The frequency of vitamin D deficiency in chronic kidney disease and its relation with baseline mineral bone markers

may be a deficiency of nutritional Vit-D (an inactive form of vitamin D) as well. 9 In that case, nutritional Vit-D (cholecalciferol / ergocalciferol) should also be replaced after ensuring its Abstract Objective: To assess the frequency of Vitamin D deficiency in chronic kidney disease and its association with baseline mineral bone markers in patients visiting nephrology clinics. Method: The observational study was conducted at the Indus Hospital, Karachi, from January 2017 to January 2018, and comprised patients of either gender aged >16 years diagnosed with chronic kidney disease stage I-V. The patients were divided into two groups on the basis of severity of vitamin D deficiency. Severe vitamin D deficiency was defined as <10ng/ml, and moderate deficiency ad 10-25ng/ml. Data was analysed using SPSS 21. Results: Of the 267 patients, 146(54.7%) were males and 189(70.8%) had vitamin D deficiency. Vitamin D-deficient patients were younger than those with normal levels (p=0.044). Serum creatinine was raised in the deficient patients compared to those with normal vitamin D level (p=0.042). Females and currently employed patients were at a higher risk of having vitamin D deficiency (p=0.048, 0.009). There was no significant association between disease stage and vitamin D deficiency (p=0.311). Conclusion: Vitamin D deficiency was found in a significant proportion of chronic kidney disease patients irrespective of the disease stage. Females, currently employed and young patients were more prone to having vitamin D deficiency.


Introduction
Vitamin D (Vit-D) is acquired both through nutritional means (10-20%) and by the cutaneous synthesis under the action of sunlight (80-90%). 1When we take vitamin D3/D2 (cholecalciferol / ergocalciferol) from dietary sources, it undergoes hydroxylation in the liver (25hydroxyvitamin D; 25[OH]D) and subsequently undergoes hydroxylation in the kidney under the influence of 1- hydroxylase enzyme to yield the biologically active, 1,25 dihydroxylated form of Vit-D.The second hydroxylation occurring in the kidney diminishes progressively with the loss of renal function to the limit where mineral bone disease manifestation starts.And this starts with low bone mass, bone fractures, vascular calcification and greater mortality. 2t-D deficiency has emerged as a pandemic. 3,4In a normal population, the prevalence of Vit-D varies between 30% and 90% in different studies. 5,6In the presence of such high Vit-D deficiency prevalence in the normal population, chronic kidney disease )CKD) is an added factor to depress its level further as CKD patients are often anorexic and have increased pigmentation which cause decreased synthesis of vitamin D. 7 The use of ergocalciferol / cholecalciferol has received relatively little attention because of an earlier held concept of the kidney as the only site of 1- hydroxylation of calcidiol and, in the failure of that, serum 25(OH)D was of less value.Recent data suggests a potential role for 25(OH)D in a number of tissues irrespective of renal conversion. 8 clinical practice, the treatment of CKD patients is often suboptimal or not well-targeted.Many doctors and general practitioners use active Vit-D to treat mineral bone disease (MBD) in CKD patients irrespective of the stage of the kidney disease.This may be partly the right approach if the patient happened to be advanced CKD stage, but along with active Vit-D deficiency, there may be a deficiency of nutritional Vit-D (an inactive form of vitamin D) as well. 9In that case, nutritional Vit-D (cholecalciferol / ergocalciferol) should also be replaced after ensuring its level in serum.Another benefit of treating with nutritional Vit-D is providing more substrate for I- hydroxylation not only in the diseased kidney, but in other sites of a body too. 10,11 CKD, supplementation with 25(OH)D is recommended at the inception of the disease, with the addition of calcitriol replacement beginning in stage III. 12By incorporating the role of nutritional Vit-D, we can reduce the cost of avoiding unnecessary use of active Vit-D which is not only costly, but also causes a dynamic bone disease if used inappropriately.
The current study was planned to assess the frequency of Vit-D deficiency in CKD and its association with baseline mineral bone markers.

