Osteosarcopenic obesity: hypothesized mechanisms
Although the etiology of osteosarcopenic obesity can only be hypothesized at this time, Fig. 2 illustrates potential mechanisms leading to progressive losses of bone and muscle mass and an increase in adipose tissue. Regardless of the initiating metabolic abnormalities, an increase in total and/or abdominal adipose tissue causes an increase in pro-inflammatory cytokines, as well as some hormonal disturbances leading to losses of both muscle and bone tissues through a variety of mechanisms which ultimately affects clinical outcomes such as increase in risk for falls and fractures and potentially a variety of other problems (discussed below). The decreases in muscle and bone are associated with decreases in physical activity; once losses hit a threshold, physical activity becomes even more limited, leading to a vicious cycle of progressive loss of muscle and bone and gain in fat, of unveiled complexity (Fig. 2).
While obesity is clearly a multifactorial condition, the primary causes appear to be related to an excess energy consumption, low levels of physical activity, and high genetic susceptibility [45]. An increase in body fat from a lack of physical activity is primarily due to the low level of energy expenditure compared to a higher level of energy intake. Interestingly, this resultant increase in adiposity is highly correlated with excess inflammation that leads to impairments in skeletal muscle function and size [46, 47]. Furthermore, while further research is needed to clarify potential mechanisms, a loss of skeletal muscle mass and function is also associated with a sedentary lifestyle [48]. Therefore, lack of activity further exacerbates the loss of muscle mass and function resulting from obesity-derived inflammation. Adding to this problem is that loss of muscle mass also accounts for a reduced level of physical activity [48]. It is clear that a sedentary lifestyle contributes to sarcopenic obesity [48, 49] and, unfortunately, leads to both disability and mobility issues greater than those with only one of these conditions [50]. The reader is referred to a comprehensive review on the topic [51].