**3. Vitamin D axis in renal disease: clinical implications**

#### **3.1. CKD-MBD: biochemical abnormalities**

Monitoring biochemical indicators in CKD should start at CKD stage 3 (GFR below 60 ml/min). The most important indicators are serum calcium, phosphate, PTH, and alkaline phosphatase. 25VD should be tested too, with further testing should be considered according to initial levels and the need for supplementation. Generally, laboratory trends are more important than single values and monitoring should be tailored to CKD stage and presence of active treatment. The suggested KDIGO frequencies for monitoring calcium, phosphate, 25VD, and PTH are shown in **Table 4**.


**Table 4.** Suggested frequency for monitoring biochemical abnormalities in renal disease [19, 21].

#### **3.2. CKD-related mineral bone disease (CKD-MBD)**

As it has already been mentioned, pathological changes in VD metabolism are present in the early CKD stages. Thus, the majority of renal patients have CKD-related bone disease, significantly higher fracture risk and higher morbidity and mortality compared to the general population are detected in patients with GFR < 60 ml/min/1.73 m2 . Bone pathology is one of the most extensively studied complications in CKD. Often CKD-related bone changes overlap with age-related and postmenopausal osteoporosis, making the picture even more complicated.

#### *3.2.1. Bone histology in CKD*

cellular membrane, thus placing the VDR-VD axis not only in the middle of genomic effects

In uremia, significant changes in VDR function occur. Low levels of 1,25VD lead to downregulation of VDR expression [36]. In addition, in areas of nodular growth in the parathyroid gland reduced VDR content was detected [37]. In CKD, there is a significant decrease in VDR-RXR binding to vitamin D reacting elements, as well as reduced RXR content in the parathyroid glands of uremic animal models, explaining increased PTH levels without the presence of hypocalcemia and hyperphosphatemia [38]. Hypocalcemia in advanced renal failure increases the parathyroid levels of calreticulin, a cytosolic protein that binds the DNAbinding domain of nuclear receptors, thus blocking VDR-mediated transactivation [39]. Higher levels of inflammatory cytokines were found to be associated with impaired binding of VDR-RXR to vitamin D reacting elements, contributing to vitamin D resistance in patients on hemodialysis [40]. Finally, hypocalcemia in CKD suppresses the calcium-sensing receptor (CaSR) in the parathyroid glands, which in turn downregulates parathyroid VDR expression. Stimulating CaSR by increasing extracellular calcium or by using calcimimetics upregulated

Monitoring biochemical indicators in CKD should start at CKD stage 3 (GFR below 60 ml/min). The most important indicators are serum calcium, phosphate, PTH, and alkaline phosphatase. 25VD should be tested too, with further testing should be considered according to initial levels and the need for supplementation. Generally, laboratory trends are more important than single values and monitoring should be tailored to CKD stage and presence of active treatment. The suggested KDIGO frequencies for monitoring calcium, phosphate, 25VD, and PTH are

**Indicator CKD stage 3 CKD stage 4 CKD stage 5 and 5D**

As it has already been mentioned, pathological changes in VD metabolism are present in the early CKD stages. Thus, the majority of renal patients have CKD-related bone disease, significantly higher fracture risk and higher morbidity and mortality compared to the

Calcium and phosphate 6–12 months 3–6 months 1–3 months PTH and alkaline phosphatase Baseline 6–12 months 3–6 months 25-Hydroxyvitamin D Baseline Baseline baseline

**Table 4.** Suggested frequency for monitoring biochemical abnormalities in renal disease [19, 21].

**3.2. CKD-related mineral bone disease (CKD-MBD)**

but also in activation of rapid transmembrane pathways [35].

**3. Vitamin D axis in renal disease: clinical implications**

VDR expression in rat models [41].

28 A Critical Evaluation of Vitamin D - Clinical Overview

shown in **Table 4**.

**3.1. CKD-MBD: biochemical abnormalities**

Bone histology in CKD represents a broad spectrum of pathological changes, which are classified according to bone turnover, mineralization, and bone volume (TMV classification) [42].

Bone turnover (T) is a parameter, corresponding to bone formation rate (BFR). BFR can be abnormally low, normal, or very high. BFR is best assessed via bone biopsy and tetracycline labeling. Other measurements that can be used for estimating BFR are osteoblastic surface, the number of active osteoblasts, and the osteoid surface, but none is as accurate as tetracycline testing [43].

