Abstract
Background and Aims
Among patients with chronic kidney disease (CKD) at predialysis stage, there is a high incidence of secondary hyperparathyroidism. Metabolic changes associated with secondary hyperparathyroidism lead to renal osteodystrophy, including osteitis fibrosa, ectopic calcification, cardiovascular disease, and the risk of death, and serum parathyroid hormone levels are influenced by nutritional variables. Non-dialyzed CKD patients are especially prone to vitamin C deficiency because of dietary restrictions and malnutrition. Vitamin C is an important antioxidant and relates to the development and maintenance of bone tissues. However, the contribution of vitamin C deficiency to parathyroid hormone secretion is unknown. Here, we performed a single-center cross-sectional study in order to assess association of serum vitamin C and parathyroid hormone in non-dialyzed CKD patients.
Method
We had 280 consecutive patients who underwent serum vitamin C and serum intact parathyroid hormone (iPTH) measurement for screening purposes from January 1st, 2013 to November 30th, 2017. We analysed a total of 128 patients (71.3±11.6 year-old, 80 males) who had an estimated glomerular filtration rate (eGFR) that remained less than 60 mL/min/1.73m2 after 152 patients were excluded because of vitamin C or vitamin D supplementation, age <20 years, dialysis, positive serostatus for HIV, hepatitis B or hepatitis C, chronic infection, or cancer.
Results
Twenty-three percent of the patients (n=29) had vitamin C levels< 2.0 μg/mL (a range seen in very deficient subjects), 53% (n=68) had levels between 2.0 and 5.5 μg/mL, and 31 patients (24%) had vitamin C levels >5.5 μg/mL, which is considered the upper limit of normal for the healthy population. Log(iPTH) significantly correlated with age (r=-0.238, p=0.00672), log(eGFR) (r=-0.625, p<0.0001), serum calcium (r=-0.609, p<0.0001), and serum phosphate (r=0.41, p<0.0001), and had a tendency to correlate with serum albumin (r=-0.146, p=0.101). Low serum vitamin C was associated with higher serum iPTH (P=0.0005, one-way analysis of variance). In a multiple linear regression model with log(iPTH) as the dependent variable, and age, gender, log(eGFR), serum levels of calcium, phosphate, albumin, and vitamin C as independent variables, the inverse relationship of log(iPTH) and serum vitamin C was confirmed (R2 = 0.568, adjusted R2 = 0.543, P<0.0001), along with other parameters influencing iPTH levels, including age, log(eGFR), serum calcium, and serum phosphate. Low vitamin C levels were also associated with increased serum alkaline phosphatase (r=-0.209, p=0.0179), a further indicator of the impact of vitamin C status on bone metabolism.
Conclusion
Vitamin C deficiency is prevalent in a significant proportion of non-dialyzed CKD patients. Low vitamin C levels contribute to secondary hyperparathyroidism, leading to increased bone turnover. This novel observation may result from effects of vitamin C on vitamin D metabolism, vitamin D binding in target tissues, and cAMP-linked signalling pathways in bone and parathyroid gland. Therapeutic intervention with supplemental vitamin C for secondary hyperparathyroidism might be a good strategy.
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