Vitamin D Supplementation and Cancer Risk: Still No Clear Benefit

Despite epidemiologic and biologic reasons to expect that vitamin D might prevent cancer, dietary supplementation studies have shown no benefit. Now, researchers randomized 2303 healthy postmenopausal women in Nebraska (mean age, 65; 99% non-Hispanic white; mean baseline serum 25-hydroxyvitamin D [25(OH)D], 33 ng/mL) to supplementation with vitamin D₃ (2000 IU daily) and calcium (1500 mg daily) or placebo. All participants were asked to limit outside supplementation; actual mean daily amounts of outside supplementation plus dietary intake were about 900 IU (vitamin D₃) and 1200 mg (calcium) in both groups.

Over 4 years, mean 25(OH)D levels increased to 44 ng/mL in the intervention group and remained constant in the placebo group. The overall incidence of new cancers was not significantly different in the intervention and placebo groups (3.9% vs. 5.6%; P=0.06). However, in a post hoc analysis of study years 2 through 4, the difference barely reached significance (3.2% vs. 4.9%; P=0.048).