SAN FRANCISCO – Taking 800 IU/day of vitamin D3 supplements for 1 year pushed serum levels into acceptable ranges in 98% of postmenopausal white women with vitamin D insufficiency, and 600 IU/day may work just as well, a study of 163 patients found.
The study was the first long-term, randomized, double-blind placebo-controlled comparison of multiple dosages of vitamin D supplements, Dr. Adarsh J. Sai and his associates reported at the annual meeting of the American College of Physicians. Many trials of vitamin D supplementation in osteoporotic women studied mainly single dosages.
Dr. Adarsh J. Sai
Participants started out with serum levels of 25 hydroxyvitamin D, or 25-(OH)D, greater than 5 ng/mL but less than 20 ng/mL. The World Health Organization and Institute of Medicine consider normal levels to be above 20 ng/mL(or greater than 50 nmol/L), although the 2011 Endocrine Society guidelines define normal as above 30 ng/mL, he said.
The study randomized participants to daily supplementation with placebo or one of seven doses of vitamin D3: 400, 800, 1,600, 2,400, 3,200, 4,000, or 4,800 IU/day. They also received calcium citrate supplements based on a 7-day food diary to increase daily calcium intake to 1,200-1,400 mg/day (Ann. Intern. Med. 2012;156:425-37).
Serum 25-(OH)D levels increased to above 20 ng/mL in 98% of women on the 800-IU dose, which is the current recommended dietary allowance of vitamin D to maintain normal serum levels in at least 98% of people, he said. A 600-IU dose, however, could achieve the same blood levels, a modeling analysis predicted, though that dose was not provided to women in the study, said Dr. Sai of Loma Linda (Calif.) University.
Prospective trials are needed to confirm whether 600 IU/day would be sufficient supplementation, he said.
After a year of supplementation, serum 25-(OH)D levels were higher in the 31 women with a normal body mass index (BMI), compared with the 65 overweight women or 76 obese women. The normal-weight women’s serum 25-(OH)D levels were 5 ng/mL higher than those of the overweight women and 7 ng/mL higher than those of the obese women.
Serum parathyroid hormone levels decreased with increasing doses of vitamin D. Hypercalcemia occurred in 9% of patients, and hypercalciuria developed in 33%. No patients developed kidney stones.
"The long-term safety of vitamin D combined with calcium needs to be considered," Dr. Sai said at the meeting. "As a reminder, in the Women’s Health Initiative study of approximately 40,000 women on vitamin D 400 IU plus calcium 2,000 mg, the kidney stones incidence increased by about 20%."
At the start of the study, patients had a mean age of 67 years and a mean body mass index of 30 kg/m2. The mean serum 25-(OH)D level was 15 ng/mL. During the study, a mean of 94% of patients adhered to vitamin D supplementation, and 91% adhered to calcium supplementation.
The study excluded women with significant comorbidities, active kidney stone disease, or a body mass index greater than 45 kg/m2, and women taking any medications that could interfere with bone or vitamin D metabolism.
The National Institute on Aging funded the study. Dr. Sai reported having no relevant financial disclosures.
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