Osteopenia/osteoporosis
Osteopenia/osteoporosis is a skeletal disorder characterized by compromised bone strength leading to increased risk of fractures. Osteoporosis affects approximately 12 million adults over the age of 50 years in the USA [4]. In 2005, more than 2 million incident fractures were reported in the USA alone, with a total cost of $17 billion. Even if rates stay the same, the aging of the world population is expected to increase the number of fractures and costs in the USA by 48 % to greater than 3 million fractures associated with a cost of $25.3 billion [5].
Although osteoporosis is a complex entity, we can measure areal bone mineral density (BMD), which is used as a proxy to define bone strength and to diagnose osteopenia/osteoporosis [6, 7]. Dual-energy X-ray absorptiometry (DXA) is the most common diagnostic imaging technique for assessing BMD. DXA uses low-radiation X-rays of two different photon energy levels that pass through the body and are identified by a photon detector that measures the amount of energy absorbed by soft tissue and bone at each pixel [8]. This method measures both bone and soft tissue [8]. Soft tissue is further subdivided into fat and lean [also called lean body mass (LBM), Table 1] based on the empiric attenuation of both pure fat and bone-free soft tissue [9]. Therefore, bone mineral content and BMD, as well as fat and fat-free soft tissues at the whole-body and regional levels can be assessed by DXA [8, 10]. Uniform standards for diagnosing osteopenia and osteoporosis by DXA do exist. According to the World Health Organization (WHO), the operational definition of osteopenia is a BMD that lies from 1.1 to 2.4 standard deviations below (a T-score of 1.1–2.4) and osteoporosis is a BMD that lies 2.5 standard deviations or more below the average value for young healthy women (a T-score of < −2.5). Increased BMD augments the strength of bone. On the contrary, significant losses in BMD predispose an individual for increased risk of fracture.
Table 1 Body composition terminology
ASM (appendicular skeletal muscle mass) Lean body mass from limbs, a surrogate measure of skeletal muscle mass. It can be expressed adjusted for height in squared meters (kg/m2) and named ASM index.
FFM (fat-free mass) Sum of LBM plus bone mineral content
FM (fat mass) Amount of fat, also known as body fat
LBM (lean body mass) Also called lean soft tissue, it is the sum of the lean compartments of the body (excluding bone mineral content) (total body water, total body protein, carbohydrate, nonfat lipid, and soft tissue minerals)
Obesity Body mass index ≥ 30 kg/m2
Osteosarcopenic Obesity Concurrent osteoporosis, low muscle mass, and obesity
Sarcopenia Low skeletal muscle mass or muscle wasting
Sarcopenic obesity Concurrent obesity and low muscle mass