2 second increase in gait speed. As recently noted by Cesari [5], the ﬁndings of Studenski et al.’s pooled analysis provide the statistical foundations to estimate expected survival in older adults based solely on gait speed [1]. Based on these consistent and robust ﬁndings, we argue that gait speed may represent a useful marker of overall health status and may also be a useful prognostic indicator of mean lifespan in old- er adults. Further, because walking places demands on mul- tiple organ systems and demands on input from the central nervous system, it may serve as the ideal target to evaluate the eﬃcacy of interventions aimed at improving health and increasing lifespan. Against the backdrop of these ﬁndings on the importance of mobility function, a growing body of literature indicates that obese, older adults are at particularly high risk of func- tional decline, marked by reductions in mobility (i.e., walk- ing speed). This is of signiﬁcant concern because the number and proportion of obese, older adults has increased dra- matically during the past two decades [6]. Recent estimates indicate that an alarming 35% of older adults are obese and another 33% are overweight, which places them at risk for obesity [6]. This paper describes the key behavioral factors contributing to the development of obesity in older adults, the major pathways through which obesity aﬀects mobil- ity function, and the relationship between important co- morbidities related to decreased lifespan and mobility func- tion in obese, older adults. Concurrently, lifestyle interven- tions, including diet and exercise, targeted toward obese older adults are described as a means to target mobility function and limit the functional limitations associated with increased mortality risk. 2. Behavioral Mechanisms of Obesity in Older Adults Both obesity and sedentary lifestyle appear to contribute to the body composition changes (i.e., increased body fat, decreased muscle mass) that promote age-related functional decline [7–9]. As such, obese, older adults may be par- ticularly susceptible to the adverse eﬀects of weight gain because of the loss of muscle mass that occurs with aging (i.e., sarcopenia) [10, 11]. Loss in muscle mass by itself is associated with impairments in mobility in older adults [12]. Moreover, the combination of muscle loss and fat gain may act synergistically to lead to further reductions in mobility in older adults [13–15]. Excessive dietary intake, speciﬁcally, is a major factor inﬂuencing the overall health and body weight of older adults. Epidemiological studies indicate that per capita en- ergy intake has increased by approximately 300 kcal per day from 1985 to 2000 [16]. Unfortunately, the trend of increasing dietary consumption also coincides with a trend to expend less energy with increasing age [17]. Physical inactiv- ity represents another major contributor to the development of obesity and obesity-related morbidity [18]. Currently, the majority of older adults in the US do not engage in even the minimum physical activity recommendations [19]. Moreover, the Centers for Disease Control and Prevention Journal of Aging Research recently reported that 40% of adults engage in no leisure- time physical activity [19]. This is of particular concern in the elderly as older adults are less active with advancing age [17]. 3. Impact of Obesity on Declines in Mobility Obesity poses several threats to mobility during aging. Most notable is the direct eﬀect of excess body weight on move- ment. As body mass increases, the energy and strength re- quired to move the body increases correspondingly. Specif- ically, work from the Baltimore Longitudinal Study of Aging showed that total knee generative mechanical work expenditure is higher in older adults with obesity [20]. Similarly, work by Messier et al. [21] demonstrated that the absolute peak vertical ground-reaction forces increase almost directly in proportion with body weight. These factors likely contribute to the slower preferred walking speed consistently found in obese, older adults compared to nonobese, older adults [20]. A growing body of evidence also indicates that relative skeletal muscle mass (i.e., skeletal muscle mass/body mass), compared to absolute body mass, is a strong predictor of impairments in mobility performance, such as walking and stair climbing [22, 23]. For example, Janssen and colleagues [22] found that there is an increased likelihood of functional impairment and disability in older adults if muscle loss progresses to the point where skeletal muscle mass, relative to body weight, reaches 30% below the mean for young adults. Alarmingly, 45% of men and 59% of women in this sample of over 4,500 older adults (age ≥ 60 years) were classiﬁed as having sarcopenia, as determined by low relative skeletal muscle mass to body weight. Sarcopenia is strongly associated with the development of functional disability and can lead to the loss of independence for aﬄicted individuals [22, 24]. Typically, older adults lose an average of 1-2% of their muscle mass yearly after the age of 50 [25, 26]. However, recently the loss in muscle strength has been shown to decline at a faster rate (3–5%) than previously thought [27], and it has been argued that muscle weakness is a stronger predictor of these changes than muscle mass [24, 28]. Additionally, the combination of muscle weakness and obesity increases the risk of mobility impairment greater than obesity or muscle weakness alone [29]. The reductions in both muscle size and the function through maximal strength are important factors in the loss of mobility among older adults [30, 31]. Somewhat paradoxically, obese, older adults typically have more absolute total muscle mass and strength than nonobese peers. Despite this fact, obese individuals also have higher amounts of intermuscular adipose tissue, a potential detriment to the muscle’s force-generating capacity [32, 33]. Furthermore, the disparity in total muscle mass is typically not suﬃcient to account for the disparity in body mass, meaning that obese persons have a lower strength to body mass ratio—an important factor in mobility. This paradoxical state of low (relative) muscle mass and high body weight has come to be known as sarcopenic obesity