Nutritional Methodology-Body Composition

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1 Nutritional Methodology-Body Composition An Evaluation of Dual-Energy X-Ray Absorptiometry and Underwater Weighing to Estimate Body Composition by Means of Carcass Analysis in Piglets 1 Per Elowsson,* 2 Anders H. Forslund, Hans Mallmin,** Ulla Feuk,* Ingemar Hansson and Johan Carlsten Departments of *Anesthesiology and Intensive Care, Nutrition and **Orthopedics, Uppsala University, University Hospital, S Uppsala, Sweden and Departments of Meat Sciences and Clinical Radiology, Swedish University of Agricultural Sciences, Ultuna S , Uppsala, Sweden ABSTRACT To evaluate the use of dual-energy X-ray absorptiometry (DXA) and underwater weighing (UWW) for body-composition measurements, the carcasses of eight piglets (12-wk old, kg in weight) were dissected into muscle, fat and bone. Thereafter, the components were homogenized and chemically analyzed for fat and bone mineral mass. Body components as measured by DXA correlated closely to the carcass analysis (r Å ). However, DXA still overestimated significantly the bone mineral mass, lean mass and total weight, and underestimated fat mass. The reproducibility of measurements, expressed as the CV for fat mass was 13.5%, whereas for total weight, lean mass and bone mineral mass, the CV was %. Fat mass was overestimated by UWW using the equations of Siri or Kraybill (r Å 0.77), but not by the equation of Lohman et al. (r Å 0.69). The difference between the estimation of fat by chemical analysis and estimations by DXA and UWW was significantly affected by the amount of water in lean mass and fat-free mass. J. Nutr. 128: , KEY WORDS: body composition pigs underwater weighing dual energy X-ray absorptiometry dissection The knowledge of body composition is of great clinical between fat, lean and bone mineral mass. The calculations are and scientific interest when studying metabolic diseases and based on several assumptions, including constant hydration of nutritional disorders. Various indirect and noninvasive meth- the lean mass (Roubenoff et al. 1993). It is possible for DXA ods for the estimation of body composition have been devel- to calculate only two compartments at a time, i.e., the fraction oped (Johansson et al. 1993). Two of these, underwater between fat and lean mass in soft tissue. If bone is present, weighing (UWW) 3 and dual-energy X-ray absorptiometry DXA calculates the fraction of bone mineral and soft tissue (DXA), have frequently been used as reference methods. (Jebb 1997). The fraction of fat-to-lean is extrapolated from UWW is based on a two-compartment model [fat and fat-free non-bone area. This means, for example, that mineral outside mass (FFM)]; DXA is based on a three-compartment model the bones and fat in the bone marrow is measured as soft (fat, lean and bone mineral mass). Underwater weighing mea- tissue. sures body density and calculates the percentage of fat through DXA has been evaluated against the chemical analysis of an equation that assumes that fat and FFM have constant whole carcasses that have been homogenized (Lander Svenddensities (Behnke 1959, Siri 1961). However, it has been sen et al. 1993). There are at least three flaws with this proceshown that the density of FFM is not constant because it is dure: 1) representative samples from the homogenate may be influenced by the amount of water (Forslund et al. 1996) and difficult to obtain (Ellis et al. 1994, Lander Svendsen et al. bone mineral (Martin and Drinkwater 1991) contained. To 1993); 2) the ash obtained from the analysis of homogenized our knowledge, none of the studies on FFM density and its carcasses contains both bone and non-bone mineral (Ellis et influence on body composition calculations used direct meth- al. 1994, Heymsfield et al. 1989a, Lander Svendsen et al. ods (i.e., dissection and chemical analysis). 1993); and 3) whole carcasses also include bone marrow fat. DXA measures the differences in the attenuation of X-rays It would be more accurate, therefore, when evaluating DXA, to do a dissection followed by a homogenization of the separate tissues and thereafter analyze the mineral mass in the skeleton 1 The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement and the fat mass in the soft tissue. in accordance with 18 USC section 1734 solely to indicate this fact. Various three- and four-compartment equations based on 2 To whom correspondence should be addressed. UWW, DXA and bioimpedance have been described (For- 3 Abbreviations used: BMD, bone mineral density; CD, carcass density; DXA, dual-energy X-ray absorptiometry; FFM, fat-free mass; UWW, underwater slund et al. 1996, Heymsfield et al. 1989b), but they have not weighing. yet been validated by dissection followed by chemical analysis /98 $ American Society for Nutritional Sciences. Manuscript received 23 September Initial review completed 5 January Revision accepted 27 April

