There was a direct link: for every 50 mg increase in magnesium per day, there was an average 6 percent decline in the chances for colorectal, colon, or rectal cancers. Six of the studies adjusted for how much calcium was in the diet and in those studies, participants who got the most magnesium were 19 percent less likely to develop colon or rectal cancer, compared to those who got the least magnesium.