https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4361695/ 上面这个文献比较了测量体脂的各种方法，提到了dxa scan中身体厚度会造成脂肪测量不准，我觉得是从侧面解释了楼主的疑惑， 还提到了水肿的问题。 Dual-Energy X-ray Absorptiometry DXA is a widely used method that assesses body composition at the molecular level. Bone mineral mass and density, as well as fat and fat-free soft tissues at the whole-body and regional levels, can be assessed.4-6 One important advantage of using DXA is its ability to estimate appendicular skeletal muscle (ASM) mass by measuring the amount of LST in the arms and legs, which is mainly muscle (except for a small amount of connective tissue and skin).6,7 ASM has been widely used in the study of sarcopenia and the establishment of cut points for the definition of this syndrome.1,6,8,9 During a DXA scan, low-radiation x-rays of 2 different photon energy levels pass through the body and are identified by a photon detector that measures the amount of energy absorbed (attenuation) by soft tissue and bone at each pixel.5 Soft tissue is further subdivided into fat and LST based on the empiric attenuation of both pure fat and bone-free soft tissue.10 DXA is a quick, noninvasive, and safe method for body composition assessment, and the radiation exposure is considered small and safe for repeated measures. Moreover, DXA measures 3 body composition compartments and can provide regional estimates of these compartments. DXA is also a very precise method for quantifying body composition; its overall precision exceeds that of any other body composition methodology.11 Although results of DXA scans, particularly fat and bone estimates, are highly correlated with cadaver analysis and in vivo neutron activation analysis,4 its accuracy depends on several factors such as patient’s body thickness and size, machine calibration procedures, software version used, and the definition of regions of interest, which is operator dependent. The thickness of the body can affect DXA results.5 Increased tissue thickness (>25 cm) causes an increased attenuation of low-energy photons, causing a disproportional shift to the high-energy photons, which may lead to an underestimation of fat mass in obese patients.12 Moreover, the actual size of an individual presents a limitation for some DXA scans, as large persons are not able to easily fit inside the scan area, which is usually 60 × 197.5 cm. Nonetheless, recently developed DXA systems have expanded the limits of the DXA table so that now a patient who is morbidly obese can be accommodated. Hydration status may affect DXA accuracy because of the programmed assumption of a constant and uniform FFM hydration. Large changes in hydration (higher than 5%) can change the attenuation of fat-free soft tissue,10 causing an overestimation of the LST compartment. Nevertheless, small changes in hydration levels do not greatly alter DXA estimates.5 Finally, the inability of DXA to decipher the different types of fat (visceral, subcutaneous, and intramuscular) and LST (muscle, organs) may also represent a practical limitation of this modality in clinical settings. In addition, orthopedic implants can create artifacts that affect DXA measurements, resulting in inaccurate identification of soft-tissue parameters,13 although little is known about the overall effect of such artifacts on whole-body composition measurements.14 A major limitation of DXA is the variability of the hardware and software packages between manufacturers. This inconstancy in technology limits the ability of healthcare professionals to compare measurements between different machines and to minimize residual calibration errors.1,14 A universal calibration of DXA machines is greatly needed to advance research in the field. Despite these limitations, DXA scans have assumed an increasingly important role in clinical settings because of its decreasing cost and its availability for measuring bone density. As DXA machines are used worldwide in the assessment of osteoporosis, healthcare professional may seek its “secondary” use (ie, perform body composition assessments), provided that software and appropriate phantom calibration is available.14
对的，去脂的体重比较稳定，即便体重起伏很大，这个变化不算大，让你体重产生巨大变化的是脂肪。总体来说体重大，去脂体重就大。像楼主这种120减到100，如果同期保证合理饮食和运动的情况，脂肪可能减的是20磅的2/3倍。
re 这个帖子真让人崩溃