USE OF BIOELECTRICAL IMPEDANCE ANALYSIS TO ASSESS BODY COMPOSITION OF SEALS

Abstract: Bioelectrical impedance analysis (BIA) measures resistance and reactance of a current as it passes through an organism. The validity of using BIA as a tool to measure body water content, and hence body composition and condition, was tested on harp and ringed seals. The resistance and reactance readings from BIA were compared to estimates of total body water (TBW) determined via tritiated water dilution. The relationship between resistance and TBW (% of body mass) was linear after logarithmic transformation and the two variables were highly correlated. We describe the electrode configuration and placements which provide reliable results in these seals. Our findings indicate that BIA has considerable potential as an inexpensive, rapid, and reliable technique for estimating body composition of phocid seals.     

ESTIMATION OF TOTAL BODY WATER IN HARBOR SEALS: HOW USEFUL IS BIOELECTRICAL IMPEDANCE ANALYSIS?

We evaluated bioelectrical impedance analysis (BIA) as a means of rapidly and inexpensively estimating total body water (TBW) of harbor seals ( Phoca vitulina ). Deuterium oxide dilution was used to estimate TBW in 17 adult females and 16 of their pups between birth and late lactation. Isotope dilution was also used to determine TBW in 12 adult males early and 10 of these males late in the breeding season. At the same time, resistance ( Rs ) and reactance ( Xc ) measurements were taken using a tetrapolar, impedance plethysmograph (Model 101 A, RJL Systems). Seals were sedated with diazepam prior to taking BIA measurements. Within-day duplicate Rs measurements on pups and adults, taken 2-240 min apart, differed by an average of 3.0%± 1.4% ( n = 42, CV = 102%). Movement of the seal during BIA measurements caused variability in both Rs and Xc values. BIA measurements were generally poor predictors of TBW. Rs was significantly correlated with TBW in pups only ( Rs = 0.93, P = 0.001, n = 11). Bioelectrical conductor volume (length2/ Rs ) was significantly correlated with TBW only in adult females ( Rs = 0.63, P = 0.02, n = 14). We conclude that BIA is not a reliable method of estimating TBW in wild harbor seals.     

Serial Measurements of Body Composition in Obese Subjects During a Very-Low-Energy Diet (VLED) Comparing Bioelectrical Impedance with Hydrodensitometry

Bioelectrical impedance analysis (BIA) is a convenient, inexpensive, and noninvasive technique for measuring body composition. BIA has been strongly correlated with total body water (TBW) and also has been validated against hydrodensitometry (HD). The accuracy and clinical utility of BIA and HD during periods of substantial weight loss remain controversial. We measured body composition in moderately and severely obese patients serially using both methods during a very-low-energy diet (VLED). Mean initial weight in these patients was 116 (± 30) kg (range, 74–196 kg). Mean weight loss was 24 (± 13) kg with a decrease in fat mass (FM) by HD of 20 kg (p<0.001) and a decrease in fat-free mass (FFM) of 3.6 kg (p<0.05). Loss of FFM is best predicted by the rate (kg/wk) of weight loss (r² = 0.86, p<0.0001). FFM, as predicted from BIA equations, was highly correlated with FFM as estimated by HD during all testing sessions (r=0.92-0.98). Although highly correlated, BIA overestimated FFM relative to HD and this difference appeared to be more pronounced for taller patients with greater truncal obesity. Although the discrepancy was no greater during weight-loss treatment, the level of disagreement was considerable. Therefore, the two methods cannot be used interchangeably to monitor relative changes in body composition in patients with obesity during treatment with VLED. The discrepancy between BIA and HD may be caused by body mass distribution considerations and by perturbations in TBW which affect the hydration quotient for FFM (BIA) and/or which affect the density constants for FFM and FM (HD).     

