Low Vitamin D Linked to High Risk for Kidney Disease in African-Americans

Norra MacReady

November 04, 2009

November 4, 2009 (San Diego, California) — Low serum levels of vitamin D are prevalent among African American adults and might be related to their high risk of developing end-stage renal disease (ESRD), Michal L. Melamed, MD, MHS, associate professor of medicine and epidemiology at Albert Einstein College of Medicine in the Bronx, New York, reported here at Renal Week 2009: American Society of Nephrology 2009 Annual Meeting.

"I think clinicians should be aware of their patients' vitamin D levels, and high-risk patients should be on supplements — even 400 IU per day would help," Dr. Melamed said in a poster session.

To test the hypothesis that low serum vitamin D levels, in the form of 25(OH)D, are a risk factor for kidney disease progression, Dr. Melamed and her coauthors analyzed data from 13,328 participants in the National Health and Nutrition Examination Survey (NHANES) III Follow-Up Study, in which 25(OH)D levels were measured from 1988 through 1994, and then participants were followed for up to 12 years. Serum 25(OH)D deficiency was defined as anything below 15 ng/mL.

Thirty-four percent of the non-Hispanic black participants were 25(OH)D-deficient, compared with 5% of non-Hispanic whites (P < .001). During a median follow-up time of 9.1 years, 65 individuals developed ESRD and began outpatient dialysis. A total of 170 participants either started dialysis or died of underlying nephropathy.

After adjustment for clinical, demographic, and socioeconomic factors, the incidence of ESRD was 2.6 times greater in people whose serum 25(OH)D was less than 15 ng/mL than in those with higher levels. When the investigators adjusted for clinical covariates other than 25(OH)D, the risk of developing ESRD was 2.83 times higher among the black than among the white participants. Adjusting for ESRD reduced the risk by 58%, leading the authors to conclude that low serum 25(OH)D levels might account for a significant proportion of the ESRD risk experienced by the black participants.

This was an observational study, so it cannot demonstrate cause and effect, said Dr. Melamed.

Vitamin D is known to inhibit the renin-angiotensin system in mice, and "there is a lot of observational data [indicating] that low vitamin D is associated with high blood pressure, kidney disease, and all-cause mortality," Dr. Melamed noted. "A recent meta-analysis of 18 trials of postmenopausal women who were randomized to receive vitamin D or placebo showed that women who took vitamin D had an 8% lower risk of death. That tells you there is something there."

One expert not involved in this research sounded the warning against jumping to conclusions prematurely. "The challenge of observational studies is that we don't know the direction of the association," said Haimanot Wasse, MD, MPH, assistant professor in the Renal Division at Emory University School of Medicine in Atlanta, Georgia.

"Patients who develop ESRD are different than those who don't, and we don't know if the progression to ESRD caused the low vitamin D, or if the low vitamin D contributed to their ESRD." Most likely, "people who develop ESRD progress because they're sicker, have a poorer diet, and have more bone disease, and low vitamin D is a part of that. But you can't conclude that from this study. There's no causality here."

Still, Dr. Wasse told Medscape Nephrology, "these findings don't surprise me at all because, as renal function declines, vitamin D levels are likely to be lower." Overall, "this is an important study because we see that vitamin D deficiency is a real problem in non-Caucasians."

Dr. Melamed and Dr. Wasse have disclosed no relevant financial relationships.

Renal Week 2009: American Society of Nephrology (ASN) 2009 Annual Meeting: Poster F-PO1101. Presented October 30, 2009.

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