% (SD). It is concluded that this new strategy provides a sufficiently accurate model for routine clinical use.

Novel Approach: Measurement of Partial Body Potassium in the Arm in Children

We introduce a novel measurement of partial body potassium in the arm (PBKArm) using a shielded, four NaI gamma ray detector system that surrounds the arm. The K measurement in the arm is directly related to the skeletal muscle (SM) mass since the amount of K in the skin and bone contributing to the total counts is small. Two cohorts (males and females) consisting of 26 pediatric subjects (7 to 19 yrs) and 50 adults (19 to 87 yrs) were measured for PBKArm, total body potassium (TBK), and lean soft tissue (LST) by DXA in the dominant arm. PBKArm in children shows a systematic increase with age although rate of increase with age differed between males and females, 0.3 g/yr and 0.06 g/yr, respectively. These increases are directly attributable to the increase in the arm SM. TBK, which obscures the arm SM, also showed a similar increase with age, although sex difference was less pronounced. In adults, PBKArm showed a decline with increasing age at the rate of 0.12 g/yr for both sexes, similar to the trends observed with TBK. In addition, PBKArm values were systematically higher for males than for females, which is due to the higher SM content in males than in females. In children, PBKArm was also strongly correlated with DXA arm LST for males (r2 = 0.90, p<9E-8) and females (r2 = 0.65, p<0.003). Whereas in adults, lower correlations were observed (male: r2 = 0.41, p<0.007, females: r2 = 0.08, p<0.1). In summary, we demonstrate the feasibility of measuring PBKArm in children, thus providing a direct measure of the SM compartment. PBKArm constitutes a new tool to the fundamental body composition measures. Support: NIH Grant PO1-DK 42618.

P0231 ANTHROPOMETRY AND ACTIVE CELL MASS IN LONG-TERM SURVIVORS OF PEDIATRIC LIVER TRANSPLANTATION

Introduction: Pre-operative nutrition is known to significantly affect morbidity and outcome in pediatric liver transplant patients. Our group and others have demonstrated gains in height and weight, both pre and post transplant1. While pre-transplant gains are almost certainly a result of nutritional rehabilitation, the nature and etiology of these gains following transplantation remains unclear Methods: Patients surviving >5years after initial liver transplantation in childhood were eligible for this study. Total body potassium measurement was used to assess the metabolically active lean cell mass (MALCM). Serial Z scores for both height and weight were calculated at listing for transplant, immediately before transplant, 12months, 60months, and up to 120months post-transplant. MALCM was adjusted for height using log-log regression. Z score of zero was considered to be normal (range > −1 to <1), with scores <−1 considered suboptimal. Results: 14 eligible patients, with mean age 24months at time of transplant (range 5–80months), consented to take part in this study. Z scores at their most recent review were suboptimal for height and weight in 5/14 and 2/14 patients respectively. 3/5 patients with low height Z scores had Alagille’s syndrome. Adjusted MALCM scores, which were all within control reference values, were not significantly different before and after transplant. This suggests that nutrition and hence body composition was well managed and could be considered to be relatively independent of the underlying liver disease. Height Z scores showed persistent increment in all patients across the full period of analysis. These results imply that although pre-transplant nutrition is important, it does not appear to be a significant factor in the delayed linear growth observed in this study. Conclusion: Long-term survivors of pediatric liver transplant exhibit persistent catch up linear growth longer than other body measurements. Adjusted MALCM scores suggest that this catch up growth is affected by factors other than nutritional disorder. We speculate that these factors include post-transplantation medication, and/or specific growth factors including deficient levels of insulin growth factors, which we previously demonstrated in end stage liver disease in childhood2.

Assessment of nutritional status, body composition, and human immunodeficiency virus-associated morphologic changes.

Nutritional status should be assessed at regular intervals as part of management of human immunodeficiency virus (HIV) infection. The simplest approach to assessment is serial weight measurement. A comprehensive nutritional assessment includes (1) anthropometric measurements of body composition; (2) biochemical measurements of serum protein, micronutrients, and metabolic parameters; (3) clinical assessment of altered nutritional requirements and social or psychological issues that may preclude adequate intake; and (4) measurement of dietary intake. Techniques for measuring body composition of fat and lean body mass include anthropometry and bioelectric impedance analysis. Other techniques, including dual X-ray absorptiometry (DXA), hydrodensitometry, total body potassium measurement, and cross-sectional computed tomography or magnetic resonance imaging are available in research centers. Anthropometry, including waist-hip ratios, regional DXA, and cross-sectional imaging, is best for detecting morphologic changes associated with fat redistribution syndrome. Nutritional assessment and intervention in children with HIV can help to prevent stunted growth and development.