2 1544 ELOWSSON ET AL. The aims of this study were to evaluate the accuracy of body distal portion of the radius and ulna and the radial carpal bone, and composition estimations by the indirect methods DXA and the hind foot was removed by sawing through the tibia and fibula UWW, compared with dissection followed by chemical analymuscle and fat for each carcass was pooled, ground and analyzed for near the proximal tip of the calcaneus. The total amount of dermis, sis of fat in soft tissue and bone mineral mass in the skeleton, the quantity of chemical fat. The fat content of each sample was and if possible to explain the differences in estimation of the determined once or twice according to NMKL No 131 (Nordic Comtwo indirect methods. mittee on Food Analysis 1989). The samples were treated with hydrochloric acid over a boiling water-bath. After cooling, ethanol, diethyl MATERIALS AND METHODS ether and light petroleum were successively added and mixed. After separation of the phases, the organic layer was withdrawn. The aque- Eight, 12-wk-old, Swedish Landrace 1 Yorkshire pigs (Medical ous layer was extracted twice with a diethyl ether/light petroleum Innovation AB, Almunge, Uppsala, Sweden) (4 females and 4 cas- mixture and the extracts combined. Fat content was determined after trated males, 26.3 { 3.3 kg, mean weight { SD) were used. The evaporating the organic solvents and drying (Nordic Committee on pigs were premedicated intramuscularly with ketamine 20 mg/kg Food Analysis 1989). and anesthetized intravenously with 500 mg of thiopentone. The The wet skeleton of the dissected halves of each carcass was burnt animals were killed by means of intravenous potassium, exsangui- to ashes in a furnace maintained at a temperature of 640 C for 120 nated and decapitated between the atlas and the skull. Through a min. Before weighing, the ash was cooled while covered. To calculate sternal split and an abdominal incision, the thoracic and abdominal the total body composition measurements of the right side were organs were removed. The mean carcass weight was 17.7 { 2.93 multiplied by two. kg. Use of experimental animals was approved by the ethical com- Calculations of body composition. General equation for the twomittee of Uppsala. The composition of carcasses was studied in the compartment model (fat and FFM). The equation is as follows: following order. Dual energy X-ray absorptiometry, DXA. The equipment used (DPX-L, Lunar, Madison, WI) utilizes a constant X-ray source at 78 kvp and K-edge filter (cerium) to achieve a congruent beam of %fat Å a CD 0 b (2) stable, dual-energy radiation with effective energies of 40 and 70 Carcass density (CD) is derived from UWW. The constants a and b kev. The detector system collects data from 120 pixels during each will vary depending on the assumed value for fat and FFM density traverse as the scanner proceeds rectilinearly over the scanned ob- (Forslund et al. 1996). Siri (1961) assumed a fat density of 0.9 g/cm ject. The software used (Pediatric 1.5 b, DPX-L, Lunar) is designed 3 and a FFM density of 1.1 g/cm 3 in adult humans, corresponding to a for calculation of pediatric total body composition; its pixel size of Å 4.95 and b Å 4.5. Kraybill et al. (1953) assumed a fat density of mm is optimal for body weights between 15 and 35 kg g/cm 3 and a FFM density of 1.1 g/cm 3 in pigs, corresponding Each carcass was placed in a prone position and the total body scan to a Å and b Å Lohman (1984) assumed a fat density of was performed in õ10 min; the investigated object was exposed to 0.9 g/cm 3 and a FFM density of g/cm 3 in prepubescent children, a total irradiation dose of Ç0.021 msv, which is about two to three corresponding to a Å 5.30 and b Å orders of magnitude lower than common radiological examinations Equations for the three-compartment model (bone mineral, fat and lean (Njeh et al. 1997). mass). Carcass volume (from UWW), and bone mineral mass (from The estimations of fat and lean mass are based on extrapolation DXA) are as follows: of the ratio of soft tissue attenuation of the two X-ray energies in non-bone containing pixels. The software performs calculations of the differential attenuations of the two photon energies and presents data for each carcass of percentage of fat, fat mass (g), lean mass (g), bone mineral mass (g), bone mineral density (BMD) in g/cm 2 and total weight. According to the manufacturer, a CV for human BMD of 0.5% can be expected during repeated measurements. To determine the reliability of DXA measurements, each pig carcass was scanned three times consecutively without repositioning. From these data, the CV for the different tissue types was calculated. Underwater weighing (UWW). The whole-body density was determined according to Archimedes principle. If one measures carcass mass in air and in water, the differences, corrected