MEASURING CHANGES IN JUVENILE GRAY SEAL BODY COMPOSITION

Determining body composition in gray seals ( Halichoerus grypus ) is important when studying their physiology and life history. In this study we investigated the predictability of total body fat (TBF) and protein (TBP) in postweaned gray seal pups from morphometric measurements, blubber thickness using ultrasonographs and bioelectrical impedance analysis (BIA). In postweaned pups, TBF (kg) could be estimated from girth measurements and sex ( n = 45, r ²= 0.878) using hydrogen isotope dilution methods as the reference. However, TBP could not be reliably estimated from morphometric data. TBF (kg) in yearlings was best predicted from mass ( n = 6, r ²= 0.776) and TBP (kg) from mass/length ( r ²= 0.949). Dorsal blubber thickness using B-mode ultrasound was also a significant predictor of TBF (kg) in postweaned pups ( r ²= 0.725) but BIA was not. Marked pups were recaptured during their first few months of life ( n = 48) and body composition changes investigated. Animals lost mass and TBF after leaving the breeding beach, largely during the first 5–6 mo of life. Postweaned pups were ∼40% TBF and ∼13% TBP whereas yearlings were ∼12% TBF and ∼20% TBP. Pups that survived beyond 6 mo of age then regained mass as protein. Morphometric measurements are a useful field indication of body condition when isotope dilution is impractical.     

RESPONSE OF ANTARCTIC FUR SEALS TO IMMOBILIZATION WITH KETAMINE, A KETAMINE-DIAZEPAM OR KETAMINE-XYLAZINE MIXTURE, AND ZOLETIL®

Adult Antarctic fur seals (Arctocephalus gazella) were immobilized with Zoletil^® ( n = 172), ketamine ( n = 30), ketamine mixed with diazepam ( n = 23) and with ketamine mixed with xylazine ( n = 45). Response to all drugs was highly variable. There was a relationship between dose rate and level of immobilization in females given Zoletil^®. Males were slightly more sensitive to Zoletil^® than females but this could have been due to the greater body mass and lower mass-specific metabolic rate of males. The dose required to achieve a level of immobilization declined with greater body mass for Zoletil^® and ketamine but not for ketamine-diatepam. Ketamine and ketamine-sedative mixtures commonly caused mild tremoring and occasionally caused convulsions. Neither reaction was seen with Zoletil^®. Mean doses were, Zoletil^® 1.5 mg/ kg, ketamine 6.9 mg/kg, ketamine-diazepam 6.3 mg/kg ketamine and 6.3 μg/kg diazepam, and ketamine-xylazine 7.3 mg/kg ketamine and 0.62 mg/ kg xylazine. Zoletil^® performs at least as well on Antarctic fur seals as ketamine but it may cause respiratory depression. The dose of ketamine required for Antarctic fur seals was greater than for most other species of seals.     

Variation in plasma leptin levels in response to fasting in Antarctic fur seals (Arctocephalus gazella)

Plasma leptin levels were determined in 8 lactating female and 20 pup Antarctic fur seals (Arctocephalus gazella) during fasting periods of normal duration. Plasma leptin levels ranged from 1.35 3.19 ng x ml(-1) in lactating females and 1.79-4.80 ng x ml(-1) in pups and were not positively correlated with body mass or condition. A negative trend, however, was observed between plasma leptin levels and body condition in lactating females upon their arrival at the colony following a foraging trip (beginning of fast). In accordance with findings in other species, plasma leptin levels dropped significantly (P < 0.02) in response to the 17-19% drop in body mass experienced by pups during fasting. In contrast, plasma leptin levels in lactating females increased during the first 24 h of fasting before decreasing throughout the remaining 48 h of the fast. This unexpected result could be due to the high level of energy expenditure by seals as they swim back to the colony (i.e. post-exercise response) or may be influenced by the intense suckling activity experienced by females during the onshore fasting periods. The results of this study support recent findings in other carnivore species which suggest the primary physiological role of leptin in these species may not necessarily be as a signal of the magnitude of body energy reserves.     

Temporal patterns of milk production in Antarctic fur seals (Arctocephalus gazella)

The timing of milk production in Antarctic fur seals was studied at Bird Island, South Georgia. Like all lactating otariid seals (Pinnipedia: Otariidae), Antarctic fur seal females alternate between short nursing periods ashore and regular foraging trips to sea. Females do not necessarily return to the colony with full mammae, which indicates that mammary volume capacity is unlikely to limit foraging trip duration. Upon arrival at the colony, milk fat (r²= 0.33, P < 0.04) and protein (r²= 0.60, P < 0.002) content were positively correlated to the time spent at sea. A similar trend was observed in the milk produced on land. The rate of milk energy production was much lower at sea (5.02 ± 0.05 MJ day^-1) than on land (23.66 ± 4.4 MJ^-1 day^-1). The rate of milk energy production during the foraging trip was negatively correlated to the time spent at sea (r²= 0.29, P < 0.05), whereas the rate of milk energy production on land was positively correlated (r²= 0.61, P < 0.001) to the duration of the preceding foraging trip. The total amount of milk energy delivered to the pup during each twoday nursing period was positively correlated (r²= 0.60, P < 0.002) to the duration of the previous foraging trip. The overall rate of milk energy delivery, however, was independent of foraging trip duration. This accords with previous observations that the growth rates of Antarctic fur seal pups are unaffected by maternal foraging trip duration patterns.     

