We found that among EPIC-Norfolk cohort participants, lower PCS scores were associated with poorer future BUA, an indicator of BMD, independent of age, BMI and other confounding factors (i.e. cigarette smoking, steroid use, lower education level, higher alcohol consumption, depression requiring treatment and HRT usage among women). However, even after adjustment for these, clear differences in BUA between those with higher and lower PCS scores remained. These findings coincide with evidence that higher physical activity in middle-late adulthood is protective against rapid deterioration in bone health [3, 6]. Another study of EPIC-Norfolk participants found that more time spent doing high-impact physical activity was strongly and positively associated with superior BUA—independent of confounding variables . Several other cohort studies have found physical capability to be a strong indicator of BMD [1, 6, 32,33,34].
Our results suggested differences in the relationship between low PCS and BUA in men compared to women. When both PCS and BUA were considered as continuous predictor and outcome variables, a strong association between PCS and BUA was evident in men but less so in women. Conversely, when both were considered as categorical variables, and relationships between self-reported physical functioning and low or very low BUA were considered, relationships were stronger in women. Changes in physical functioning in men may be a stronger marker of osteoporosis than in women; however, in this cohort few men had low or very low BUA and results were thus underpowered. As a group, women are more likely to have low BMD; hence, categorical outcomes showed a stronger relationship for women. Other studies investigating physical function measures and BUA have noticed a stark difference between sex [25, 35]. Jakes et al. found high physical activity in men conferred a BUA of around 9.5% higher than that of those who reported low physical activity. However, in women this was only 3.4% higher and similar in size to that of a 4-year age-difference . This is likely to be attributed to higher BMI in women with a sedentary lifestyle, which is protective against rapid BMD deterioration after menopause due to increased oestrogens production by adipose tissue and greater mechanical loading [25, 36]. A Brazilian study found the impact of prolonged sedentary time and lower physical activity on BMD varied greatly between men and women, with benefits of shorter bouts of sedentary behaviour only observed in women .
Our findings suggest that a PCS score of less than 1SD below the sex-specific mean was a weak to moderate predictor of low BUA, indicative of at least osteopenia. Osteopenia is BMD of 1 to 2.5SD below mean peak levels (i.e. young-adult BMD) and describes low, but not yet critical, bone mass [27, 37]. Early detection and management of osteopenia, including increased weight-bearing exercise and increasing calcium and vitamin D intake, has been demonstrated to reduce fracture risk and improve quality of life [27, 37]. When the outcome was low or very low BUA, indicative of osteopenia or osteoporosis, low PCS was demonstrated to be a strong predictor, although more so in women. This is likely to be due to increased chance of low BMD among women compared to men in the cohort. The number of individuals with very low BUA, particularly in men, was small which resulted in underpowered results.
Older adults constitute the world’s fastest growing population, making osteoporosis a significant social and economic burden . Therefore, risk assessing this population is crucial, and although fracture risk assessment tool (FRAX) is an effective tool for older white women, it is less effective among men and non-Caucasians . Furthermore, FRAX assesses fracture risk, not overall bone health status . Indicators of poorer BMD provided by self-reported physical functional health could make a significant contribution to tackle this public health issue. Furthermore, the SF-36 can potentially predict numerous measures besides BMD status, from surgical outcomes [7, 8] to risk of death from heart disease and stroke [9, 10], emphasising the versatility of the tool as a measure of health status and risk.
To our knowledge, this study is the first to investigate self-reported physical functional health as a predictor of future BMD in older adults, allowing for early identification of people at risk of suboptimal BMD potentially earlier than FRAX can . We used a large population‐based cohort with validated follow-up methods , with the ability to control for a range of confounders including sociodemographic and lifestyle factors. Data were collected prospectively, minimising potential for recall bias, and follow-up was over a critical period of time in the participants’ lives when BMD typically deteriorates. This highlighted the effectiveness of PCS score as an early, inexpensive and non-invasive indicator of BMD and therefore risk of future osteopenia and osteoporosis.
The existence of healthy volunteer bias is possible given that EPIC-Norfolk is a volunteer study consisting of predominantly white, middle-class and health-conscious individuals. However, previous literature suggest that EPIC-Norfolk sample characteristics are representative of the UK population  and mean SF-36 scores are comparable to mean scores in other population-based studies . As a secondary data analysis of an observational study, there may be unknown confounders that were not adjusted for. Potential confounders measured at baseline or 2HC may vary during follow‐up, e.g. alcohol consumption. The WHO osteopenia and osteoporosis criteria consider spine, hip or forearm BMD measures by DEXA and compare this to the young-adult BMD mean . Here we assessed BMD using CUBA which is a cheaper, safer and relatively precise alternative to DEXA, although can be limited by poor foot positioning [24, 40]. We compared the sex-specific mean which was in older, rather than younger, adults where peak BMD will have already deteriorated, potentially underestimating the number of participants with low or very low BMD [3, 41]. Furthermore, use of CUBA estimates BMD in the calcaneus, rather than the spine, hip or forearm . Although calibration of devices were regularly checked and measurements taken at least twice in each foot , random measurement error is possible. Despite these limitations, using a CUBA devise is an inexpensive and accurate method to evaluate BMD, without any radiation exposure . Time between PCS and BUA assessment varied among participants, with the shortest interval being 18 months. This relatively short time interval in some participants will result in an estimation of what current/near future BMD is, rather than predicting BMD status several years in advance.
The use of PCS score as a predictor of BMD in later life warrants further investigation. Despite having a different function to the FRAX tool, direct comparison between PCS score and FRAX is required to evaluate the effectiveness of each. SF-36 has the potential to predict many health outcomes, therefore may prove to be a useful tool in clinical practice.
Study findings indicate that self-reported physical functioning is a tool capable of predicting future BMD and identifying at-risk individuals in an apparently healthy population, especially in women. In the current climate, where increased sedentary activity and a reduction of routine medical appointments due to COVID-19 will have implications on bone health, self-reported functional health may prove a useful and inexpensive indicator to stratify populations by risk of low BMD. Further validation is required to gain insight into the role of PCS score in clinical practice.