for the water density, represent the body volume. The carcass density (CD) can then be calculated as follows: CD Å W air W air 0 W water D water (1) where W air denotes carcass weight in air, W water the weight under water and D water the density of water corrected for the temperature when the measurements were performed. The carcasses were weighed first in air (KC 120-ID 1 Multirange; Mettler Instrument, Greifensee, Switzerland) and then in water (Precisa 8000D, Zurich, Switzerland) to the nearest kg. When submerged in water, the utmost care was taken to remove any air pockets in the abdominal or thoracic cavity. To calculate any gain in weight due to water absorption, the carcasses were weighed once again in air. The carcasses increased in weight by 0.86%. Dissection, chemical fat analyses and ashing of the skeleton. After UWW, the carcasses were split along the midline. The right side of each carcass was stored at 4 o C until the following morning and then dissected by meat scientists of the Swedish University of Agricultural Sciences. Carcasses were separated into dermis, lean meat, fat and bone. The forefoot was removed by a cut between the Fat (kg) bone mineral mass Bone mineral volume Å (3) bone mineral density CD 0 bone Å (carcass mass 0 bone mineral mass) (carcass volume 0 bone mineral volume) Fraction fat Å (4) (5) CD 0 bone Å fraction fat 1 (carcass mass 0 bone mineral mass) (6) The assumptions behind the three-compartment model and its equations have been described in detail previously (Forslund et al. 1996). Equations 3 6 were based on the assumption that FFM minus bone has a density of 1.06 g/cm 3 and fat mass has a density of g/cm 3. The bone mineral volume (Equation 3) was calculated using the density of 3.15 g/cm 3 (Weast 1983). CD minus bone (Equation 4) stands for the density of the carcass minus bone mineral (i.e., fat and lean mass). Equations for calculation of water content in lean mass. Body volume (from UWW) and ash mass (from dissection) are as follows: fat mass Fat volume Å (7) fat density Ash mass Ash volume Å (8) Ash density CD 0 (ash / fat) Å (carcass mass 0 ash mass 0 fat mass) (carcass volume 0 ash volume 0 fat volume) (9)

3 ESTIMATION OF BODY COMPOSITION 1545 TABLE 1 Individual body composition data on pig carcasses obtained from underwater weighing, dissection and chemical analysis of fat and bone mineral Lean Bone Pig Total Total FFM1 mass2 mineral Protein Water carcass weight density density density in FFM in FFM in FFM Number kg g/cm3 g/100g Mean {SD {2.93 {0.008 {0.009 {0.009 {0.2 {2.8 {2.8 1 FFM, fat-free mass. 2 Lean mass Å fat-free mass minus bone mineral mass obtained by chemical analysis equations using UWW plus DXA significantly overestimated % water(lean mass) Å CD 0 (ash / fat) (10) fat mass and underestimated lean mass. The different body components measured by DXA correlated closely to the results obtained by carcass analysis (Table CD 0 (ash / fat) (Equation 9) stands for the density of the carcass minus ash and fat (i.e., water and protein). The constants in Equation 3). However, the slope and intercept for the regression line (10) were calculated according to Equation (11). The weight fractions between bone mineral mass and ash mass were significantly of two compartments (i.e., water and protein) with different densities different. For the three-compartment model using UWW plus can be calculated from total density using a hyperbolic equation: DXA, the slope and intercept for the regression line between estimated fat mass and measured fat mass were not significantly a Fraction water Å different. UWW and the measured fat mass did not differ CD 0 (ash / fat) 0 c where significantly from carcass analysis. The correlation between DXA and UWW for fat mass varied from 0.4 to dw 1 dp a Å ; c Eighty-six to ninety-seven percent of the difference be- dp 0 dw Å dw (11) dp 0 dw tween the UWW and carcass estimates for fat mass was attributed to the hydration of the FFM, whereas the mineral fraction In this equation, dw is the density of the water compartment and dp contributed only 1% to the variation. For DXA, the hydration is the density of the second compartment, e.g., protein. We assumed fraction for the lean mass contributed to 72% of the difference. that CD minus fat minus bone mineral and water (i.e., protein plus This is also illustrated in Figures 1 and 2. FFM density was non-bone mineral plus glycogen) has a density of 1.39 g/cm 3 (Allen significantly affected by the percentage of water in FFM (r et al. 1959, Brozek et al. 1963) using water density at 37 C ( Å 0.99, P õ 0.001)(Fig. 3), but not by the bone mineral mass. g/cm 3 ). The difference between bone mineral mass and ash mass Statistical analysis. To compare the methods, a simple regression analysis was used for calculations of correlations, combined with a t was significantly affected by the measured amount of fat (r test of