Although dual-energy X-ray absorptiometry (DXA) is widely used in clinical research as a means of quantifying body composition, there remains at present little published information that reviews the method's underlying physical basis. Because a clear understanding of DXA physical concepts is integral to appropriate use and interpretation, we present here a three-section review that includes both relevant in vitro and in vivo experimental demonstrations. In the first section we describe the main physical principles on which DXA is based. The section that follows presents a step-by-step analysis of the DXA two-component soft tissue model. In the final section we demonstrate how knowledge of physical concepts can lead to resolution of important methodological concerns, such as the influence of hydration changes on DXA fat estimates. A thorough understanding of DXA physical concepts provides a basis for appropriate interpretation of measurement results and stimulates many new and important research questions.

Objectives: Air displacement plethysmography (ADP) may provide a partial alternative to body density (B d ) and therefore body composition measurement compared to conventional hydrodensitometry (H d ) in children. As there are no evaluation studies of ADP in children, this study had a two-fold objective: to compare B d estimates by ADP and H d ; and to compare fat estimates by both ADP and H d to fat estimates by another reference method, dual energy X-ray absorptiometry (DXA). Setting: Obesity Research Center, St. Luke'saRoosevelt Hospital, New York, USA. Subjects: One hundred and twenty subjects (66 femalesa54 males) who ranged in age from 6 ± 86 y and in body mass index (BMI, kgam 2 ) from 14.1 ± 40.0 kgam 2 met study entry criteria. Study Design: Cross-sectional study of healthy children (age 19 y) and adult group for comparison to earlier studies. Each subject completed ADP, H d , and DXA studies on the same day. Only subjects with subjectivelyjudged successful H d studies were entered into the study cohort.Results: There was a high correlation between B d by ADP and H d (B d Hd 0.11 0.8966B d ADP; r 0.93, SEE 0.008 gacm 3 , P`0.0001), although the regression line slope and intercept differed signi®cantly from 1 and 0, respectively. Additional analyses localized a small-magnitude B d bias in the child (n 48) subgroup. Both ADP and H d %fat estimates were highly correlated (r b 0.9, P`0.0001) with %fat by DXA in child and adult subgroups. Bland ± Altman analyses revealed no signi®cant %fat bias by either ADP or H d vs DXA in either children or adults, although a bias trend (P 0.11) was detected in the child subgroup. Conclusion: With additional re®nements, the air displacement plethysmography system has the potential of providing an accurate and practical method of quantifying body fat in children as it now does in adults. Sponsorship: This study was in-part supported by NIH Grants RR00645, NIDDK 42618 and NIDDK 37352.

The study of human body composition is now a distinct research area consisting of three interconnected parts: the five-level model and associated rules that govern the relations between components, body-composition methodology, and biological factors that influence body composition. In this overview we summarize fundamental concepts that relate to the five-level model and body-composition methods. We show how these concepts can be used to outline the essential features needed to critically evaluate the bioelectrical impedance analysis method. Body-composition research is a rapidly expanding area and in-depth systematic evaluation of new methods is a vital aspect of the field's growth.

Obesity prevalence rates are increasing worldwide and one prevailing hypothesis is that physical activity levels of modern humans are markedly reduced compared to those of our Paleolithic ancestors. We examine this hypothesis by deriving relative activity energy expenditure from available doubly labeled water and indirect calorimetry data in free-ranging non-human mammals. Our results, given the constraints posed by limited data availability, suggest that a low physical activity level, much less than that observed in free-ranging non-human mammals or highly active humans, is present in modern adult humans living within advanced settings. Our observations lend support to the hypothesis that low activity-related energy expenditure levels contribute to the rising worldwide prevalence of obesity.

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Microalgae are highly efficient photosynthetic organisms that hold enormous potential as sources of renewable energy. In particular, Chlorella pyrenoidosa displays a rapid growth rate, high tolerance to light, and high lipid content, making it especially valuable for applications such as flue gas CO2 fixation, biofuel production, and nutritional extracts. In order to unveil its full potential, it is necessary to characterize its subcellular architecture. Here, we achieved three-dimensional (3D) visualization of the architectures of C. pyrenoidosa cells, by combining focused ion beam scanning electron microscopy (FIB/SEM), cryo-FIB milling, and cryo-electron tomography (cryo-ET). These high-resolution images bring to light intricate features of intact organelles, including thylakoid membranes, pyrenoid, starch granules, mitochondria, nucleus, lipid droplets and vacuoles, as well as the fine architectures within the chloroplast, including the concave-convex pyrenoid, plastoglobules, thylakoid tips, and convergence zones. Significantly, comparative analysis of wild-type and nuclear-irradiated mutagenic strains determined that cell volume and surface area of mutant cells have increased substantially to 2.2-fold and 1.7-fold, respectively, consistent with up-regulation of the enzyme Rubisco and enhanced photosynthetic metabolic processes. Moreover, quantitative analysis established that the thylakoid membrane width in mutant cells increased to 1.3-fold, while the membrane gap decreased to 0.8-fold, possibly contributing to the higher biomass growth rate of mutant cells. Our work reveals the first 3D subcellular architectures of C. pyrenoidosa cell and provides a structural framework for unlocking the higher growth rate in microalgae relevant to a wide range of industrial applications.

Purpose: Traditionally, patients treated with total skin electrons are required to remove all clothing. This is assumed to be best for the treatment of superficial skin lesions out of concern clothing may significantly perturb the electron dose delivered. However, no known peer‐reviewed studies have been done to validate this. We investigate what effect cloth and paper gowns will have on dose near the skin surface. Method and Materials: Using a Markus ion chamber and Gafchromic‐EBT2 film, dose to a cylindrical phantom was measured with a cloth hospital gown, a paper gown, and a tri‐layer cloth (to simulate folds and seams) compared to no covering. A 6‐MeV electron beam with spoiler accessory was used at a distance of about 4‐meters, with gantry angled at 248° and 292°. The materials covered the films which were placed onto the phantom at 1000‐MU per shot. The phantom was rotated at −60°, 0°, and 60° relative to the beam's central axis simulating the six‐field Stanford technique. Similar procedure was followed for Markus chamber with 1‐mm buildup cap in a Lucite phantom. Effect of air gap of 0, 3, and 5‐cm between full‐size gowns and ion chamber also investigated. Results: Compared to no covering, the films revealed a 0.8% increase in dose for cloth, 1.8% for tri‐layered cloth, and 0.7% for paper. Markus chamber readings revealed at 0°, 60° orientation −0.03%, −1.5% for cloth, 1.4%, −4.1% for tri‐layered cloth, and 0.2%, −0.2% for paper, respectively. Air gaps had differences by less than 0.7% for cloth and 0.2% for paper. Conclusion: For cloth and paper gowns there was a variability of less than 1.5% in dose delivered. Air gaps did not substantially affect dose but folds and seams in cloth may perturb dose more than 4%. For patient comfort, paper gowns may be most appropriate covering option.