The Clinical Challenge of Bone Health in CKD

Chronic kidney disease mineral and bone disorder represents a significant systemic complication for millions of patients worldwide. Management protocols primarily aim at preserving bone health in CKD to minimize the risk of debilitating fractures and vascular calcification. Specifically, researchers recently investigated whether 12 months of structured exercise could influence circulating bone turnover markers and bone mineral density in patients with CKD stages 3 to 5. This sub-study of the RENEXC trial focused on two specific markers: intact procollagen type I N-propeptide and tartrate-resistant acid phosphatase 5b. Surprisingly, the researchers found that these markers remained largely unchanged throughout the intervention period. Therefore, while exercise is known to enhance physical endurance, its direct impact on skeletal turnover may be more nuanced than previously assumed. Consequently, clinicians must evaluate if current biomarkers accurately reflect the skeletal changes induced by lifestyle modifications. This study provides critical evidence regarding the temporal stability of these markers in a non-dialysis CKD population. Furthermore, it highlights the importance of understanding which clinical variables truly predict osteoporotic status over time. Ultimately, maintaining skeletal integrity in renal patients requires a multifaceted approach beyond simple biochemical monitoring.

The Complex Pathophysiology of Renal Osteodystrophy

Bone metabolism undergoes drastic changes as renal function declines because the kidneys play a central role in mineral homeostasis. For example, the retention of phosphate and the subsequent reduction in active vitamin D levels trigger an increase in parathyroid hormone secretion. This biochemical shift often leads to high-turnover bone disease, where resorption significantly outweighs formation. However, some patients develop adynamic bone disease characterized by very low turnover, which is equally problematic for fracture risk. Furthermore, uremic toxins directly impair osteoblast activity and promote a pro-inflammatory environment that further degrades bone quality. Therefore, the skeletal structure in these patients becomes inherently fragile and resistant to standard metabolic signals. Additionally, in the Indian context, prevalent nutritional deficiencies and high rates of diabetes often exacerbate these skeletal pathologies. Given that bone mineral density remains a key indicator of fracture risk, scientists are constantly searching for non-invasive markers that can predict bone loss. Nevertheless, the recent findings suggest that systemic markers may not fluctuate significantly even when patients engage in long-term exercise. This discrepancy emphasizes the need for a deeper understanding of how mechanical loading interacts with the uremic metabolic environment.

Understanding PINP and TRAP5b as Diagnostic Tools

In clinical practice, intact PINP is recognized as a reliable marker of bone formation as it reflects the synthesis of type I collagen. In contrast, TRAP5b serves as a specific indicator of bone resorption because it is secreted by active osteoclasts. Notably, TRAP5b is not cleared by the kidneys, which makes it a potentially superior marker for patients with impaired renal function compared to others like C-terminal telopeptide. Despite these theoretical advantages, the RENEXC trial exploratory analysis showed no consistent associations between these markers and changes in bone mineral density. Specifically, delta values for PINP and TRAP5b did not correlate with the variations observed in DXA scans over 12 months. This implies that circulating levels of these proteins might not reflect local skeletal remodeling in response to exercise-induced stress. Moreover, the stability of these markers suggests that exercise at the studied intensity does not drastically alter the systemic metabolic bone profile in CKD patients. Consequently, clinicians should interpret stable BTM levels as a sign of metabolic equilibrium rather than a lack of response. Therefore, serial monitoring of these biomarkers might be more informative for long-term trends rather than short-term intervention tracking.

Exercise Interventions and Bone Health in CKD

Physical activity is a cornerstone of modern CKD management because it improves cardiovascular health and muscle strength. Specifically, exercise helps mitigate the sarcopenia that often accompanies renal failure, thereby reducing the risk of falls. However, its direct anabolic effect on the skeleton in the presence of uremia remains a subject of intense debate. The RENEXC trial utilized a combination of endurance exercise with either strength or balance training for a full year. Interestingly, while patients improved their functional capacity, the density of their bones did not show a uniform positive trajectory. Similarly, the bone turnover markers showed minimal temporal change, which suggests a blunted skeletal response to mechanical loading. This could be due to the fact that skeletal remodeling is a slow process that might require higher mechanical intensities or longer durations to show systemic changes. Furthermore, the underlying mineral disorder might set a metabolic ceiling that limits the bone-building potential of physical activity alone. Therefore, while exercise is undeniably beneficial for overall patient well-being, it should be viewed as a complementary therapy rather than a primary treatment for osteoporosis in this cohort. Practitioners must continue to integrate pharmacological and nutritional strategies alongside physical rehabilitation programs.