Serum chemistry and antibodies against pathogens in antarctic fur seals, Weddell seals, crabeater seals, and Ross seals

Information on health parameters, such as antibody prevalences and serum chemistry that can reveal exposure to pathogens, disease, and abnormal physiologic conditions, is scarce for Antarctic seal species. Serum samples from Antarctic fur seals (Arctocephalus gazella, n=88) from Bouvet?ya (2000-2001 and 2001-2002), and from Weddell seals (Leptonychotes weddellii, n=20), Ross seals (Ommatophoca rossii, n=20), and crabeater seals (Lobodon carcinophagus, n=9) from the pack-ice off Queen Maud Land, Antarctica (2001) were analyzed for enzyme activity, and concentrations of protein, metabolites, minerals, and cortisol. Adult Antarctic fur seal males had elevated levels of total protein (range 64-99 g/l) compared to adult females and pups (range 52-79 g/l). Antarctic fur seals had higher enzyme activities of creatine kinase, lactate dehydrogenase, and amylase, compared to Weddell, Ross, and crabeater seals. Antibodies against Brucella spp. were detected in Weddell seals (37%), Ross seals (5%), and crabeater seals (11%), but not in Antarctic fur seals. Antibodies against phocine herpesvirus 1 were detected in all species examined (Antarctic fur seals, 58%; Weddell seals, 100%; Ross seals, 15%; and crabeater seals, 44%). No antibodies against Trichinella spp., Toxoplasma, or phocine distemper virus (PDV) were detected (Antarctic fur seals were not tested for PDV antibodies). Antarctic seals are challenged by reduced sea ice and increasing temperatures due to climate change, and increased anthropogenic activity can introduce new pathogens to these vulnerable ecosystems and represent a threat for these animals. Our data provide a baseline for future monitoring of health parameters of these Antarctic seal species, for tracking the impact of environmental, climatic, and anthropogenic changes in Antarctica over time.     

Vagrant Antarctic pinnipeds at Gough Island

Opportunistic sightings of Antarctic fur seals Arctocephalus gazella and a leopard seal Hydrurga leptonyx at Gough Island (40°20′S, 09°54′W) in the South Atlantic Ocean represent the northernmost island records for vagrant Antarctic fur seals, and only the second for a vagrant leopard seal at Gough Island. Some ten different individual Antarctic fur seals were sighted on a single day, up to seven on a single beach. An overall total of 18 individuals were recorded over a 7-week period in October/November 2005, before the onset of the breeding (pupping) season of the resident population of Subantarctic fur seals Arctocephalus tropicalis. Most were immature, male and mostly in good condition (n = 11, 61%); one mature male and six putative females were also present. All had departed after 23 November, although only a fraction of the available beaches was searched.     

The cost and benefits of territorial tenure, and factors affecting mating success in male Antarctic fur seals

The timing, location and duration of territorial tenure, and the mating success and return rates of male Antarctic fur seals ( Arctocephalus gazella ) were measured over four consecutive breeding seasons (1984–87) on Bird Island (54°00'S, 38°02'W), South Georgia. Tenure duration (days) followed a heavily skewed, Poisson-like distribution (median 13.08 days, maximum 75 days) and was positively related to the number of years of tenure (r_s= 0.52, P < 0.0001). Mating success was also biased to a few individuals and was positively correlated to both duration of tenure (days) and the previous number of years in which tenure was achieved ( P < 0.0001 in both cases). The timing and location of territorial tenure had no measurable effect on mating success ( P > 0.05 in both cases). The probability of a male returning to hold a territory in the next year was not related to the number of years tenure that had been achieved ( P > 0.7) or to the level of mating success in the current year ( P > 0.15). It was, however, positively related to the duration of tenure in the current year ( P < 0.0001). The overall annual return rate was 43% which is not significantly different from the survival rate for the general male population and suggests that territorial tenure does not contribute to increased mortality in male Antarctic fur seals.     

