Inflammatory bowel diseases (IBD) include Crohn's disease (CD), ulcerative colitis (UC) and microscopic inflammatory bowel disease. They go through periods of remission and exacerbation. In CD, inflammation can be localized in any part of the gastrointestinal tract, from the mouth, through the esophagus, stomach, small intestine, large intestine, and rectum. Inflammation can affect the entire thickness of the gastrointestinal wall. In UC, inflammation usually begins in the rectum and spreads proximally to the large intestine. The nature of inflammatory changes is continuous and involves the mucosa and submucosa (1, 2).

IBDs are characterized by recurrent immune disorders involving both anti-inflammatory and pro-inflammatory factors. An abnormal inflammatory response can lead to a range of clinical manifestations (3). The goal of treatment is to achieve and maintain deep remission (healing of the intestinal mucosa), thus reducing the risk of the need for surgical treatment. When considering the implementation of appropriate treatment, the activity and severity of the disease course, the response to previously used medications, and concomitant extraintestinal symptoms, among other factors, should be taken into account. Treatment modalities can be divided into pharmacological and surgical. An adjunctive component of primary treatment is nutritional therapy (4). The most common pharmacological treatment is based on traditional drugs (5-ASA, Sulfasalazine) and biological treatment based on the use of anti-TNF-α drugs (3).

The prevalence of low bone density in patients with CD and UC is about 42% (5). IBD has a higher risk of reduced bone density than the general population. This is due in part to the prolonged active phase of the disease. During the occurrence of inflammation, cytokines are produced that enhance bone resorption (increase RANKL—RANK receptor ligand to OPG—osteoprotegerin). During prolonged remission of the disease, patients may increase bone mineral density (BMD) (6). Patients with IBD often have progressive clinical symptoms due to worsening nutritional deficiencies, the treatment used, the inflammation present, or a genetic predisposition (7, 8). After achieving remission in patients with IBD, without other risk factors for osteoporosis, bone turnover markers should return to normal. However, whether this is also the case in patients treated with TNF-alpha blocking drugs remains an open question. It is known that in terms of preventing bone loss, anti-TNF drugs do not have an advantage over traditional IBD treatment (9, 10). However, more recent data indicate that anti-TNFα therapy is associated with increased bone turnover markers and decreased BMD in the femoral neck (11).

The aim of the study was to determine whether bone mineral density and selected regulatory molecules [OC (osteocalcin), PTH (parathyroid hormone), SOST (sclerostin), FGF23 (fibroblast growth factor 23), DKK1 (Dickkopf-related protein 1), OPG, OPN (osteopontin)] in patients with IBD during remission maintenance treatment are normalized or remain outside normal limits.

There were no significant differences between the groups (CD, UC and the control group) in terms of age, height, weight, body mass index (BMI) and, basic laboratory assessments like hemoglobin, high sensitivity C-reactive protein (hs-CRP) and calprotectin. The mean time to disease diagnosis was comparable between CD and UC (Supplementary material).

All patients had no clinical symptoms of CD or UC followed by normal values of inflammatory markers, which confirmed remission of the disease. Moreover, within the last 6 months, all patients passed endoscopic examination which also confirmed endoscopic remission. The criteria for CD remission were a simple Endoscopic Score for Crohn's Disease (SES-CD) of no greater than 4, with at least a 2-point reduction compared to the baseline of induction and no subscore greater than 1 (17), and for UC as a Mayo endoscopy Subscore (MES) (18).

A detailed post-hoc analysis (Dunne's test with Bonferroni correction) revealed different patterns of significance for the markers studied. OPN values were significantly higher in the CD and UC groups compared to the control group (p < 0.001). In addition, statistically significant differences between groups were observed in OC levels (p = 0.028). The other markers (DKK1, OPG, SOST, PTH, and FGF23) did not show statistically significant differences (Table 1).

There were no significant differences in L2–L4 BMD, L2–L4 T-score, L2–L4 Z-score, Total Z-score, and BMC values between all study groups. Only the total T-score showed a significant difference between the groups (p = 0.005), with the highest values in the control group (Table 2).