Patients and Methods
The observational study was conducted at the Indus Hospital, Karachi, from January 2017 to January 2018.
After approval from the institutional review board, the sample size was calculated using OpenEpi software while assuming confidence interval (CI) of 95%, desired precision 6%, and the prevalence of Vit-D deficiency in CKD patients 50.8%. 13All patients aged >16 years of either gender with established CKD (pre-dialysis) on regular follow-up consenting to participate were included.All patients on dialysis, with chronic liver disease, and already on Vit-D or calcium (Ca)-containing drugs or at least stopped such medications for the preceding one month were excluded.Detailed demographic and clinical history was obtained from each subject and general physical examination was conducted.
Routine renal and other biochemical investigations, including serum creatinine (mg/dl), serum calcium (mg/dl), serum phosphorus (mg/dl), serum alkaline phosphatase, intact parathyroid hormone (IPTH) level (pg./ml), Vit-D level, serum albumin (mg/dl), were carried out as per the standard method used in the biochemistry laboratory of the hospital.Estimated glomerular filtration rate (eGFR) was calculated using the four-variable modification of diet and renal disease (MDRD) equation.The patients were divided into two groups on the basis of severity of Vit-D deficiency.Severe Vit-D deficiency was defined as level <10ng/ml, and moderate deficiency as 10-25ng/ml.Data was analysed using SPSS 21.Mean ± standard deviation (SD) / Median with interquartile range (IQR) were calculated, as appropriate, for quantitative variables age, height, weight, body mass index (BMI), serum albumin, serum calcium, serum phosphate, alkaline phosphate, and IPTH.Independent sample t-test / Mann-Whitney U test / Kruskal Wallis were applied, as appropriate, to assess the significant difference in quantitative variables between Vit-D and CKD stages.Chisquare / Fisher-exact tests were applied, as appropriate, to find significant association of various categorical variables, like gender, CKD stage, co-morbid, with Vit-D status.P<0.05 was considered significant.

Discussion
Vitamin D is the major steroid hormone which ensures bone health in conjunction with calcium, phosphorus and IPTH.It helps in the absorption of calcium from the intestine and reabsorption from tubules of kidneys and in conjugation with calcium mobilisation from skeleton by IPTH. 14This delicate balance of Vit-D, calcium, phosphorus and IPTH is disturbed when CKD progresses and this leads to increased risk of not only bone disease but also cardiovascular disease. 15 part of the typical lifestyle, humans have a combination of vitamins D2 and D3 available from ambient ultraviolet (UV) rays, habitual dietary intake of vitamin D3-rich foods (egg yolks and oily fish), fortified foods like margarine and breakfast cereals which generally have vitamin D2 fortification. 16The prolonged deficiency of 1,25 dihydroxy vitamin D together with parathyroid abnormality leads to renal osteo-dystrophy (ROD) and CKD-mineral bone disorder (CKD-MBD), which can be manifested by any one or combination of abnormalities of calcium, phosphorus, PTH, and Vit-D metabolism, leading to increase in secondary hyperparathyroidism, bone disease, vascular or soft tissue calcification. 9,17 the current study, hypertension was noted as the most common comorbid followed by diabetes, ischaemic heart disease, and nephrolithiasis.Furthermore, Vit-D deficiency was found to be more in diabetic and hypertensive patients.Comparing the results with an earlier study, diabetes came as not only the most common causative agent of CKD but Vit-D deficiency was also seen in higher proportion in diabetics than in non-diabetica. 13In our study, serum calcium and serum albumin were significantly lower in Vit-D deficient group but phosphorus and alkaline phosphatase didn't show a significant change in either group in the study.Also, Vit-D-deficient patients had higher creatinine and mean IPTH level.
Mean IPTH remained above normal in all CKD stages and was higher in stage V.This signifies the development of secondary hyperparathyroidism when nephron mass declines.Phosphorus level also increased with the severity of CKD.Deficiency of Vit-D was seen in significant proportion (70.8%) as we defined the cut-off level at <25ng/ml.This was further categorised as moderate deficiency (10-25ng/ml) and severe deficiency (<10ng/ml).
In this way, we found 57% who came in the severe deficiency group.This finding was comparable with studies done in Italian and Indian populations. 14,18One study found 87% Vit-D deficiency. 19 the Indian population, 18 active Vit-D was also checked and was found to be more deficient (>90% population) which we could not check in our population.Active Vit-D is supposed to be more declined in advanced CKD as it is activated in functional renal tissue. Th best laboratory indicator of Vit-D adequacy is serum 25(OH)D concentration.20 The Vit-D level declined significantly as CKD progressed from stage III to stage V in our study which is similar to an earlier finding.14 Females were found to have a 1.7 times higher likelihood of being Vit-Ddeficient compared to males in our study which was in line with literature.13,21,22 Although Vit-D deficiency was significant in CKD population, if we compare it with the prevalence in the general population of Pakistan, the results are not surprising as more than 50% deficiency has already been seen in the general population.[23][24][25] This fact is also highlighted by a study which did not find significant difference in Vit-D deficiency in between CKD patients and the normal population.24 We observed that the Vit-D-deficient group was relatively younger than the normal Vit-D group, indicating that there is relatively decreased Vit-D production in the skin of the elderly compared to the young.26 A study found no difference in the severity of Vit-D deficiency in terms of age.This might be due to decreased Vit-D consumption in food and relatively less sun exposure.27

Conclusion
Vitamin D deficiency was found in a significant proportion in CKD patients irrespective of the disease stage.Females, currently employed and young patients were more prone to having Vit-D deficiency.