Weaning mass affects changes in body composition and food intake in harbour seal pups during the first month of independence

In phocid seals, the transition to nutritional independence is abrupt, with females abandoning their offspring after weaning and returning to sea. We hypothesized that body size at weaning may play an important role in the nature of this transition. We studied the changes in body composition and water flux of newly weaning harbour seals over the first 4-6 wk postweaning. Thirty-three pups were dosed with deuterium oxide to estimate total body water (TBW) and a subset of 24 was dosed twice to estimate changes in body composition and water flux. All pups lost body mass over the study period, but TBW increased during the period of mass loss, indicating continued lean tissue growth. Combined data from this and our early study indicated that heavy (>median mass) pups were relatively fatter (41.0% vs. 37.1%) and had significantly greater total body energy at weaning than did light (< or = median mass) pups. Percentage TBW declined linearly over time in light pups but was constant in heavy pups for the first 19 d postweaning and then declined linearly. Both the temporal pattern and composition of mass loss differed between light and heavy pups. Estimated food intake increased in the second 2 wk of study compared to the first 2 wk, in both light and heavy pups, reflecting increased foraging success but at levels still insufficient to meet daily expenditures of most individuals.     

Noninvasive estimation of body composition in small mammals: a comparison of conductive and morphometric techniques

Body fat stores may serve as an index of condition in mammals. Thus, techniques that measure fat content accurately are important for assessing the ecological correlates of condition in mammal populations. We compared the ability of two conductive techniques, bioelectrical impedance analysis (BIA) and total body electrical conductivity (TOBEC), to predict body composition with that of morphometric methods in three small mammal species: red squirrels (n=13), snowshoe hares (n=30), and yellow-bellied marmots (n=4). Animals were livetrapped in northern Idaho; BIA (all subjects) and TOBEC (squirrels only) measurements were taken following chemical immobilization in the field, and morphometric measurements were taken postmortem. Information provided by BIA and TOBEC failed to improve upon the predictive power of morphometric equations for total body water (TBW) and lean body mass (LBM) in squirrels and hares, which do not store substantial amounts of fat (<5% body mass comprised of fat). Although the same pattern held with respect to LBM in marmots, which accumulate substantial amounts of body fat (>10% body mass), a BIA-based model proved best at estimating TBW, suggesting that the usefulness of conductive techniques may be a function of fat deposition. However, regardless of the technique used to predict body composition, estimates of body fat furnished by our equations failed to approximate actual fat levels accurately in all three test species, probably because these techniques only provide indirect estimates of fat content. These results highlight the limitations inherent in contemporary methods of animal fat estimation and underscore the need for the development of direct and accurate measures of body fat in mammals.     

Body composition assessment in extreme obesity and after massive weight loss induced by gastric bypass surgery

Body composition methods were examined in 20 women [body mass index (BMI) 48.7 +/- 8.8 kg/m(2)] before and after weight loss [-44.8 +/- 14.6 (SD) kg] after gastric bypass (GBP) surgery. The reference method, a three-compartment (3C) model using body density by air displacement plethysmography and total body water (TBW) by H(2)18O dilution (3C-H(2)18O), showed a decrease in percent body fat (%BF) from 51.4 to 34.6%. Fat-free mass hydration was significantly higher than the reference value (0.738) in extreme obesity (0.756; P < 0.001) but not after weight reduction (0.747; P = 0.16). %BF by H(2)18O dilution and air displacement plethysmography differed significantly from %BF by 3C-H(2)18O in extreme obesity (P < 0.05) and 3C models using (2)H(2)O or bioelectrical impedance analysis (BIA) to determine TBW improved mean %BF estimates over most other methods at both time points. BIA results varied with the equation used, but BIA better predicted %BF than did BMI at both time points. All methods except BIA using the Segal equation were comparable to the reference method for determining changes over time. A simple 3C model utilizing air displacement plethysmography and BIA is useful for clinical evaluation in this population.     