Body composition and muscle constituents during weight loss: studies in obese patients following gastroplasty.

The influence of rapid and prolonged weight loss on body composition and muscle constituents in the obese patient is not well known. There are serious complications related to rapid and prolonged weight loss. It is of general interest to increase the understanding of the mechanisms and consequences of significant weight loss in man. In 40 obese patients, the body composition and muscle constituents were studied before and during 1 year of weight loss following gastroplasty. The study was undertaken in two groups (A and B) of obese patients, comprising 32 women and eight men, body weight 82-175 kg and aged 24-49 years. Mean BMI in group A and B was 45 (W/H(2)) and 43 (W/H(2)) respectively. Body composition was assessed by total body potassium measurements and muscle constituents were determined by analyses of muscle specimens obtained percutaneously. The preoperative body composition was found to be equal parts of lean body mass and body fat. Preoperatively, muscle constituents revealed a higher protein content per cell and a lower potassium concentration related to fat-free solids. The loss of 18-28% body fat and lean body mass occurred in equal proportions during the first 3 postoperative months of rapid weight loss, followed by a continuous decrease of body fat but not of lean body mass. The concentrations of proteins and potassium per muscle cell revealed a reduction during the period of rapid weight loss. The RNA/DNA ratio 1 year after surgery was still reduced, indicating a low protein synthesis rate. Preoperatively mean body fat accounted for 50% of the body weight in obese patients. Following weight loss, body fat, lean body mass and concentrations of proteins were reduced compared to preoperative values. After the period of rapid weight loss, with reduction of lean body mass and body fat in parallel, a progressive reduction of body fat was observed whereas the lean body mass did not decrease further. Protein synthesis rate was still low 12 months after surgery.

Comparison of total body chlorine, potassium, and water measurements in children with cystic fibrosis

Symptoms of cystic fibrosis (CF) may limit the utility of total body chlorine (TBCl) and total body potassium (TBK) measurements for assessing body fluid compartments of children. This study assessed relations among independent measurements of TBCl, TBK, and total body water (TBW) in children with CF. We compared cross-sectional measurements of TBCl by in vivo neutron activation analysis, TBK by whole-body counting of (40)K, TBW by D(2)O dilution [TBW(D(2)O)], and TBW from TBCl and TBK [TBW(Cl + K)] in 19 prepubertal children (13 boys) aged 7.6-12.5 y who had mild symptoms of CF. Body-composition measurements were compared with data from previous studies of healthy children. Subjects with CF had deficits in TBCl, TBK, TBW, and body weight compared with control reference data (P < 0.05). The ratios (TBCl + TBK)/TBW and TBCl/TBK were not significantly different from control reference values, and plasma chlorine and potassium concentrations were within control reference ranges. The sum of TBCl and TBK correlated with TBW(D(2)O) (r(2) = 0.79, P < 0.001), and TBW(Cl + K) correlated with TBW(D(2)O) (r(2) = 0.78, P < 0.001). TBW(Cl + K) was similar to TBW(D(2)O) (mean +/- SEM: 19.0 +/- 0.5 compared with 19.4 +/- 0.5 L; NS). Prepubertal children with mild symptoms of CF can develop deficits in TBCl, TBK, and TBW that reflect chronic energy malnutrition. Mild symptoms of CF do not appear to affect normal relations among TBCl, TBK, and TBW. Measurements of TBCl and TBK may be used to assess body fluid compartments in these patients.