Mineralization (M) is the second parameter. Normally, the osteoblasts lay down new collagen and direct mineralization of the matrix. This process is impaired in CKD, leading to thickened osteoid. Mineralization is measured by osteoid maturation time and mineralization lag time. The osteoid maturation time is the osteoid width divided by the distance between labels per day. The mineralization lag time is the osteoid maturation time adjusted for the percentage of osteoid surface that has a tetracycline label [21, 43]. Mineralization is classified as normal and abnormal. A typical example of abnormal mineralization is osteomalacia (OM), where increased osteoid volume, increased osteoid maturation time, or increased mineralization lag time are detected.

Bone volume (V) sums up bone formation and resorption rates. It is generally accepted that bone volume is expressed as bone volume per tissue volume and ranges between low and high. There are different ways to measure bone volume—dual-energy X-ray absorptiometry (DEXA) and quantitative computed tomography (QCT). These two methods, however, have disadvantages in cases with low mineralization, as they reflect primarily bone density. In addition, there are differences in the microstructure in different diseases. In idiopathic osteoporosis, both cortical and cancellous bone volumes decrease, whereas in hemodialysis patients the cortical compartment is reduced, but the cancellous one is increased [44]. Bone volume is classified as low, normal, and high [42].

Evidently, the above-mentioned parameters are linked between each other and can be most accurately assessed via bone biopsy. The procedure, however, has its disadvantages. First, its invasive character is associated with pain, bleeding, and infection. The more serious problem is the representativeness of the obtained sample. Finally, few centers can use the bone-specific histomorphological staining techniques. Hence, though recommended by the KDIGO group, bone biopsy is not routinely performed in everyday renal practice. There are several indications for bone biopsy [42] (**Table 5**).


**Table 5.** Suggested indications for bone biopsy.

Once the individual parameters for TMV are obtained, personal graph can be formed.

Osteomalacia is currently described as a low-turnover bone with abnormal mineralization. The bone volume may be low to medium, depending on the severity and duration of the process and other factors that affect bone. Adynamic bone disease (AD) is currently defined as low-turnover bone with normal mineralization. Bone volume can be at the lower end of the spectrum, but in some patients with normal mineralization and low turnover it will be normal. Mild HPT (mild hyperparathyroid-related bone disease) and osteitis fibrosa (or advanced hyperparathyroid-related bone disease) are currently used as distinct categories, but in actuality represent a range of abnormalities along a continuum of medium to high turnover, and any bone volume depending on the duration of the disease process. Mixed uremic osteodystrophy is variably defined internationally, but generally it represents with combination of the above-mentioned biopsy findings. For example, it can present as high-turnover, normal bone volume, with abnormal mineralization [42].

In summary, the TMV classification system more precisely describes the range of pathologic abnormalities that can occur in patients with CKD. However, due to the difficulties in performing bone biopsy, the older clinical classification is still used, based on bone turnover high-turnover mineral bone disease (HTMBD) and low-turnover mineral bone disease (LTMBD). The KDIGO work group currently does not support the use of DEXA measurement of bone density and bone-derived turnover markers of collagen synthesis and breakdown in GFR below 60 ml/min/1.73 m2 as these parameters do not predict the type of renal osteodystrophy. In contrast, in patients with CKD stages 3–5D, measurements of serum PTH or bonespecific alkaline phosphatase can be used to evaluate bone disease because markedly high or low values predict underlying bone turnover [21].

#### *3.2.2. High-turnover mineral bone disease (HTMBD)*

The typical histological substrates of HTMBD are osteitis fibrosa and mild HPT-related bone disease. The prevalence of HTMBD has remained stable over the last years, ranging between 40 and 50% in CKD patients [45].

Generally, HTMBD is asymptomatic and is preceded by laboratory and X-ray changes. Clinical presentation is usually not typical and consists of nonspecific pain in the lower back, hips, and legs, aggravated by weight bearing. Bone fractures and bone deformities are also common. In addition, symptoms associated with impaired calcium levels (hypocalcemia or hypercalcemia) are present. Extraskeletal calcifications are commonly detected, as well as pruritus. Rarely calciphylaxis (calcific uremic arteriolopathy) is detected. Increased mortality is present.