Analysis of the impact of treatment type (anti-TNFα vs. conventional therapy) on bone turnover markers showed a statistically significant difference only in the case of OC. Patients treated conventionally had significantly higher median OC concentrations compared to patients receiving biological treatment (p = 0.041). No significant differences were observed between the subgroups for the other biochemical markers (p > 0.05; Table 3).

There were no differences in L2–L4 BMD, L2–L4 T-score, L2–L4 Z-score, Total T-score, Total Z-score, and BMC values between the biologically and conventionally treated groups (Table 4).

The only significant correlation in patients treated with anti-TNFα was a negative correlation between FGF23 concentration and BMC (rho = −0.36; p < 0.05). However, no significant correlations were found between the other bone markers (DKK1, OPG, OC, OPN, SOST, and PTH) and bone density indices in this subgroup. Within the biochemical markers themselves, a statistically significant positive correlation was observed between DKK1 protein and OPG (rho = 0.41; p < 0.05) and a significant negative correlation between SOST and OC (rho = −0.43; p < 0.05; Table 5).

In the group of patients treated conventionally, a negative correlation was found between FGF23 and BMC (rho = −0.59; p < 0.05). In addition, a strong positive correlation was observed between PTH and SOST (rho = 0.61; p < 0.05). However, no direct correlations were found between the other markers (DKK1, OPG, OC, and OPN) and bone density indices (Table 6).

Chronic inflammation persistent in IBD patients may significantly impact bone turnover. Although it was shown in both CD and UC that bone loss is associated with the action of pro-inflammatory (19–21). However, patients with CD at higher risk of skeletal pathology compared to those with UC (22). Bone marrow TNF-α cells show the ability to promote osteoclastogenesis and excessive bone resorption (23). CX3CL1 expression can promote M1 macrophage polarization and osteoclast differentiation, while inhibition of CX3CL1 expression can reduce inflammation, thereby reducing the predisposition to reduced bone density (24, 25). IBD treatment aims to reduce inflammation, resulting in reduced bone resorption and increased BMD (26, 27). Treatment with anti-TNFα through various mechanisms affects the activity of both osteoclasts and osteoblasts (28).

In our study, BMD (lumbar) and BMC values were similar between the study groups, with the exception of the T-score, which was lower in patients with IBD, especially in CD. Similar results were obtained by Cortés-Berdonces et al. (29) which confirms a certain regularity. However, no differences in BMD and BMC were observed between patients depending on the therapy used, although some data suggest an improvement in BMD after anti-TNFα treatment (30, 31). As shown by Pazianas et al. (31), an increase in BMD was observed mainly with the simultaneous use of infliximab and bisphosphonates, while infliximab treatment alone did not affect BMD.

The most pronounced difference between the groups was elevated OPN levels in patients with CD and UC compared to controls. Although the results of the studies are inconsistent, some authors confirm similar observations (32, 33). Furthermore, Mishima et al. (34) found a positive correlation between serum OPN levels and clinical activity indices in patients with IBD, and therefore propose that OPN be used as a clinical marker of disease activity. OPN reflected inflammation in the colon and rectum, as its plasma levels were reduced and correlations completely disappeared after proctocolectomy (34). In our study, patients were in remission, which may explain the absence of such correlations. However, elevated OPN may reflect increased bone cell activity and constitute a late marker of bone formation, which does not necessarily normalize after anti-TNFα therapy, despite reduced disease activity (35).