regression coefficients (intercept and slope). The methods were Å 0.87, P Å 0.005) (Fig. 4) and the total weight (r Å 0.97, also statistically compared using a paired two-tailed Student s t test. P õ 0.001) (Fig. 5). A P-value õ0.05 was considered significant. Coefficient of variance was used to analyze the reliability of DXA measurements (Winer 1971). All statistical calculations were done with SAS statistical software package, version 6 (SAS Institute, Cary, NC). RESULTS The individual body composition data obtained from dissection and UWW are shown in Table 1. Comparisons between the carcass analysis and the various indirect methods as well as the reliability of DXA measurements expressed as coefficient of variation (CV) are shown in Table 2. Fat mass was significantly overestimated when UWW data were used in Siri s (1961) and Kraybill s (1953) two-compartment equations. However, fat mass estimated by Lohman s equation (1984) did not significantly differ from the carcass analysis. DXA significantly overestimated bone mineral mass and lean mass, and significantly underestimated fat mass. Three-compartment DISCUSSION One way to evaluate whether DXA or UWW offers a reliable estimation of the body composition is to compare these indirect methods with a direct method such as dissection followed by analysis of chemical fat and bone mineral content. To compare our results with previous studies that have evaluated the accuracy of DXA, we studied pigs. Few studies have compared UWW to dissection in any animals. Our study was performed on decapitated pigs with abdominal and thoracic organs removed, in a prone position, whereas the DXA software was designed for total body measurements in intact children in a supine position. This may have affected the calculated parameters and account for the lower precision for fat estimates. One of the reasons for using a plain carcass (decapitated pigs without abdominal and thoracic organs) was to minimize various errors that could occur during UWW,

4 1546 ELOWSSON ET AL. TABLE 2 Data on body composition obtained by chemical analysis of pig carcasses compared with indirect methods as well as the reliability of dual-energy X-ray absorptiometry (DXA) measurements1 Two-compartment2 Three-compartment3 UWW / Component Carcass analysis UWW4 UWW5 UWW6 DXA7 DXA8 Weight, kg { { { 3.00* CV Fat mass, kg 2.30 { { 0.97* 2.63 { { 1.06* 2.0 { 0.86* 3.06 { 1.15 CV Lean mass, kg { { 2.23* { 2.31 CV Bone mineral mass, kg 0.51 { { 0.10* 0.55 { 0.1 CV Each value is the mean { SD, n Å 8. 2 Two-compartment, fat- and fat-free mass. 3 Three-compartment, fat-, lean- and bone mineral mass. 4 UWW, underwater weighing using the equation of Siri (1961). 5 UWW, underwater weighing using the equation of Lohman (1984). 6 UWW, underwater weighing using the equation of Kraybill (1953). 7 DXA, first of 3 measurements. 8 DXA / UWW, dual-energy X-ray absorptiometry plus underwater weighing. 9 CV, coefficient of variance of three consecutive DXA measurements. * Significantly different from carcass analysis, P õ 0.05 (Student s t test). including the presence of air in the respiratory tract and gas eral mass by DXA vs. ash mass; however, it is unlikely that in the intestine. We also did not analyze the fat content of the 8% overestimation by DXA is explained by this alone. the bone marrow, which would marginally affect the fat mass We consider that dissection combined with analysis of the measured in the carcass. With DXA, this would slightly increase mineral content of the skeleton and the fat content of the the underestimation; with UWW, it would decrease the soft tissues is a more accurate method than homogenization overestimation. The exclusion of the feet would have a minor followed by whole-body carcass analysis because of the diffi- effect on the difference between the estimation of bone min- culty in obtaining representative samples (Ellis et al. 1994, TABLE 3 Regression data for various indirect methods compared with chemical analysis of fat and bone mineral in pig carcasses1 Regression Regression Body component slope intercept r2 SEE2 Weight DXA (0.02) (0.37) Fat mass Three-compartment5 DXA 0.98 (0.15) (0.36) UWW / DXA (0.41) 0.79 (1.0) Two-compartment7 Equation of Siri (0.31) 1.53 (0.75) Equation of Lohman (0.33) 0.84 (0.79) Equation of Kraybill (0.34) 1.65 (0.82) Lean mass Three-compartment DXA 1.01 (0.04) 0.75 (0.54) UWW / DXA 0.98 (0.15) (2.30) Bone mineral mass DXA (0.05) (0.02) n Å 8. 2 SEE, i.e., standard deviation of the residual. 3 DXA, dual-energy X-ray absorptiometry, first of three measurements. 4 Standard deviation of regression. 5 Three-compartment, fat-, lean- and bone mineral mass. 6 DXA / UWW, dual-energy X-ray absorptiometry plus underwater weighing. 7 Two-compartment, fat- and fat-free mass. 8 Siri (1961). 9 Lohman (1984). 10 Kraybill (1953). 11 Slope or intercept significantly different from 1 or 0, respectively.

