In this study, the individual and joint associations of obesity and the MetS on incident mobility limitation were examined in initially well-functioning older adults for more than 6.5 years. Both the obesity and the MetS independently predicted the risk of developing mobility limitation in women, but obesity only and not the MetS predicted the development of mobility limitation in men. Furthermore, having the MetS increased the risk of developing mobility limitation in nonobese women, but in obese women and men, the MetS did not significantly increase the risk of mobility limitation beyond the effects of obesity.
These results confirm earlier findings on the association between obesity and mobility limitation in old age (4
) and further support the notion that both the obesity (4
) and the MetS seem to expose women to greater risk of mobility limitation than men (10
). As far as we know, this is the first study to report that obesity per se independent of its metabolic consequences is a stronger risk factor for mobility limitation in older obese adults. Thus, we cannot compare our findings with past work.
It has been suggested that obese persons are not a homogeneous group and that the effect of obesity on health may be substantially different when obesity is associated with metabolic dysregulation (15
). Previous studies have shown a higher risk of cardiovascular events and mortality among obese persons with metabolic alterations (30
). Interestingly, although we identified a subgroup of obese persons who did not have the MetS, sometimes referred to as “metabolically healthy obese,” their adjusted risk for mobility limitation was similar as the risk of obese persons who additionally had the MetS. A possible interpretation of our findings is that obesity-related factors other than metabolic consequences are more important in increasing the risk of mobility limitation. For example, excess body weight can cause biomechanical stress on the lower extremity joints leading to pain, osteoarthritis, reduced physical activity, and impaired muscle strength, all of which can predispose an individual to mobility limitation (33
). In addition, in older obese persons, the lower extremity muscle strength (35
) or cardiorespiratory fitness may be inadequate to perform weight-bearing activities without difficulties.
However, when interpreting our findings, it must be emphasized that the prevalence of metabolically healthy obese was relatively low (9% of women and 7% of men) in the present study population; thus, we may lack power to show significant difference in the risk of mobility limitation between obese persons with and without the MetS. Another explanation for the nonsignificant effect of the MetS among obese participants is selective survival. Although there was no difference in survival among those who entered the study, it is possibly that due to the strict inclusion criteria of the Health ABC Study and the relatively older age of the study participants, obese persons with more serious obesity-related consequences, including the MetS and related cardiovascular conditions, were excluded from this study. Thus, the effect of obesity and MetS on incident mobility limitation may have been underestimated in this study. Future studies in a general population, including younger participants, are needed to confirm our findings.
Although the presence of the MetS did not present additional risk of mobility limitation in obese participants, nonobese women with the MetS had 1.5 times higher risk of developing mobility limitation compared with those without the MetS. In nonobese men, the MetS did not increase risk of mobility limitation. Previous studies have shown that the MetS is associated with poorer physical functioning and predicts the development of mobility limitation (10
), but the effect of general obesity (measured with BMI) on the association of the MetS and mobility limitation was not addressed in these studies. Potential explanation for the sex difference in the present study is that women and men exhibit a different pattern of factors that constitutes the MetS. Nearly 90% of nonobese women with the MetS have high waist circumference, whereas in men, the corresponding proportion is 59%. As abdominal obesity, independent of general obesity, is a known risk factor for mobility limitation (4
) and because mobility limitation is more prevalent in women, this may explain the found differences between men and women.
Finally, our study suggests that elevated inflammatory markers partly explain the association between obesity, the MetS, and mobility limitation. The role of heightened inflammatory state was especially clear in explaining the additional mobility limitation risk related to the MetS. This is in accordance with current knowledge about the association of chronic subclinical inflammation with both the MetS (20
) and the mobility limitation (21
). The role of inflammation as a risk factor of functional decline has proven to be very important. Increasing evidence suggests that proinflammatory cytokines have catabolic effects on muscle, thus decreasing muscle mass and strength (21
) and further predisposing older people to functional decline (21
In conclusion, this prospective study provides evidence that obesity itself, independent of its metabolic consequences, is a risk factor for mobility limitation among obese older adults. In addition, having the MetS increases the risk of functional decline only in nonobese women. Our study implies that it is important to recognize the MetS in the nonobese population, especially in women. Furthermore, in addition to lifelong control of healthy body weight, interventions targeting nonmetabolic consequences of obesity, such as reduction of pain, treatment of lower extremity joint problems, improvement of muscle strength, and cardiorespiratory fitness, may be useful in preventing and delaying mobility decline in older obese adults.