Measurement of the body composition of living gray seals by hydrogen isotope dilution

The body composition of living gray seals (Halichoerus grypus) can be accurately predicted from a two-step model that involves measurement of total body water (TBW) by 2H or 3H dilution and application of predictive relationships between body components and TBW that were derived empirically by slaughter chemical analysis. TBW was overestimated by both 2HHO and 3HHO dilution; mean overestimates were 2.8 +/- 0.9% (SE) with 2H and 4.0 +/- 0.6% with 3H. The relationships for prediction of total body fat (TBF), protein (TBP), gross energy (TBGE), and ash (TBA) were as follows: %TBF = 105.1 - 1.47 (%TBW); %TBP = 0.42 (%TBW) - 4.75; TBGE (MJ) = 40.8 (mass in kg) - 48.5 (TBW in kg) - 0.4; and TBA (kg) = 0.1 - 0.008 (mass in kg) + 0.05 (TBW in kg). These relationships are applicable to gray seals of both sexes over a wide range of age and body conditions, and they predict the body composition of gray seals more accurately than the predictive equations derived from ringed seals (Pusa hispida) (Stirling et al., Can. J. Zool. 53: 1021-1027, 1975) and from the equation of Pace and Rathbun (J. Biol. Chem. 158: 685-691, 1945), which has been reported to be generally applicable to mammals.     

King-size fast food for Antarctic fur seals

The study of scats of adult male Antarctic fur seals Arctocephalus gazella (AFS) revealed occasional frequent capture of penguins. Although AFS adult males have been occasionally reported to kill king penguins Aptenodytes patagonicus at the shore, here we report the first observations of at-sea predation by AFS on adult king penguins in the Crozet archipelago, southern Indian Ocean. During our 20 days survey, we observed 17 penguins attacked and either severely injured or killed and consumed. Only AFS adult males were seen catching king penguins successfully. Some adult females and sub-adults also attempted to catch penguins, either at sea or in land, and so did subantarctic fur seals Arctocephalus tropicalis individuals. Our results confirm the ability of fur seals to catch and predate large seabirds, even at sea.     

Reliability in estimates of body composition of birds: oxygen-18 versus deuterium dilution

Body composition in birds was evaluated indirectly by 18O and 2H dilution. Body composition was determined by whole-body chemical analysis of eight adult roosters (Gallus gallus). In vivo measurements of total body water (TBW) were carried out using doubly labeled water (2H2 18O). Estimated dilution spaces using both the plateau and intercept approaches were compared with the results obtained by carcass lyophilization. Both 18O and 2H slightly overestimated TBW compared with the results obtained by lyophilization, by 2.2%+/-1.9% and 5.7%+/-0.2%, respectively; both differences were statistically significant (P<0.01). The difference between these isotope estimations was significant (P<0.001). However, isotope dilution spaces and TBW were highly correlated. There was a strong inverse relationship between total body fat and TBW percentages (r2=0.98, P<0.0001). The relation between TBW and body protein was significant. Water content in lean body mass (72.8%) obtained in our study was very close to that reported in mammals, demonstrating no fundamental difference in tissue water content between birds and mammals. Estimated body fat and protein values from isotopic dilution did not significantly differ from values obtained by direct chemical analysis (P>0.05), except for body fat in the Pace and Rathbun approach (Table 3). Although estimation of TBW and body composition by isotope dilution is time consuming and expensive, deuterium offers a reliable and low-cost alternative compared with 18O. The advantage of in vivo estimation of TBW with isotopic dilution in combination with the regression approach is that it permits repeated measurements of body composition on the same birds under laboratory and free-living conditions.     

Vagrant Subantarctic fur seal at Cape Shirreff,Livingston Island,Antarctica

A vagrant adult male Subantarctic fur seal Arctocephalus tropicalis was observed among Antarctic fur seals A. gazella at Cape Shirreff, Livingston Island, Antarctica, which is located to ~4,190 and ~5,939 km from the nearest breeding colonies of Subantarctic fur seals. Although the colony of origin of this animal and the reason for its movement outside its distribution range are unknown, this sighting shows the high dispersal capacity of this species and provides an insight into possible changes in its distribution. Although this vagrant was not observed with females Antarctic fur seal, news sightings in the future could result in viable hybrid, and introgressive hybridization could represent a threat for Cape Shirreff population recovery, if still the population way to go to recover to presailing levels.     

