Deficiency in 25-hydroxyvitamin D, which is common in patients with end-stage renal disease, is nearly universal among patients with hypoalbuminemia initiating dialysis in winter, especially blacks, researchers have concluded.
Ishir Bhan, MD, of Massachusetts General Hospital in Boston, and colleagues studied 908 dialysis patients participating in the ArMORR (Accelerated Mortality on Renal Replacement) study who had 25-hydroxyvitamin D levels measured. ArMORR included patients who began dialysis between July 1, 2004 and July 30, 2005.
The researchers established three different threshold of 25-hydroxyvitamin D deficiency: below 30, below 20, and below 10 ng/mL. Of the 908 subjects, 79% had levels below 30, 57% had levels below 20, and 20% had levels below 10.
“Regardless of the 25-hydroxyvitamin D threshold used to define deficiency, winter season and hypoalbuminemia increased the likelihood of identifying vitamin D deficiency,” the authors wrote in the Clinical Journal of the American Society of Nephrology (2010;5:460-467). The investigators defined hypoalbuminemia as a serum albumin level of 3.1 g/dL or less.
All black patients with hypoalbuminemia starting dialysis in winter had hydroxyvitamin D levels below 20, compared with 90% of black women and 85% of black men at baseline, the investigators reported. In addition, 94% and 88% of white women who had hypoalbuminemia and started dialysis in winter had levels below 30 and below 20, respectively (compared with 82% and 62% at baseline). For hypoalbuminemic white men starting dialysis in winter, the proportions were 92% and 67% (an increase from 92% and 67% ).
Dr. Bhan’s group concluded that black race was the leading independent predictor of vitamin D deficiency based on 25-hydroxyvitamin D thresholds of 10 and 20 and remained a significant predictor even when they defined deficiency with a threshold of 30. Regarding this association, the researchers pointed out that melanin in skin absorbs ultraviolent radiation and decreases the energy available for conversion of 7-dehydrocholesterol to previtamin D.
As to why starting dialysis in winter predicts vitamin D deficiency, the researchers said this is likely related to decreased exposure to ultraviolet light during that season.
With regard to the association between low serum albumin and vitamin D deficiency, the authors explained that albumin “is likely to reflect the level of nutrition, and low serum albumin might correlate with lower intake of dietary sources of vitamin D.”
Another possibility is that low albumin could represent reduced carrying capacity for vitamin D, which largely circulates in protein-bound form, they observed. Alternatively, patients might have a disorder that predisposes to both hypoalbuminemia and vitamin D deficiency.
The presence of hypoalbuminemia, particularly in women and blacks, may lessen the need to measure serum 25-hydroxyvitamin D levels in patients initiating dialysis between October and March, according to the investigators. Given the high cost of 25-hydroxyvitamin D testing relative to treatment, they stated, empiric therapy with a nutritional form of vitamin D could be considered in these patients if their results are validated in other studies.
The ArMORR cohort in the new study had a median age of 64 years and was 53% male, 61% white, 32% black, and 7% other races.