A recent study suggests that vitamin D deficiency in renal transplant recipients is associated with excessive body fat, decreased estimated glomerular filtration rate, and increased proteinuria, but not the risk factors for cardiovascular disease.
Vitamin D, also known as the sunshine vitamin, is essential for strong bones, plays an important role in the regulation of calcium and phosphorus absorption and provides a protective effect against many diseases. It is estimated that more than a billion people in the world are vitamin D deficient or insufficient. Measurement of 25-hydroxyvitamin D (25(OH)D), the circulating form of vitamin D, is used to evaluate vitamin D status
Humans get majority of their vitamin D from sunlight (approximately 80%), and the rest comes from diet and supplements. Therefore, the most important reason for vitamin D deficiency is inadequate exposure to sunlight, which could be due to shorter exposure time, season, latitude, altitude, clothing, and use of sunscreen.
Vitamin D plays an important role in calcium and phosphorus homeostasis. 25(OH)D needs to be converted into the active form, 1,25-dihydroxyvitamin D by undergoing hydroxylation. Kidneys are the primary site for hydroxylation of vitamin D. Impaired kidney function can lead to low circulating levels of 25(OH)D. Vitamin D deficiency is common among renal transplant recipients (RTR) due to avoidance of sunlight to reduce the risk of skin cancer. In addition, the use of immunosuppressive drugs such as corticosteroids accelerate vitamin D catabolism. Another reason for vitamin D deficiency in these patients is excessive body adiposity because post-transplant (post-tx) obesity occurs in up to 50% of patients in the first 12 months post transplant.
Recent evidence suggests that in general population, obesity increases the risk of cardiovascular disease (CVD) morbidity and mortality, end-stage renal disease (ESRD) and proteinuria. Additionally, obesity is also associated with CVD risk factors and graft failure in RTR.
A recent cross-sectional study published in the British Journal of Nutrition evaluated vitamin D status and its association with body adiposity, CVD risk factors, estimated glomerular filtration rate (eGFR) and proteinuria in RTR. The study conducted by Kelli T.C. Rosina and colleagues in the low latitude city of Rio de Janeiro, Brazil included 195 RTR men and women between 18 and 65 years of age. Risk factors for CVD were hypertension, diabetes, dyslipidemia and the metabolic syndrome. Body adiposity was evaluated using dual energy X-ray absorptiometry (DXA). Nutritional evaluation included anthropometry and DXA to measure BMI, total body fat (BF), waist circumference (WC), waist:height ratio, and waist:hip ratio (WHR). Blood samples were collected to analyze all laboratory parameters.
Vitamin D status of participants revealed that 10% had vitamin D deficiency (< 16ng/ml), 43% insufficiency (16-30ng/ml) and 47% sufficiency (>30 ng/ml). The percentage of women was highest in the vitamin D deficient group. Nutritional parameters were significantly higher in participants with vitamin D deficiency but did not significantly differ in the sufficient and insufficient groups.
The analysis of nutritional parameters revealed that RTR with vitamin D deficiency presented the highest values of all adiposity parameters, with WHR as the only parameter that did not significantly differ in the three groups. Multivariate regression analysis used to determine the percentage of BF that was associated with vitamin D deficiency revealed that vitamin D deficiency was the dependent variable and the covariates for adjustment were age, sex, and eGFR. On the contrary, percentage of BF, independent of age, sex, and eGFR was associated with vitamin D deficiency. Furthermore, the vitamin D deficient group showed significantly lower serum albumin levels compared to the sufficient group. There was no significant difference observed for glucose metabolism, lipid profile and blood pressure in three groups. Lower levels of 25(OH)D were found to be associated with increased proteinuria and decreased eGFR. Based on these results, the authors suggest that hypovitaminosis (low vitamins) in RTR is associated with excessive body fat, decreased eGFR and increased proteinuria.
Strengths of this study included adequate evaluation of body adiposity and appropriate adjustments of statistical analysis. However, one of the main limitations of the study is it’s cross-sectional design that limits the likelihood of causalities being determined.
In conclusion, this study suggests vitamin D deficiency is common in RTR, even in a low latitude city with high solar radiation, and is associated with excessive body fat, decreased eGFR and increased proteinuria. However, the study results indicate that vitamin D deficiency is not associated with classical risk factors for CVD. Longitudinal and intervention studies should be conducted in the future to further clarify the associations observed in this study.
Written By: Preeti Paul, MS Biochemistry