Aging is associated with a number of structural and functional changes that are conducive to increased disability, frailty and falls (Baumgartner et al., 1998). Contributing factors for the physical impairments are a gradual deterioration of bone (osteoporosis) (WHO Scientific Group, 2003) and a progressive decline in muscle mass (sarcopenia) (Santilli et al., 2014).
Osteoporosis is a disease characterized by low bone mass and micro-architectural deterioration of bone tissue, leading to enhanced bone fragility and a consequent increase in fracture risk. Bone mineral density (BMD) is influenced by ethnicity, geographic factors, diet, exercise, family history, and other lifestyle factors. For example, BMD values in white females are lower than in black females (Kao et al., 1994), and values in Asian females are lower than in white females (RussellAulet et al., 1993).
Sarcopenia is characterized by an abnormal loss of muscle mass with qualitative changes to the muscle, resulting in a loss of strength (Santilli et al., 2014). The loss of muscle mass from sarcopenia is thought to affect functional ability as shown by the positive association between muscle mass and lower extremity function, and the negative association between muscle mass and use of a cane or walker (Visser et al., 2005), history of falling (Landi et al., 2012), and self-reported difficulty with gait and daily activities (Estrada et al., 2007).
The prevalence of osteoporosis and sarcopenia increases with age, in conjunction with physical inactivity (Ryu et al., 2013), malnutrition (Vandewoude et al., 2012) and body fatness (Hsu et al., 2006). Korea's population is rapidly aging. In 2010, people 65 years and older comprised 11% of the population, and the number is projected to increase to 24.3% in 2030 and reach 37.4% in 2050 (Korean National Statistical Office, 2012). In a nationwide survey of Koreans 65 years and older, the prevalence of sarcopenia was estimated to be 9.7% for men and 11.8% for women (Kim et al., 2012). The prevalence of osteoporosis was 7.5% for men and 35.5% for women (Choi et al., 2012). Despite the high prevalence, few studies have investigated the potential association between sarcopenia and osteopenia/osteoporosis, especially in older Korean women. Further, the majority of existing studies on the association between sarcopenia and osteoporosis have been from Western societies.
Given the close association between sarcopenia and osteoporosis, physical activity should be encouraged throughout life as an efficient strategy to maintain bone and muscle health. For example, compelling data support the efficacy of physical activity, resistance exercise in particular, in maintaining muscle mass and muscle function in aging populations (Ryu et al., 2013; Fiatarone et al., 1994). In addition, physical activity regulates bone maintenance and stimulates bone formation, including the accumulation of mineral, strengthening of muscles, and improvement of balance, and thus reduces the overall risk of falls and fractures (Moayyeri, 2008). Together, the previous findings suggest physical activity as a mediator in the association between sarcopenia and osteoporosis in older adults. Further, in older women, sarcopenia and osteoporosis may act together. However, there is little information on exactly how modifiable risk factors such as body fatness and physical inactivity mediate the association between sarcopenia and osteoporosis.
In this study we examined: 1) the association between sarcopenia and osteopenia and/or osteoporosis in community-dwelling women 65 years and older and 2) whether covariates such as body fatness and physical activity modulate the association between sarcopenia and osteopenia and/or osteoporosis in this study population.
Initially, 280 study participants were recruited via advertisements (i.e., local newspapers and flyers) from local community centers in the Northwestern Gyeonggi Province of South Korea between December 2014 and December 2015. Eligibility criteria included: women aged 65 years or older; self-report of no difficulty walking; no difficulty performing basic activities of daily living; no reported use of cane, walker, crutches or other special equipment to get around; no history of active treatment for cancer in the prior 3 years; and no enrollment in a lifestyle intervention trial.
After receiving information describing the study, potential participants were screened for the eligibility criteria described above. Of the 280 participants, 7 were excluded due to mobility limitations and/or use of cane, walker, and crutches. An additional 4 participants refused to participate in body composition and/or physical activity assessments due to personal reason(s). Consequently, a total of 269 participants were available for analysis.
Data was collected during three separate visits. During the first visit, the participants' demographic characteristics and general health statuses were assessed with a standardized self-administered questionnaire modified from ACSM's Guidelines for Exercise Testing and Prescription (American College of Sports Medicine, 2014) and interviews were conducted by geriatric nurses. One week after the first visit, each participant's body composition was analyzed using dual-energy X-ray absorptiometry (DEXA). Finally, daily physical activity was monitored for seven consecutive days; the subjects were asked to wear the device from the time they woke up in the morning until they went to bed at night, except while bathing, for the full 7-day data collection period. The Sungkyunkwan University Institutional Review Board, in accordance with the Declaration of Helsinki of the World Medical Association, approved the study protocol. All participants provided written informed consent to participate in the study.
Total body composition and bone densitometry by DEXA was performed at Yongin University on equipment branded QDR 4500A (Hologic, Waltham, MA, USA). The DEXA measurement methods and validation have been tested and reported by Visser et al. (1997) and Salamone et al. (2000).
The performance of the densitometries of the whole body, lumbar spine, femoral neck, and total hip followed the manufacturer's instructions and were performed by the same operator. The results of the bone densitometry were expressed in g/[cm.sup.2] and T-score, calculated by the device itself, and analyzed according to the criteria of the World Health Organization (WHO Scientific Group, 2003). In brief...