To our knowledge, this is the first two-sample MR study to analyze the bidirectional causal relationship between osteoporosis and stroke. In this study, we used bidirectional MR to test the causal relationship between osteoporosis and the risk of stroke, IS, LV-IS, SV-IS, CE-IS, and ICH. Using the largest publicly available GWAS summary data, we found no bidirectional causal relationship between genetically predicted BMD and the increased risk of stroke and its subtypes. Sensitivity analyses supported the reliability of our results.
There is a complex interaction between osteoporosis and stroke, with previous studies indicating that patients with osteoporosis have a higher risk of stroke [29]. The close relationship between osteoporosis and stroke raises the question of whether osteoporosis is a marker rather than a risk factor for stroke. However, our results suggest that osteoporosis does not have a direct causal relationship with stroke but rather serves as a marker. They share common risk factors, pathophysiological pathways, and genetic factors.
Systemic inflammation is a common risk factor for both osteoporosis and arterial diseases. Osteoporosis is associated with systemic inflammation, with elevated levels of inflammatory markers such as C-reactive protein (CRP), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) [30,31]. This chronic inflammatory state can promote the differentiation and maturation of osteoclasts, leading to accelerated bone resorption and osteoporosis. On the other hand, current evidence suggests that inflammation may play a crucial role in the development of stroke [32,33]. Inflammation is also important in the development and progression of peripheral artery disease . Elevated levels of circulating CRP, IL-6, and TNF-α are associated with a high risk of peripheral artery disease [34,35]. Thus, dysregulation of inflammatory signaling pathways may represent a common mechanism between osteoporosis and stroke.
The connection between osteoporosis and stroke is also linked through vitamin D. Studies have shown that vitamin D receptors are expressed in cardiomyocytes, vascular smooth muscle cells, and endothelial cells [36]. Vitamin D can reduce oxidative stress in endothelial cells and inhibit the formation of atherosclerotic plaques [37,38]. A meta-analysis of limited existing data indicated that moderate to high doses of vitamin D supplementation could reduce cardiovascular risk. Epidemiological studies have suggested that vitamin D deficiency is associated with an increased risk of peripheral artery disease [39]. The link between osteoporosis and stroke may be mediated through the common role of vitamin D.
BMD is a measure of lifelong estrogen exposure. Bone degeneration results from age-related changes in other organs, particularly due to the decline in ovarian function in postmenopausal women. The decrease in estrogen levels after menopause is associated with other diseases, such as cardiovascular disease. BMD can be influenced by estrogen levels and physical activity, particularly in postmenopausal women, and is directly related to arterial calcification [40,41]. The observed age and gender differences in the relationship between BMD and stroke risk may stem from differences in endogenous estrogen levels, which can influence cardiovascular risk [42]. Menopause increases the risk of osteoporosis and cardiovascular diseases, such as atherosclerosis [43,44]. Thus, decreased estrogen levels may simultaneously contribute to both osteoporosis and stroke.
Lack of physical activity is also considered a risk factor for peripheral artery disease [45]. This may be because insufficient physical activity is associated with increased oxidative stress and endothelial dysfunction [46]. At the same time, a lack of physical activity accelerates bone resorption, increasing the risk of osteoporosis [47]. Therefore, it is a common risk factor for both osteoporosis and peripheral artery disease, which could explain the link between the two.
In the opposite direction, the relationship between stroke and osteoporosis is also of interest in the medical field, with various hypotheses proposed to explain their connection. Although we did not find a genetic causal relationship between stroke and osteoporosis, previous observational studies have shown a strong correlation between stroke and osteoporosis [15]. A cohort study found that patients with hemorrhagic stroke had a 2.06 times higher risk of osteoporosis (95% CI 1.83-2.33), and patients with ischemic stroke had a 1.77 times higher risk of osteoporosis (95% CI 1.65-1.89) compared to non-stroke cohorts [48]. Based on our MR study results, which found no genetic causal relationship between stroke and osteoporosis, we believe that the clinical association observed is likely due to secondary effects of stroke rather than an intrinsic direct causal relationship. Many underlying mechanisms lead to bone loss after a stroke, mainly including decreased mobility, vitamin D deficiency, and increased inflammation.
Reduced mobility is one of the main reasons for osteoporosis after a stroke. Evidence over the past few decades suggests that stroke survivors experience a significant decrease in BMD, especially in paralyzed limbs [49,50]. The difference in BMD between paralyzed and non-paralyzed limbs after a stroke can be attributed to various factors, primarily reduced mobility and mechanical loading[51]. Immobilization and muscle weakness promote bone resorption and hinder bone formation, leading to rapid bone loss [52]. Notably, some studies have shown that BMD actually increases on the non-paralyzed side, possibly due to compensatory physical activity on the unaffected side [53]. Yamada et al. reported an increase in lumbar spine BMD in 100 immobilized stroke patients [54]. A meta-analysis showed reduced BMD in the femoral neck of the non-paralyzed side, but no reduction in distal radius BMD of the non-paralyzed side [55]. The independence of upper limb activity might mitigate bone loss through compensatory use of the non-paralyzed side in daily tasks [56,57]. These studies support our findings that there is no direct, intrinsic causal relationship between stroke and osteoporosis. Stroke affects osteoporosis through secondary effects.
Another major factor affecting bone health after a stroke is calcium homeostasis, particularly vitamin D deficiency. Older individuals are less exposed to sunlight, and their skin has a reduced ability to produce vitamin D precursors [58]. Stroke patients are at higher risk because they are more likely to stay indoors, thus receiving less sunlight exposure and having poorer dietary intake [59]. Poole et al. found that most acute stroke patients had vitamin D deficiency throughout the year [60]. This also results from secondary effects post-stroke rather than a direct impact of stroke on osteoporosis.
However, there are some limitations of our study.Selecting SNPs from different large-sample GWAS studies may increase the risk of sample overlap between exposure and outcome variables, potentially biasing the results. Additionally, due to a lack of suitable SNPs for ICH, we were unable to explore the potential causal effect of ICH on osteoporosis. Finally, the MR study population was of European descent. Whether these results can be replicated in Asian populations remains to be explored.
In conclusion, our study found no causal relationship between osteoporosis and stroke in any direction. Based on our findings, although there is no causal relationship, they share similar pathophysiological mechanisms and pathways.