Determination of skeletal muscle and fat-free mass by nuclear and dual-energy X-ray absorptiometry methods in men and women aged 51–84 y

Skeletal muscle mass (SMM) and fat-free mass (FFM) are important variables in nutritional studies. Accurate techniques for measuring these variables have not been thoroughly validated in elderly subjects. The objectives of this study were to 1) compare SMM values derived from dual-energy X-ray absorptiometry (DXA) with those calculated by a nuclear method from total body potassium (TBK) and total body nitrogen (TBN) measurement (both: KN) in older subjects, and 2) assess the accuracy of FFM measurement by DXA in these subjects. TBK, TBN, DXA (model XR36; Norland, Fort Atkinson, WI), bioimpedance, and anthropometric measurements were performed on healthy women (n = 50) and men (n = 25) aged 51-84 y. Mean SMM by KN was not significantly different from SMM by DXA in either sex. SMM by KN predicted SMM by DXA with an SEE of 2.1 kg (r = 0.95, P < 0.0001 for women and men together). In the men, FFM by DXA agreed well with FFM estimated by TBK, skinfold thicknesses, bioimpedance analysis, and a multicompartment model. In women, FFM by DXA was 4-5 kg less than that by the other methods (P < 0.01). Truncal fat was related to intermethod FFM differences (r = 0.58, P < 0.0001). These data indicate that 1) either the nuclear or the DXA method can be applied to estimate SMM in healthy older subjects, and 2) the Norland DXA instrument significantly underestimates FFM in older women, in part, because of the influence of truncal adiposity.

The Effects of 10 Years of Recombinant Human Growth Hormone (GH) in Adult GH-Deficient Patients

The long term effects of GH replacement in adult GH-deficient (GHD) patients have not yet been clarified. We studied 21 GHD adults who originally took part in a randomized, double blind, placebo-controlled trial of GH treatment in 1987. After completion of that trial, 10 patients received continuous GH replacement for the subsequent 10 yr, whereas 11 did not. A group of 11 age- and sex-matched normal controls were also studied in 1987 and 1997. Lean body mass, as assessed by total body potassium measurement and computed tomography scanning of the dominant thigh, increased in the GH-treated group (P < 0.01 for both) only (P < 0.05 between groups for total body potassium). Low density lipoprotein cholesterol decreased in the GH-treated group (P < 0.05) only. Carotid intima media thickness was significantly greater (P < 0.05) in the untreated group than in the GH-treated group. Assessment of psychological well-being using the Nottingham Health Profile revealed improvement in overall score, energy levels, and emotional reaction in the GH-treated group compared with those in the untreated group (P < 0.02). In conclusion, GH treatment for 10 yr in GHD adults resulted in increased lean body and muscle mass, a less atherogenic lipid profile, reduced carotid intima media thickness, and improved psychological well-being.

NUTRITIONAL STATUS AND BODY COMPOSITION IN INFANTS (&lt; 2 YEARS) WITH END STAGE LIVER DISEASE.

157 Aims: The aim of this study was to compare body composition and nutritional status in infants with end stage liver disease (ESLD) and normal infants, and to relate changes in body composition to alterations in energy expenditure. We studied infants less than two years of age using a four compartment model Methods: Eight infants (4 boys) awaiting liver transplantation and 10 normal infants took part in the study. Body fat% was assessed by four-site skinfold thickness, body cell mass by total body potassium (TBK) measurement, fat free extracellular solids (FFECS) by the method of Brook1, and extracellular water (ECW) by subtraction. Resting energy expenditure (REE) and respiratory quotient (RQ) were measured by indirect calorimetry. Results: Infants with ESLD had significantly increased BCM as a percentage of body weight (32.67±1.57% vs 29.11±1.02%, p<0.05) and REE per unit weight (0.35±0.02 vs 0.21±0.08 MJ/kg/day, p<0.01) compared with control infants. However, there was no difference in REE when it was expressed per unit body cell mass, nor was there any difference in height or weight for age z-scores, body fat% or ECW%. Weight for height z-score was directly correlated with body fat% in ESLD infants (r = 0.91, p<0.05). There was no direct relationship between fat stores and RQ, and no significant difference in RQ beween ESLD and control infants. Conclusions: Despite good nutritional status in this cohort, infants with ESLD still display significant alterations in body composition and increased energy requirements per kg body weight, as a greater proportion of weight is constituted by the metabolically active compartment. Close attention to clinical nutrition remains a management priority in children awaiting transplant.