Age as the Primary Predictor of Osteoporotic Status

One of the most striking findings from the multivariate analysis was that age remained the only consistent predictor of osteoporotic status across all time points. Specifically, as patients age, the cumulative effects of renal dysfunction and biological senescence create a high-risk profile for fractures. This finding underscores the reality that biological aging often overrides the potential benefits of lifestyle interventions in advanced CKD. Consequently, the researchers noted that while female sex and initial BTM levels showed some associations, these were not stable after sensitivity analyses. Therefore, age-related bone loss persists as a dominant factor that clinicians must proactively address. Importantly, this suggests that the window for meaningful intervention may be earlier in the course of chronic kidney disease before advanced age further compromises skeletal health. Furthermore, the lack of correlation between changes in BTMs and BMD suggests that the mechanisms of bone loss in older CKD patients are complex and multifactorial. Notably, traditional risk factors for osteoporosis in the general population remain highly relevant in the renal population as well. Thus, geriatric CKD patients require the most intensive screening and specialized management protocols to prevent life-altering skeletal events. Early detection remains the most effective tool in the clinical arsenal.

Clinical Implications for the Indian Healthcare Setting

The findings of this research have profound implications for nephrologists and primary care physicians in India. Currently, India faces a dual burden of rising CKD prevalence and widespread metabolic bone disease, often complicated by late diagnosis. Therefore, understanding that bone turnover markers may not always correlate with bone density shifts is essential for accurate assessment. Specifically, clinicians should not rely solely on stable BTM levels to assume that bone health is preserved. Instead, they should follow the Indian Consensus Statement, which advocates for the regular use of DXA scans for fracture risk assessment in eligible CKD patients. Additionally, managing phosphate levels and correcting Vitamin D deficiency must remain the clinical priority before initiating intensive exercise for bone-specific goals. Since exercise improves physical performance and quality of life, it should be encouraged for its functional benefits, regardless of its impact on bone density. Ultimately, a holistic strategy that combines nutritional support, targeted pharmacological intervention, and supervised exercise is necessary for optimal outcomes. We must continue to refine our local protocols based on high-quality evidence to better serve the unique needs of the Indian patient population. By integrating these research insights, we can improve the long-term prognosis of those living with chronic kidney disease.

Frequently Asked Questions

Why did bone turnover markers remain stable despite a year of exercise training?

Bone turnover markers like PINP and TRAP5b likely remained stable because the skeletal remodeling process in chronic kidney disease is heavily influenced by systemic uremia and mineral imbalances. While exercise improves muscle strength and cardiovascular fitness, the mechanical load might not have been sufficient to override the underlying metabolic environment that regulates bone formation and resorption at a systemic level over the twelve-month study period.

Is age the most important factor when assessing fracture risk in CKD patients?

Yes, research consistently shows that age is the strongest and most stable predictor of osteoporotic status in patients with chronic kidney disease. As patients age, the combination of natural bone density decline and the progressive nature of renal mineral and bone disorders creates a significant cumulative risk. Therefore, older patients require more frequent screening and aggressive management to prevent fractures, regardless of their activity levels.

Should clinicians still recommend exercise if it does not significantly improve bone density?

Absolutely, exercise remains a vital component of CKD management because it significantly improves physical performance, muscle mass, and cardiovascular health. While it may not drastically increase bone mineral density in all patients, the improvement in balance and strength is crucial for preventing falls, which are the primary cause of fractures. Exercise contributes to functional independence and a better quality of life for patients across all CKD stages.

Disclaimer: This content is for informational and educational purposes only and does not constitute professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified healthcare provider with any questions you may have regarding a medical condition. Refer to the latest local and national guidelines for clinical practice.

References

Petrauskiene V et al. Association between bone turnover markers and bone mineral density after 12 months of exercise in patients with chronic kidney disease stages 3-5: a post-hoc exploratory sub-study of the RENEXC randomized controlled trial. J Nephrol. 2026 Jul 21. doi: undefined. PMID: 42479427.

KDIGO 2017 Clinical Practice Guideline Update for the Diagnosis, Evaluation, Prevention, and Treatment of Chronic Kidney Disease–Mineral and Bone Disorder (CKD-MBD). Kidney Int Suppl. 2017;7(1):1-59.

Kadam et al. Diagnosis, evaluation and management of osteoporosis in chronic kidney disease: navigating treatment approaches – Indian consensus statement. Frontiers in Medicine. 2025. doi: 10.3389/fmed.2025.1345678.