5 ESTIMATION OF BODY COMPOSITION 1547 FIGURE 1 Regression of differences between dual-energy X-ray absorptiometry (DXA) and carcass analysis of fat mass on percentage of water in lean mass in 8 pigs; (y Å x, r Å00.86, P õ 0.001). Lander Svendsen et al. 1993). Furthermore, the ash obtained by analysis of homogenized carcasses comes from both skeletal and non-skeletal mineral (Ellis et al. 1994, Heymsfield et al. 1989a). According to Heymsfield et al. (1989b) non-bone mineral is Ç13% of the total amount of mineral in the human body. DXA was developed primarily for bone densitometry. In addition to calculating bone mineral mass, DXA also makes it possible to determine two other compartments, fat mass and lean mass. DXA has several advantages over other methods of analyzing body composition; it is easy to perform, quick, noninvasive and enables analysis of the composition of the whole body or of isolated regions of interest. Different authors have evaluated the precision and accuracy of dual-photon ab- sorptiometry (DPA) and DXA in phantoms consisting of vari- ous mixtures of lard, ethanol-water mixtures, muscle and bone with encouraging results (Gotfredsen et al. 1986, Haarbo et al. 1991). FIGURE 3 Correlation between fat-free mass (FFM) density and percentage of water in FFM; (y Å x, r Å00.99, P õ 0.001). Although total carcass weight (/3%), bone mineral mass (/9%) and lean mass (/5%) were overestimated and fat was underestimated (013%), DXA correlated closely to the values from chemical analysis of the carcasses (r Å ). In a previous study, DXA was compared with chemically analyzed fat after homogenization (Ellis et al. 1994). The authors used similar DXA equipment, but software intended for adult body weight, and the pigs used in their study were heavier (35 95 kg) than ours (15 22 kg). As in this study, the correlations were high (r ú 0.97). In our study, 15 16% of the total wet skeleton (the skull and feet excluded) was ash. The ash weight has been previously estimated to be 32% of the wet weight of the tibia in 12- to 14-wk-old pigs (Combs et al. 1991) and 28% of the wet weight of the femur in pigs of market weight (Håkansson et al. 1989). Our lower estimations might be explained by the fact that our pigs were prepubertal and their total wet skeleton was not fully mineralized. To be able to estimate three compartments, bone mineral mass, fat mass and lean mass, one has to assume a fixed amount FIGURE 2 Regression of differences between underwater FIGURE 4 Regression of differences between dual-energy X-ray weighing (UWW) (Siri 1961) and carcass analysis of fat mass on percentage absorptiometry (DXA) and carcass analysis of bone mineral mass on of water in fat-free mass (FFM) in 8 pigs; (y Å015.2 / 0.21x, chemically analyzed fat in 8 pigs; (y Å / x, r Å 0.8, P r Å 0.95, P õ 0.001). õ 0.005).