Effect of potassium concentration in dialysate on total body potassium

Objective: To investigate the effects of the presence/absence of potassium in the dialysate on total body potassium content in stable hemodialysis patients. Design: Randomized selection. Setting: Outpatient chronic hemodialysis unit at the James A. Haley VA Hospital in Tampa, Florida. Patients: Six adult hemodialysis patients (mean age 48 ± 11.61 years, range 32–65 years) participated in this study. They were all males, four African-Americans and two Caucasians. Intervention: Subjects in a random order received dialysis using a dialysate containing no potassium for 3 months, followed by a 2 mEq/L of potassium for another 3 months. Total body potassium measurements and routine blood analysis were taken at baseline and after each three month period. All subjects received diet instruction pre and monthly on a 3 g/day potassium dietary restriction. Main outcome measured: There were no significant differences in the amount of total body potassium concentration between the two different dialysates containing 0 or 2 mEq/L potassium. Mean values of serum albumin were significantly higher when the subjects were dialyzed on a 0 potassium bath. Conclusion: The use of dialysate containing 0 or 2 mEq/L potassium concentration does not make a significant difference with regard to total body potassium concentration and maybe advantageous for the patients in terms of more freedom in nutritional intake as demonstrated by a significant increase in serum albumin when they were dialyzed on a 0 potassium bath.

Total body potassium and body fat: relevance to aging

Understanding the mechanisms that govern sarcopenia (depletion of muscle mass with age) may suggest ways to preserve lean tissue and functional capacity, and to maintain quality of life in the elderly. We investigated the body-composition changes in normal aging in a cross-sectional study of 188 healthy volunteers aged 20-89 y, which examines the differences in body cell mass and fat as a function of age. In aging, the assumptions of indirect body-composition-measurement techniques, such as the "constant" hydration coefficient of lean body mass or the "constant" density of fat-free mass, may not hold. Therefore, we selected body-composition-measurement techniques that are not sensitive to assumptions about the composition of lean tissue. Cellular mass, lean body mass, and fat were assessed "directly" by total body potassium (TBK) measurements and neutron inelastic scattering. Our results show that TBK content declines at a rate of 7.20 +/- 1.00 mg K.kg body wt-1.y-1 for females (r = 0.601, P < or = 0.001) and 9.16 +/- 0.96 mg K.kg body wt-1.y-1 for males (r = 0.710, P < or = 0.001). Body fat measurements by neutron inelastic scattering have shown a significant increase of percentage body fat with age for female volunteers between the ages of 20 and 50 y and a continuous increase for male volunteers throughout adult life.

Use of Bioelectric Impedance Analysis and Dual-Energy X-Ray Absorptiometry for Monitoring the Nutrition Status of Dialysis Patients

Despite recent technological advances, inadequate nutrition has been clearly identified as a significant risk factor to survival of patients undergoing chronic maintenance dialysis therapy. Although body density by underwater weight and residual lung volume, total body water by isotope dilution, bone mineral content, and total body potassium measurements will provide a very accurate multicompartmental analysis of body composition, they are not applicable to routine clinical or field work. Because of availability and simplicity, in addition to anthropometry, bioelectrical impedance and dual energy X-ray absorptiometry have received the most attention from the renal community. Several studies have validated the use of total body water by BEI as a surrogate for isotope dilution methods in dialysis patients, whereas others have established an excellent correlation with the volume of distribution of urea as measured by urea kinetic volume. Bioimpedance analysis has been extensively validated in stable healthy populations for measurement of lean body mass. Results are similar to those obtained with hydrodensitometry and total body potassium. Several studies in dialysis patients have compared lean body mass measurements by BEI and DEXA. Although the number of patients studied is relatively small, there is a high degree of correlation and concordance between the two methods. Nevertheless, selective equations for specific patient populations may be required with both methodologies for individual clinical applications. Longitudinal follow-up of body composition using BEI and DEXA in dialysis patients is contingent on a stable hydration status and/or accurate estimation of extracellular volume status for appropriate corrections. Consistency of technique and standardization of BEI and DEXA equipment is essential for reproducibility of results. Equations used in calculations must be age, sex, race, body habitus, and population specific whenever possible. Multiple compartment models including BEI, DEXA and isotopic dilution provide the best current "gold standard" for body composition analysis. BEI methodology is a practical bedside tool for assessment of total body water, and provides more consistent and reproducible results than anthropometry.