**•** inconsistencies among biochemical parameters

30 A Critical Evaluation of Vitamin D - Clinical Overview

**•** unexplained hypercalcemia and hypophosphatemia

**•** before parathyroidectomy if there has been significant exposure to aluminum in the past or if the results of biochemical

Once the individual parameters for TMV are obtained, personal graph can be formed.

Osteomalacia is currently described as a low-turnover bone with abnormal mineralization. The bone volume may be low to medium, depending on the severity and duration of the process and other factors that affect bone. Adynamic bone disease (AD) is currently defined as low-turnover bone with normal mineralization. Bone volume can be at the lower end of the spectrum, but in some patients with normal mineralization and low turnover it will be normal. Mild HPT (mild hyperparathyroid-related bone disease) and osteitis fibrosa (or advanced hyperparathyroid-related bone disease) are currently used as distinct categories, but in actuality represent a range of abnormalities along a continuum of medium to high turnover, and any bone volume depending on the duration of the disease process. Mixed uremic osteodystrophy is variably defined internationally, but generally it represents with combination of the above-mentioned biopsy findings. For example, it can present as high-turnover,

In summary, the TMV classification system more precisely describes the range of pathologic abnormalities that can occur in patients with CKD. However, due to the difficulties in performing bone biopsy, the older clinical classification is still used, based on bone turnover high-turnover mineral bone disease (HTMBD) and low-turnover mineral bone disease (LTMBD). The KDIGO work group currently does not support the use of DEXA measurement of bone density and bone-derived turnover markers of collagen synthesis and breakdown in

trophy. In contrast, in patients with CKD stages 3–5D, measurements of serum PTH or bonespecific alkaline phosphatase can be used to evaluate bone disease because markedly high or

The typical histological substrates of HTMBD are osteitis fibrosa and mild HPT-related bone disease. The prevalence of HTMBD has remained stable over the last years, ranging between

as these parameters do not predict the type of renal osteodys-

determinations are not consistent with advanced secondary or tertiary hyperparathyroidism

**•** suspicion of overload or toxicity from aluminum

**•** before beginning treatment with bisphosphonates

normal bone volume, with abnormal mineralization [42].

low values predict underlying bone turnover [21].

*3.2.2. High-turnover mineral bone disease (HTMBD)*

**Table 5.** Suggested indications for bone biopsy.

GFR below 60 ml/min/1.73 m2

40 and 50% in CKD patients [45].

**•** unexplained skeletal fracture or bone pain **•** severe progressive vascular calcification

> Laboratory changes comprise of low calcium and high phosphate levels (in advanced CKD stages), elevated bone-specific alkaline phosphatase (BAP), and PTH. Calcium levels can be elevated due to calcium oral supplementation or vitamin D overdose. BAP and PTH are currently the most reliable noninvasive markers for bone turnover.

> Routine radiology imaging is insensitive for the type of osteodystrophy and is performed only when symptoms appear. However, certain findings are described, which are not specific for CKD-related bone disease [46].

**Table 6** summarizes the clinical, laboratory, and radiology findings in HTMBD.


**Table 6.** Clinical presentation and laboratory/radiology findings in high turnover mineral bone disease (HTMBD).

#### *3.2.3. Low-turnover mineral bone disease (LTMBD)*

LTMBD encompasses two entities in bone pathomorphology in CKD—osteomalacia and adynamic bone disease. Osteomalacia presents with low turnover and abnormal mineralization, increased osteoid matrix, and is more frequently associated with aluminum toxicity. AD is characterized with low turnover and bone acellularity. In AD even the matrix formation is reduced, thus the mineralization seems unchanged [21]. OM in CKD prevalence is decreasing significantly since 1995 and currently is bordering at 0%. However, AD is getting more common than all other subtypes of renal osteodystrophy, with prevalence peaking to 40% [44].

AD increases in importance not only due to its higher incidence, but also because of its clinical role in CKD. AD is associated with cardiovascular calcification, increased mortality, and higher fracture risk compared to HTMBD [46]. The major factors contributing for AD development are increased use of calcium-containing phosphate binders, excessive use of calcitriol/vitamin D analogs (VDAs), and excessive PTH suppression. Increased patients' age, diabetes mellitus, and peritoneal dialysis as renal replacement therapy were found to be important contributors too. All possible factors leading to AD are listed in **Table 7**.

