T2DM is a global health problem characterized by irregularity of carbohydrate, lipid, and protein metabolism resulting from impaired insulin secretion, insulin resistance, or a combination of both.^[1] Serum sclerostin levels are elevated in patients with T2DM. This increase is thought to be associated with disease duration and blood glucose bad control (HbA1c), and this molecule is at least a potential mediator in the development of diabetes-related bone disease.^[2] Sclerostin is the key molecular coordinator of both bone formation and bone resorption. Irregularity of sclerostin expression forms the basis of pathophysiology in skeletal disorders characterized by loss of bone mass and the damaging effects of some cancers on bone tissue.‌^[‌3] Immunohistochemical approaches state that sclerostin is expressed in osteocytes in human and rodent bones.^[4] Vitamin D is very important for bone metabolism. Vitamin D deficiency may cause abnormalities in calcium and phosphorus levels.^[5] Oxidant-antioxidant balance is important for the homeostasis of an organism. Oxidative stress occurs when this balance is disrupted. Oxidative stress involves macromolecular oxidative damage, induces tissue protein denaturation, DNA damage, and lipid peroxidation, and interferes with the normal metabolic activity of the body, leading to the emergence and/or development of diseases.^[6] Information on the role of sclerostin in the pathogenesis of T2DM in humans remains insufficient, and the relationship between sclerostin and oxidative stress remains unclear.

This study aims to compare the relationship between serum sclerostin level and oxidative status, HbA1c, folate, B12, ferritin, vitamin D, Ca, P, Mg in T2DM patients and healthy people.

The mean age, BMI, and biochemical results of the 90 people included in our study are given in Table 1. There was no significance when Ca (9.49±0.59 mg/dl; min: 8.6, max: 11.2), P (3.64±1.02 mg/dl; min: 2.9, max: 5.03), Mg (2.04±0.31 mg/dl; min: 1.56, max: 3.15), B12 (624±290 pg/ml; min: 202, max: 1184), folate (9.99±5.5 ng/ml; min: 5.43, max: 23.5), and vitamin D (24.1±11.8 µg/L; min: 7.2, max: 77.69) levels of the T2DM group and Ca (9.25±0.35 mg/dl; min: 1.5, max: 10.6), P (3.30±0.67 mg/dl; min: 2.6, max: 4.5), Mg (2.11±0.15 mg/dl; min: 1.9, max: 2.5), B12 (683.9±155.3 pg/ml; min: 321, max: 900), folate (11.6±3.03 ng/ml; min: 7, max: 17), and vitamin D (25.2 ±6.5 µg/L; min: 11.2, max: 41.94) levels of the control group were compared. The TAS (0.88±0.29 mmol Trolox equivalent/l min: 0.16, max: 1.39) and albumin (3.75±0.5 g/dl; min: 3.80, max: 5) levels of the T2DM group were significantly lower than the TAS (1.88±0.24 mmol Trolox equivalent/l min: 1.29, max: 2.40) and albumin (4.6±0.61 g/dl; min: 3.6, max: 5.2) levels of the control group (Figs 1, 2).

The HbA1c (10.58±3.22%; min: 6.83, max: 14) and TOS (402.8±150.7 μmol; H₂O₂ equivalent/l min: 70.93, max: 633.9) levels of the T2DM group were significantly higher than the HbA1c (3.92±0.63%; min: 2.21, max: 5.23) and TOS (41.2±25.03 μmol; H₂O₂ equivalent/l min: 16.1, max: 143.2) levels of the control group (p<0.05). Sclerostin level (168.4±74.9; pg/ml min: 31.59 max: 283) was found to be significantly higher in the T2DM group than in the control group (121.09±71.3; min: 20.32, max: 256) (p<0.05) (Table 1, Fig. 3).

Finally, there was a significant positive correlation between HbA1c values of the groups and TOS and sclerostin levels (r=0.77, p=<0.01; 0.23, p=0.026). There was a significant negative correlation between the HbA1c and TAS levels of the groups (r=−0.77, p=<0.01). There was a significant negative correlation between Mg values and TOS values of the groups (r=−0.23, p=0.03). There was a positive correlation between TAS values whereas there was a negative correlation between folate values and TOS values of the groups (r=−0.30, p=0.004; r=0.30, p=0.005) (Table 2).

