An increase in the use of colonoscopies as a surveillance tool has reduced the incidence of colorectal cancer. Previous studies have found a link between vitamin D deficiency and colorectal cancer. In the current study, which was a double-blind randomized controlled trial, the researchers recruited 240 participants with a history of colorectal polyps, a risk factor for colorectal cancer. The dose of oral magnesium (magnesium glycinate) given was based on participants’ calcium-to-magnesium intake ratio, which is generally considered to be around 2:1 calcium to magnesium on a weight basis. The extra bacteria didn’t explain increases in vitamin D, though, which suggested that magnesium works on vitamin D metabolism both directly and through the microbiome, but via separate mechanisms.