Kidney Disease Research Updates
Low Vitamin D Levels Linked to Increased Risk of Kidney Failure
May Explain Higher Risk among African Americans
Low vitamin D levels may explain African Americans’ increased risk of kidney failure, called end-stage renal disease (ESRD), according to a study funded by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).
“Participants with 25-hydroxyvitamin D [25(OH)D] levels less than 15 nanograms per milliliter (ng/ml) were 2.6 times more likely to progress to ESRD compared with participants with higher 25(OH)D levels,” wrote Michal L. Melamed, M.D., assistant professor, Department of Medicine and Department of Epidemiology and Population Health, Albert Einstein College of Medicine, and co-authors in their December 2009 report in the Journal of the American Society of Nephrology. The normal range of 25(OH)D, the main circulating form of vitamin D, is 30 to 74 ng/ml. “The increased prevalence of 25(OH)D less than 15 ng/ml among non-Hispanic black individuals seems to explain a substantial proportion of their excess risk for ESRD,” Melamed said.
Chronic kidney disease (CKD), a complication of diabetes and obesity, has increased 30 percent during the last decade. CKD afflicts an estimated 23 million Americans, many of whom are unaware they have the disease. Kidney disease is the ninth leading cause of death in the United States. CKD may progress to ESRD. Having CKD raises the risk of cardiovascular disease (CVD), with more CKD patients dying of a heart attack or stroke before their kidney disease progresses to ESRD.
Vitamin D controls calcium and phosphate, vital minerals the body needs to accomplish a wide variety of tasks, including keeping bones and muscles strong and preventing blood clots. Vitamin D deficiency causes rickets, a condition in children that softens the bones and leads to skeletal abnormalities. Low vitamin D levels have been linked to high blood pressure, type 2 diabetes, and CVD.
Despite a CKD prevalence similar to Caucasians, African Americans have a much higher incidence of ESRD: African Americans represent 12 percent of the U.S. population but account for 28 percent of new ESRD cases.
Possible explanations for African Americans’ increased risk of ESRD compared with Caucasians include socioeconomic status, access to health care, poor response to anti-hypertensive therapy, faster progression to ESRD, less mortality in the late stages of CKD, and now, according to Melamed and colleagues, vitamin D deficiency, which is defined as a 25(OH)D blood level less than 15 ng/ml.
Melamed and colleagues analyzed 25(OH)D blood levels of 13,328 participants in the 1988–1994 National Health and Nutrition Examination Survey (NHANES), a health survey sponsored by the Centers for Disease Control and Prevention that is representative of the U.S. population. They were able to assess ESRD incidence based on participants’ Medicare claims because CKD patients become eligible for Medicare when they are diagnosed with ESRD and begin dialysis.
During a median follow-up of about 9 years, 65 participants developed ESRD. Participants with vitamin D deficiency were more likely to be female, non-Caucasian, and of lower socioeconomic status than participants without. Those with vitamin D deficiency developed ESRD at 2.64 times the rate of those with 25(OH)D blood levels higher than 15 ng/ml.
African Americans developed ESRD at a rate 2.83 times that of Caucasians, after adjusting for ESRD risk factors such as diabetes, body mass index, age, and hypertension. When the researchers adjusted for vitamin D deficiency, however, the incidence rate dropped to 1.77 times that of Caucasians. Melamed and co-authors calculated that vitamin D deficiency accounted for 58 percent of the increased risk of ESRD among African American participants.
“We present these results as potentially hypothesis generating but urge caution in their interpretation because of the small sample sizes on which they are based,” wrote Melamed and co-authors. “If the association we observed between 25(OH)D deficiency and progression of kidney disease is seen in other observational studies and treatment with vitamin D is shown to retard the progression of kidney disease in randomized clinical trials, then this would have far-reaching public health implications for a readily available, safe medication for kidney disease.”
Humans acquire vitamin D through supplements and foods such as fish and eggs. An important source of vitamin D is sunlight. A few minutes of sun exposure is all that is needed for a full day’s supply of vitamin D, but many people, especially those who work inside most of the day, lack adequate exposure to the sun. During winter months, vitamin D blood levels go down among those who live at higher latitudes. African Americans’ highly pigmented skin filters the ultraviolet light required to synthesize vitamin D. As a result, many African Americans are at risk for vitamin D deficiency.
More information about research projects funded by the National Institutes of Health can be found by using the Research Portfolio Online Reporting Tools (RePORT) Expenditures and Results (RePORTER) tool located at www.projectreporter.nih.gov/reporter.cfm. The research described in this article is funded under NIDDK grant numbers 3K23DK078774–03S1 and 5K23DK078774–03.
The National Kidney and Urologic Diseases Information Clearinghouse, an information dissemination service of the NIDDK, has fact sheets and easy-to-read booklets about kidney diseases, including a fact sheet titled Chronic Kidney Disease-Mineral and Bone Disorder. For more information and to obtain copies, visit www.kidney.niddk.nih.gov.
NIH Publication No. 10-4531