This is the first study to reveal the relationship between serum sclerostin levels and oxidative stress and other biochemical outcomes in T2DM patients and healthy people. Our study showed that high sclerostin levels were associated with oxidative stress in patients with T2DM. T2DM is a metabolic disorder characterized by hyperglycemia with pathophysiological factors resulting essentially from the combination of insulin resistance and insufficient insulin secretion.^[9] Sclerostin responds to the mechanical stress applied to the skeleton and plays an important role in regulating the reshaping of the bone. Sclerostin, which is largely expressed by the SOST gene in osteocytes from bone cells, is a small protein. In addition, sclerostin has been reported to act at a certain distance to regulate adipocytes, energy homeostasis, and mineral metabolism in the kidney.^{[10, 11]} In addition, immunohistochemical studies in human and rodent bones have shown that sclerostin is not expressed in osteoblasts or lining cells.^[12] Lack of sclerostin expression or secretion in humans causes hereditary, high bone mass formation characterized by exaggerated bone formation such as sclerosteosis, Van Buchem disease, and craniodiaphyseal dysplasia.^[13] Sclerostin quickly became a promising molecular target for the treatment of osteoporosis and other skeletal diseases. Useful skeletal results were observed in animal studies and clinical trials using neutralizing antibodies to sclerostin.^[3] Compounds that inhibit sclerostin have been shown to stimulate bone formation, reduce osteoporosis, and induce robust increase in bone mineral density.^[10] Sclerostin levels have been positively associated with abdominal fat, dyslipidemia, especially HDL and LDL cholesterol levels, and HbA1c. This suggests that sclerostin plays a role in the pathogenesis of the metabolic disease. ^{[14, 15]} Serum sclerostin levels in our study were found to be higher in the T2DM group than those in the control group (Fig. 1). Significantly higher serum sclerostin levels were detected in male and female patients with T2DM compared to non-diabetic controls in previous studies.^{[2, 16, 17]} Circulating sclerostin levels have been shown to increase and sclerostin levels have been reported to be positively associated with fasting glucose production and insulin resistance measurements even in the case of prediabetes when insulin resistance and insulin secretion first change.^[18] Higher sclerostin levels are produced by osteoclast cultures formed from the old bone marrow compared to young mice, suggesting that sclerostin produced from the osteoclast may contribute to a decrease in bone formation caused by aging.^[19] There are contradictions in the reports revealing the relationship between serum sclerostin and bone mass even though most studies agree that circulating sclerostin increases with age.^[20–22] It is associated with metabolic syndrome and diabetes in humans, similar to mice overexpressing sclerostin. The relationship between sclerostin and HbA1c and diabetes duration has been previously demonstrated and can be explained by strengthening the process of overproduction of reactive oxygen species (ROS) and the formation of advanced glycation end products (AGES) as a result of varying glucose metabolism.^[23] High levels of glucose in T2DM lead to the accumulation of advanced glycosylation end products (AGEs) in the organic bone matrix by a process known as non-enzymatic glycation. HbA1c is a common example of early-stage glycation.^[24] High glucose concentrations cause oxidative stress. Permanent hyperglycemia and increased oxidative stress play an important role in the development of secondary diabetic complications.^[25] Oxidative stress is the imbalance between antioxidants and oxidants. Excessive cellular levels of ROS damage proteins, nucleic acids, lipids, membranes, and organelles. This damage has been associated with various diseases such as diabetes.^{[26, 27]} The main sources of oxidative stress are mitochondria in diabetes mellitus. Production of high glucose mitochondrial oxygen radicals is thought to be an important factor underlying diabetes complications.^{[28, 29]} In addition, another study reported that glucose and its metabolites react with hydrogen peroxide in the presence of iron and copper ions, forming hydroxyl radicals during auto-oxidation in diabetes and thus supporting ROS formation and the development of diabetic complications.^[27]TOS is usually used to detect the general oxidation status of the body whereas the TAS is used to detect the general antioxidant status of the body.^{[7, 8]} The T2DM group was found to have low TAS compared to the control group whereas TOS was found to be high in our study (Figs 2, 3). Our results are consistent with some previous studies. Picu et al. investigated the values of the parameters characterizing the oxidant/antioxidant balance of T2DM patients and reported that TOS levels were higher in patients with T2DM compared to healthy individuals.^[30] Vitamin D is a steroid hormone that plays a role in the regulation of musculoskeletal function.^[31] Vitamin D is a key molecule in calcium and phosphate homeostasis.^[32] Vitamin D deficiency affects osteocytes and changes serum sclerostin levels. An inverse relationship was reported between serum 25-OHD and sclerostin levels in healthy postmenopausal women in one study.^[22] Cidem et al. investigated the effects of vitamin D3 treatment on serum sclerostin levels in young adult women with significant vitamin D deficiency. Twenty-six subjects were treated orally with calcium (1,200 mg/day for 2 months) and vitamin D3 (300,000 IU/week for 1 month). They found that sclerostin levels decreased significantly due to the increase in vitamin D when they examined post-treatment sclerostin levels.^[33] However, sclerostin levels were higher in the T2DM group even though there was no significant difference between vitamin D, Ca, P values of the T2DM group and vitamin D, Ca, P values of the control group. In addition, we found that high sclerostin levels were not associated with vitamin D, Ca, P levels in T2DM patients. Furthermore, the high TOS value in patients with T2DM suggested that this might be associated with high sclerostin levels. Several studies on ROS, oxidative stress, and bone have hypothesized that aging-induced oxidative stress antagonizes the Wnt signaling pathway leading to reduced bone formation.^[34]

This study showed that serum sclerostin levels were elevated in patients with T2DM and that high sclerostin levels were also associated with oxidative stress. We determined a positive relationship between high oxidative stress and high sclerostin levels in patients diagnosed with T2DM. It was determined that high sclerostin levels may be a stress factor in the pathophysiological process of T2DM.