Vitamin D levels above 20 ng/mL were not associated with lower mortality rates in patients with and without kidney disease; however, levels below 12 ng/mL were associated with higher mortality rates in these patients, according to the results of a study published Oct. 24 in the online journal PLoS One.
The minimal difference in mortality rates for individuals with vitamin D levels between 20 ng/mL and 30 ng/mL suggests that vitamin D supplements may not be necessary for approximately 3 million adults with chronic kidney disease and 75 million adults without kidney disease, said Dr. Holly Kramer of Loyola University Medical Center in Maywood, Ill., and her colleagues.
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The minimal difference in mortality rates for individuals with vitamin D levels between 20 ng/mL and 30 ng/mL suggests that supplements may not be necessary for approximately 3 million adults with chronic kidney disease, said the investigators.
To examine the impact of vitamin D levels on mortality, the researchers reviewed data from 15,099 adults who were part of the Third National Health and Nutrition Examination Study (NHANES III). The study population included 1,097 adults with chronic kidney disease, which was defined as an estimated glomerular filtration rate of less than 60 mL/min per 1.73 m2.
In order for mortality rates to be compared, the researchers divided the study population into groups based on vitamin D levels, ranging from less than 12 ng/mL to greater than 40 ng/mL, using the 24- to 29.9-ng/mL group as the reference group.
"This group was selected as the referent, because it includes 25[OH]D levels which are above the threshold for ‘risk of insufficiency,’ defined by the Institute of Medicine Committee to Review Dietary Reference Intakes for Vitamin D and Calcium, yet below the thresholds defined as ‘insufficient’ in previous analyses," the researchers wrote.
Overall, about one-third of the adults with kidney disease (35%) and of those without kidney disease (30%) had insufficient levels of vitamin D, based on Institute of Medicine recommendations.
The median vitamin D levels for each of the groups were 10.0 ng/mL, 14.1 ng/mL, 18.0 ng/mL, 21.9 ng/mL, 26.5 ng/mL, 33.9 ng/mL, and 43.6 ng/mL.
After the investigators controlled for risk factors including age, race, sex, smoking status, and comorbid conditions, the all-cause mortality rate was determined for the patients with kidney disease. The mortality rate was 153/1,000 person-years for those with vitamin D levels less than 12 ng/mL, 121/1,000 person-years for those in the 12- to 16-ng/mL group, and 108/1,000 person years for those in the 24- to 29.9-ng/mL range.
Mortality rates were similar for those with kidney disease and vitamin D levels greater than 20 ng/mL, the researchers noted, with the lowest mortality rate of 97/1,000 person-years seen in those in the highest vitamin D group.
Among patients without kidney disease, the adjusted all-cause mortality rate was 17/1,000 person-years among those with vitamin D levels less than 12 ng/mL, compared with 13 for those in the 24- to 29.9-ng/mL range and 12 among patients with vitamin D levels greater than 40 ng/mL.
The study was limited by its observational design, and the results may not be generalizable to nursing home residents, individuals on dialysis, or anyone who has had a kidney transplant, the researchers noted.
Vitamin D supplementation has been linked to an increased risk of cancer and kidney stones, and clinical trials are needed to assess the risks versus benefits in individuals with and without kidney disease, the researchers added.
The National Institutes of Health supported the study.