Among well-functioning, community-dwelling, diabetic older adults, higher serum PTH levels appear to identify persons at increased risk of falling in the subsequent year independent of other risk factors for falls. Among participants who reported falling in the subsequent year, the mean baseline serum PTH level was approximately 18% higher compared to those who did not report falling. For every one standard deviation increment in baseline serum PTH, there was approximately a 30% increase in the likelihood of reporting falls in the subsequent year after adjusting for demographic characteristics, health behaviors, kidney function, health conditions, and medication and dietary supplement use. Even though serum PTH was associated with lower extremity physical performance at baseline, physical performance did not appear to mediate the association between serum PTH and incident falls. A trend also remained between serum PTH and incident falls after further adjustment for reported falls in the previous year, a major risk factor for subsequent falls.
A limited number of studies have examined the association between serum PTH and muscle strength or physical performance with inconsistent results (11
). Among older ambulatory women, serum PTH was significantly correlated with gait speed and balance but not with knee extension or knee flexion strength (19
). However, among older patients attending a falls clinic, serum PTH was a significant predictor of isometric quadriceps strength independent of vitamin D levels (11
). Older community-dwelling adults with high serum PTH levels were significantly more likely to experience loss of grip strength and appendicular skeletal muscle mass over three years of follow-up (18
). However, among moderately to severely disabled older women, serum PTH was not associated with muscle strength or physical performance over three years of follow-up (20
). Furthermore, serum PTH was not associated with baseline or four-year change in quadriceps strength, chair stand pace, gait speed, or tandem walk time in older, community-dwelling, vitamin-D replete women (21
). In contrast, this study found significant correlations between serum PTH and physical performance, particularly for standing balance and gait speed, among older, well-functioning diabetic men and women.
There are only a few studies that have examined the association between serum PTH and falls (10
). Among older men and women living in assisted living or nursing home facilities, median serum PTH levels were approximately 30% higher among those who reported falling (10
). In longitudinal analyses, higher serum PTH levels were a significant predictor of time to first fall among older adults residing in assisted living or nursing home facilities independent of vitamin D levels (12
). However, among older community-dwelling, vitamin-D replete women, serum PTH was not associated with falls over four years of follow-up (21
). In this study, serum PTH was associated with greater odds of falling over one year of follow-up among older, well-functioning diabetic men and women.
The prevalence of falls over one year of follow-up was similar in blacks and whites. Previous studies have also found similar rates of falling in blacks and whites (30
). Consistent with other studies (32
), the current study found that blacks had higher mean serum PTH levels than whites. Higher PTH levels in blacks are primarily the result of increased skin pigmentation and the concomitant reduction in vitamin D production in the skin along with low intakes of vitamin D (35
). Although blacks had higher serum PTH levels on average, we did not find an interaction between race and serum PTH and the likelihood of reporting falls associated with a one standard deviation increment in serum PTH did not appear to be higher among blacks compared to whites. Previously, studies have suggested that blacks may have a skeletal resistance to PTH-stimulated bone resorption (33
). Resistance to the effects of elevated PTH in blacks may extend to muscle function and falls.
There are important characteristics of this Health ABC sub-study of bone health that limit the generalization of these findings. Health ABC participants were well-functioning and free of lower-extremity functional limitations at baseline. It is possible that the associations between serum PTH and muscle strength, physical performance, and falls may have been stronger had participants of all stages of functional ability been included. Only participants with diabetes were included in this sub-study. Participants were asked to recall falls over the previous 12 months. It is possible that some participants may not have recalled falling; however, in a meta-analysis, falls recalled over the previous year compared favorably to more resource intensive strategies (e.g., fall diaries, weekly or monthly fall postcards) (38
). Furthermore, elevated serum PTH could be a marker for low vitamin D levels which have previously been associated with increased risk of falls (12
). A major limitation of this study is that vitamin D levels were not measured as part of this sub-study; thus, we could not distinguish between falls due to vitamin D deficiency versus falls due to elevated PTH levels. It is possible that the association between PTH and falls was driven by low vitamin D status. However, previous studies have shown an association between PTH and falls independent of vitamin D status (10
). Finally, the observational design of our study does not allow us to evaluate a causal association between serum PTH and falls. However, the association is biologically plausible as PTH may play a role in falls through several different biological mechanisms. In animal models, infusion of PTH has been shown to increase protein catabolism and decrease the number of type 2 muscle fibers, intracellular energy-rich phosphate compounds, and mitochondrial oxygen uptake (16
In summary, serum PTH was associated with incident falls among older, well-functioning diabetic men and women. An association between serum PTH and incident falls remained after adjusting for physical performance suggesting that the association between PTH and falls was not mediated by physical performance. Further investigation aimed at understanding the underlying mechanism for the association between serum PTH on muscle strength, physical performance, and falls is needed.