Comparison of DXA and CT in the Assessment of Body Composition in Premenopausal Women With Obesity and Anorexia Nervosa

Accurate methods for assessing body composition in subjects with obesity and anorexia nervosa (AN) are important for determination of metabolic and cardiovascular risk factors and to monitor therapeutic interventions. The purpose of our study was to assess the accuracy of dual‐energy X‐ray absorptiometry (DXA) for measuring abdominal and thigh fat, and thigh muscle mass in premenopausal women with obesity, AN, and normal weight compared to computed tomography (CT). In addition, we wanted to assess the impact of hydration on DXA‐derived measures of body composition by using bioelectrical impedance analysis (BIA). We studied a total of 91 premenopausal women (34 obese, 39 with AN, and 18 lean controls). Our results demonstrate strong correlations between DXA‐ and CT‐derived body composition measurements in AN, obese, and lean controls (r = 0.77–0.95, P < 0.0001). After controlling for total body water (TBW), the correlation coefficients were comparable. DXA trunk fat correlated with CT visceral fat (r = 0.51–0.70, P < 0.0001). DXA underestimated trunk and thigh fat and overestimated thigh muscle mass and this error increased with increasing weight. Our study showed that DXA is a useful method for assessing body composition in premenopausal women within the phenotypic spectrum ranging from obesity to AN. However, it is important to recognize that DXA may not accurately assess body composition in markedly obese women. The level of hydration does not significantly affect most DXA body composition measurements, with the exceptions of thigh fat.     

Assessment of nutritional status in rhesus monkeys: comparison of dual-energy X-ray absorptiometry and stable isotope dilution

Body composition estimates from dual-energy X-ray absorptiometry and stable isotope dilution ((2)H and (18)O) were compared in 61 rhesus monkeys (Macaca mulatta) from the ongoing long-term energy restriction study at the University of Wisconsin. Their average age was 18.9 +/- 2.5 y/o. Of the animals, 51% were in the energy restricted group and 38% were females. Although the correlation between methods was highly significant for fat mass (R(2) = 0.97, SEE = 0.25 kg or 7.5%, P < 0.0001) and fat-free mass (R(2) = 0.98, SEE = 0.29 kg or 3.6%, P < 0.0001), we observed that dual-energy X-ray absorptiometry underestimated fat mass by 0.67 +/- 0.26 kg (7.5%, P < 0.0001) and overestimated fat-free mass by 0.57 +/- 0.29 kg (20%, P < 0.0001) when compared with isotope dilution. Taken together with data from the literature, the present results emphasize the usefulness of dual-energy X-ray absorptiometry to derive body composition and thus nutritional status in monkeys, but demonstrate the importance of validation experiments for a given DXA model and software.     

Dual-energy X-ray absorptiometry lean soft tissue hydration: independent contributions of intra- and extracellular water

Dual-energy X-ray absorptiometry (DEXA) provides a measure of lean soft tissue (LST). LST hydration, often assumed to be constant, is relevant to several aspects of DEXA body composition estimates. The aims of this study were to develop a theoretical model of LST total body water (TBW) content and to examine hydration effects with empirically derived model coefficients and then to experimentally test the model's prediction that, in healthy adults, LST hydration is not constant but varies as a function of extra- and intracellular water distribution (E/I). The initial phase involved TBW/LST model development and application with empirically derived model coefficients. Model predictions were then tested in a cross-sectional study of 215 healthy adults. LST was measured by DEXA, extracellular water (ECW) by NaBr dilution, intracellular water (ICW) by whole body (40)K counting, and TBW by (2)H(2)O dilution. TBW estimates, calculated as ECW + ICW, were highly correlated with (r = 0.97, SEE = 2.1 kg, P < 0.001) and showed no significant bias compared with TBW measured by (2)H(2)O. Model-predicted TBW/LST was almost identical to experimentally derived values (means +/- SD) in the total group (0.767 vs. 0.764 +/- 0.028). LST hydration was significantly correlated with E/I (total group, r = 0.30, SEE = 0.027, P < 0.001). Although E/I increased with age (men, r = 0.48; women, r = 0.37; both P < 0.001), the association between TBW/LST and age was nonsignificant. Hydration of the DEXA-derived LST compartment is thus not constant but varies predictably with ECW and ICW distribution. This observation has implications for the accuracy of body fat measurements by DEXA and the use of TBW as a means of checking DEXA system calibration.