#### **Iatrogenic factors**


#### **Other factors**


**Table 7.** Factors for adynamic bone disease in CKD.

Similarly to HTMBD, the symptoms in AD are nonspecific. In most of the cases the disease is asymptomatic, pain in the bones, fractures, and bone deformities are one of the most common symptoms. In addition, signs of hypercalcemia, extraskeletal calcifications, and pruritus are also present. In cases of aluminum-associated OM, anemia and dementia can be detected [47].

*3.2.3. Low-turnover mineral bone disease (LTMBD)*

32 A Critical Evaluation of Vitamin D - Clinical Overview

too. All possible factors leading to AD are listed in **Table 7**.

**Iatrogenic factors**

**•** Peritoneal dialysis **•** Aluminum treatment

**•** Diabetes mellitus

**•** Hypogonadism **•** Malnutrition

**•** Low thyroid hormone levels

**•** Altered growth factors and cytokines

**•** Vitamin D receptor polymorphisms

**Table 7.** Factors for adynamic bone disease in CKD.

**Other factors**

**•** Age

**•** Excessive vitamin D treatment **•** Excessive calcium binders use **•** Excessive PTH suppression

**•** High calcium concentration in dialysate fluids

LTMBD encompasses two entities in bone pathomorphology in CKD—osteomalacia and adynamic bone disease. Osteomalacia presents with low turnover and abnormal mineralization, increased osteoid matrix, and is more frequently associated with aluminum toxicity. AD is characterized with low turnover and bone acellularity. In AD even the matrix formation is reduced, thus the mineralization seems unchanged [21]. OM in CKD prevalence is decreasing significantly since 1995 and currently is bordering at 0%. However, AD is getting more common

AD increases in importance not only due to its higher incidence, but also because of its clinical role in CKD. AD is associated with cardiovascular calcification, increased mortality, and higher fracture risk compared to HTMBD [46]. The major factors contributing for AD development are increased use of calcium-containing phosphate binders, excessive use of calcitriol/vitamin D analogs (VDAs), and excessive PTH suppression. Increased patients' age, diabetes mellitus, and peritoneal dialysis as renal replacement therapy were found to be important contributors

Similarly to HTMBD, the symptoms in AD are nonspecific. In most of the cases the disease is asymptomatic, pain in the bones, fractures, and bone deformities are one of the most common

than all other subtypes of renal osteodystrophy, with prevalence peaking to 40% [44].

Bone imaging detects fractures, looser zones, deformities, osteoporosis, and osteopenia. Laboratory findings are the key to the differential diagnosis between HTMBD and LTMBD. In LTMBD, lower PTH and bone-specific AP are present, as well as higher calcium and low phosphate levels. However, high PTH in biopsy-proven AD can be detected; therefore, PTH levels cannot be regarded as the best marker of differentiation between LTMBD and HTMBD [48]. Yet, PTH and PAP are significantly lower in LTMBD and a downward trend in these parameters indicates development of AD [45]. The clinical, laboratory, and radiologic findings in LTMBD are summarized in **Table 8**.


**Table 8.** Clinical presentation, laboratory, and radiologic findings in low turnover mineral bone disease (LTMBD).

#### *3.2.4. Soft tissue and vascular calcifications*

Soft tissue calcification is the third component of the diagnosis of CKD-related mineral bone disease and is more prevalent in CKD patients compared to the general population [49]. The most dangerous locations of extraskeletal calcification are the vasculature and the heart, increasing the risk for cardiovascular event. In renal disease, the pathogenesis of the deposits is not only passive deposition of calcium and phosphate, but also involves active cellular osteogenic transformation [50].

#### **3.3. Pleiotropic effects in CKD patients**

Poorer VD status is associated with a broad spectrum of nonskeletal clinical effects, probably due to the widely spread VDR and the presence of nonrenal 1-α hydroxylase. As already mentioned, VD is linked to renin-angiotensin aldosterone system suppression, renal protection, antiproteinuric effects, improved diabetes control, and reduced cancer risk. Pleiotropic effects were detected in CKD patients too—treatment with VD and vitamin D analogs in patients with renal disease led to reduced proteinuria; similar findings were reported in patients with diabetic nephropathy with a relatively low risk for hypercalcemia [51, 52]. However, the studies dealing with CKD patients are relatively few, compared to those reporting VD pleiotropy in the general population. Furthermore, no clear-cut data is present what VD treatment dose and target levels are needed to achieve the extraskeletal effects.