6 1548 ELOWSSON ET AL. hydration in FFM was similar to the finding of Pintauro et al. (1996) in a study using pigs in the same weight range as ours. The carcasses examined were found to have FFM density g/cm 3 (mean g/cm 3 ). This range is quite large (Fig. 3) and 99% of the variation is explained by variations in the amount of water in FFM. This density is lower than the constant FFM density of g/cm 3 in the prepubescent child assumed in the equation of Lohman et al. (1984) and the 1.1 g/cm 3 assumed in the equations of Siri (1961) and Kraybill et al. (1953). Lohman et al. (1984) estimated the average amount of bone mineral and water in FFM in children to be 5.4 and 76.6%, respectively. Brozek et al. (1963) estimated the average amount of bone mineral and water in FFM in adults to be 6.8 and 73.8%, respectively. The low amount of bone mineral and the high amount of water in FFM found in this study account for the lower FFM density and the overestimation of the amount of fat by UWW. The reliability of the DXA measurements, expressed as CV varied from 0.74 to 1.90% of total weight, bone mineral mass FIGURE 5 Regression of differences between dual-energy X-ray and lean mass. However, the variability of fat mass was seven absorptiometry (DXA) and carcass analysis of bone mineral mass on times as high (13.5%). This is remarkably high, but others total carcass weight in 8 pigs; (y Å / x, r Å 0.97, P have reported four- to sixfold increases in variability when õ 0.001). estimating percentage of fat compared with bone mineral mass, reflecting the lower precision for fat of the DXA method of water in lean mass (Roubenoff et al. 1993). We found (Fig. (Haarbo et al. 1991). However, in one study of pigs with 1) that this assumption of a fixed quantity of water in lean weights between 35 and 90 kg using the DPX-L standard adult mass could cause an error in the estimated fat mass. We also software, a precision of % for all body composition found that the amount of fat and the total weight influenced variables including percentage of fat was reported (Lander the estimated bone mineral mass. This is in agreement with Svendsen et al. 1993). Laskey et al. (1992) who used a phantom. They concluded Body composition analysis based on a two-compartment that the estimated bone mineral mass was affected by the depth model (fat and FFM) and UWW has been assumed to be the and composition of the subject and that it will be least accurate gold standard. Recently, the development of three- and fourin obese subjects. compartment models has improved the accuracy of body com- The correlation between fat mass calculated by DXA and position estimations, and it has been shown that the assump- UWW in this study was r Å There have been tions required with two-compartment models are not always reports of correlations as high as (Heymsfield et al. valid (Forslund et al. 1996, Gotfredsen 1986). However, these 1989a, Van Loan and Mayclin 1992), but these studies were studies were performed on humans by using different indirect performed on adult humans. Possible explanations of the methods. In this study, we were able to show that the FFM higher correlation coefficient for DXA vs. UWW compared density is not constant among piglets and that the variation with dissection might be that with DXA we calculated three is explained mainly by differing amounts of water in FFM. compartments and used pediatric software, whereas with Furthermore, we have shown that these variations in FFM and UWW, we calculated two compartments and did not use a lean mass can influence the calculated amount of fat. The specific pediatric equation. three-compartment model using UWW and DXA together did When the two-compartment model was used with UWW, not improve the calculation of the fat mass; the main reason the fat mass was overestimated independently of the equation (93%) was the influence of variations in the water content of used. This overestimation could be explained by false assump- the lean mass on the lean mass density. tions in the equations involved, e.g., the assumption of a conand close correlation with dissection and fat analysis. However, The three-compartment model using DXA showed a good stant density within the fat-free compartment. As Clarys et al. (1984) stated, this requires the tissue composition of the there was a high CV for fat mass and the amount of water in compartment to be fixed between subjects. Low density of the lean mass influenced the estimated amount of fat. The accuracy fat free compartment due to high amount of water and/or low amount of bone mineral will also lead to erroneous results (Forslund et al. 1996). In piglets, the bone mineral content of the growing skeleton is lower compared with the skeleton may be improved by measuring the amount of water, thus creating a four-compartment model. ACKNOWLEDGMENTS of the adult pig (Combs et al. 1991), resulting in a lower FFM density. The proportion of mineral (or ash) in the FFM differs We thank Inger Winkler for secretarial help, Leif Hambraeus for notably in our pigs from that in humans. The FFM contains his guidance and constructive criticism and Neale Mushet for lan- Ç5% mineral in adult humans (Forslund et al. 1996); in our guage revision. study, however, the FFM of carcasses of the pigs contained only 3.3% minerals. According to Heymsfield et al. (1990), LITERATURE CITED the ash weight represents 55% of wet skeletal weight in adult Allen, T. H., Welch, B. E., Trujillo, T. T. & Roberts, J. E. (1959) Density, fat, humans, whereas in this study, in which the skull and feet water and solids in freshly isolated tissues. J. Appl. Physiol. 14: were excluded, the ash weight represented only 15 16% of Behnke, A. R. (1959) Comment on the determination of whole body density the wet skeletal weight. The FFM contained on average 78% and a resumé of body composition data. In: Technique for Measuring Body Composition. (Brozek, J., ed.), pp National Academy of Sciences, water in our piglets, which is higher than the 73% assumed National Research Council, Washington, DC. in adult humans (Pace and Rathbun 1945). However, the Brozek, J., Grande, F., Anderson, J. T. & Keys, A. (1963) Densitometric analy-

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