Body composition derived from whole body counting of potassium in growth hormone-deficient adults: a possible low intracellular potassium concentration.

The validity of total body potassium (TBK) measurement in estimating fat mass and fat-free mass (FFM) in GHD adults was assessed by comparison with the reference technique of dual energy x-ray absorptiometry (DEXA). The TBK and FFM values determined by DEXA were used to calculate the potassium concentration per kg FFM in GH-deficient (GHD) adults and compared with standard values for normal subjects of 59.6 mmol for females and 66.4 mmol for males. There were considerable differences between predicted and measured TBK values for both males (3972 vs. 3577 mmol; P < 0.001) and females (2526 vs. 2277 mmol; P < 0.001). Similarly, the estimation of FFM and fat mass by TBK measurement was significantly inaccurate for both sexes compared to values determined by DEXA. These discrepancies may be accounted for by the lower calculated potassium concentrations compared with standard values for both males (56.2 vs. 66.4 mmol; P < 0.001) and females (53.1 vs. 59.6 mmol; P < 0.001). These observations suggest that caution should be exercised in the interpretation of TBK in GHD adults, and the reduced potassium concentrations would alleviate inaccuracies in the estimation of body composition. Secondly, the decreased intracellular potassium concentration of GHD adults may account for the decreased muscle strength and ease of fatigueability seen in GHD adults.

The Effects of Two Different Periods of Weight-Reduction on Selected Performance Parameters in Elite Lightweight Oarswomen

Six successful members of the British Women's Lightweight Rowing Team were assessed before and after two-month (1990) and four-month (1991) periods of weight-reduction controlled by reduced caloric intake, while engaged in their normal physical training. Fat free mass (FFM) was calculated from body weight (BW) by utilising total body potassium measurements. Maximal oxygen intake (VO2max), respiratory anaerobic threshold (Tvent), upper body anaerobic peak power (PP) and mean power (MP) outputs, and knee flexor (KF) and extensor (KE) isokinetic peak torques were among the physiological parameters assessed. No statistical differences were noted between the data obtained prior to the two weight-reduction periods, and both periods resulted in lower BW (p < 0.001) and FFM (p < 0.05); approximately 50% of the lost BW was FFM. At the end of the two-month weight-reduction period Tvent (p < 0.02) and KF (p < 0.02) decreased. In contrast, a similar BW loss during the four-month period was associated with higher VO2max (p < 0.01) and PP (p < 0.05) compared with values prior to weight reduction. Comparisons between the percentage changes pre to post BW loss showed that the longer weight-reduction period was associated with significantly improved VO2max (p < 0.01), Tvent (p < 0.005), PP (p < 0.05) and KF (p < 0.05). We conclude: a) the proportion (50%) of weight lost as FFM in the present elite rowers is higher than the suggested optimal figure of 22%, and b) compared to four months, 6-7% of BW loss over two months may adversely influence fitness-related parameters in international lightweight oarswomen.

Estimation of body cell mass using bioelectrical impedance in patients with chronic pancreatitis compared with total body potassium measurement

Estimation of body cell mass using bioelectrical impedance in patients with chronic pancreatitis compared with total body potassium measurement

Whole body potassium measurements independent of body size.

The measurement of total body potassium (TBK) has long been recognized as an accurate index of the body’s cell mass, that is, the metabolically active tissues of the body. Whole-body counters, therefore, have been used extensively to monitor TBK in a wide range of healthy and diseased populations. Thus, we undertook the task of designing a counter that could accurately measure body potassium in a range of body sizes from a very-low-birth-weight (VLBW) infant to an obese adult. A strong emphasis was placed on the in vivo measurement of TBK in healthy infants, children, and teenagers. We describe the different counting configurations and performance characteristics of the Children’s Nutrition Research Center (CNRC) whole-body counter.

Total body potassium in the infant

We have examined several whole body counting geometries for exclusive use in total body potassium (TBK) measurements in preterm infants. This paper describes two counters, one for very-low-birth-weight (VLBW) infants with body weights under 2000 g, and a second counter for larger preterm infants. Both systems use large-volume NaI(Tl) detectors (10.2 cm × 10.2 cm × 45.7 cm) and are operated in a low background room. The standard error for a TBK measurement decreases from 5% at 1 kg body weight to 2% at 4.5 kg weight.