With regard to OC, OPG, SOST, PTH, and FGF23, no significant differences were found between patients with CD, UC, and controls, which may indicate a beneficial effect of maintaining remission on bone metabolism parameters. OPG, as an inhibitor of bone resorption by blocking RANKL-RANK interaction (36, 37), should theoretically be modulated by TNF-α and anti-TNFα treatment (38, 39). However, the literature is inconsistent: some studies indicate lower OPG levels (22, 40), others higher in selected groups (41–43). Miheller et al. (44) demonstrated a decrease in OPG after anti-TNFα treatment, whereas in our study, the levels were similar between the groups, with slightly lower values in CD. It is likely that the diagnostic value of OPG may be limited in the assessment of bone metabolic activity in this group of patients

With regard to the Wnt pathway, DKK-1 plays a role in the pathogenesis of CD and bone metabolism (45, 46). The reduction in DKK-1 in pediatric patients treated with anti-TNFα (45) indicates the potential involvement of this pathway in epithelial regeneration and bone remodeling. In our study, DKK-1 levels did not differ significantly between treatment groups, which may reflect the complex regulation of this molecule. The observed positive correlation of DKK-1 with OPG in patients treated with anti-TNFα can be interpreted as a compensatory element in response to chronic inflammation. Similarly, SOST, which acts by inhibiting the Wnt pathway (47), showed no differences between treatment groups, although its values were lower in CD, consistent with other reports (45, 48). Anti-SOST antibody therapy has the potential to restore bone mass (46), but in our study, OC, which is functionally related to osteoblast activity, did not differ significantly between groups and was lower in patients treated with biologics, although the literature indicates a possible increase in OC after anti-TNFα therapy (44, 49).

Furthermore, OC was negatively correlated with SOST in this group of patients, which may suggest that higher SOST activity may be associated with inhibition of osteoblast activity. However, reduced values of these parameters may suggest an imbalance in bone metabolism resulting from chronic inflammation. Marini et al. (50) indicate that treatment with monoclonal antibodies against SOST may promote bone mass restoration, thus preventing bone fractures caused by bone fragility. PTH has been shown to increase calcium reabsorption and inhibit phosphate reabsorption (51).

With regard to PTH, we observed negative correlations between PTH and some DXA parameters, in line with previous reports (52). However, no changes in PTH were found depending on the therapy, which is consistent with some studies (53), although others suggest an increase in OC after anti-TNFα therapy (54). FGF23, synthesized by osteocytes and osteoblasts, may correlate with inflammation and bone turnover (55). Exacerbation of IBD is associated with elevated FGF23 and lower BMD (56). In our study, we observed a negative correlation between FGF23 and BMD in patients treated with anti-TNFα, consistent with previous observations (56). At the same time, its levels did not differ between therapies, and the positive correlation of FGF23 with BMC may indicate secondary, compensatory changes in the hormonal axis (57–59).

The results of our study confirm the presence of bone metabolism abnormalities, which may be present during clinical remission. Therefore, it is worth considering a multimineral supplement for such patients with IBD (60). Biological treatment did not have a clear effect on most of the bone markers we studied, suggesting a varied response to therapies and multifactorial regulation of bone metabolism in IBD. This points to the need for further studies evaluating the long-term effects of chronic inflammation and the treatment used on the skeletal system, taking into account the clinical stage of the disease and the differences between CD and UC. However, individual conclusions cannot be extrapolated to the entire remission.

Our study also has certain limitations. It was a cross-sectional study, and all biochemical and densitometric measurements were taken at a single point in time, during remission of the disease. Therefore, the results obtained reflect the state of bone turnover during maintenance treatment and do not allow for the assessment of changes over time or for conclusions to be drawn about the impact of therapy. In the future, it would also be useful to include information on the individual clinical data of all participants, including detailed disease activity indices (e.g., the Mayo scale) and a complete history of supplementation. The lack of this information may have been an uncontrolled confounding factor affecting the assessed markers of bone turnover. In addition, the study did not measure reference markers of bone turnover, such as P1NP and CTX, due to technical limitations and the availability of laboratory methods. This limits the ability to directly compare the results obtained with other studies using these standard parameters.

Our results indicate that the choice of anti-TNFα therapy should not be based on the expected differential effects of individual drugs on bone metabolism. Decisions regarding biological therapy should therefore continue to focus primarily on controlling disease activity, while bone status requires parallel and independent assessment and monitoring. In this context, bone turnover markers can serve as complementary tools for identifying patients who require more intensive monitoring or additional supportive interventions to prevent long-term bone complications.