Does breast cancer risk decrease as 25-hydroxyl vitamin D blood levels increase? Yes, they do. The blood levels drop as the blood levels of 25 HD go higher and higher in cross-sectional or observational studies, we see lower and lower incidence of breast cancer. We can’t make a completely cause-effect relationship just yet because there’s some pending studies that will help us to say for sure that it’s direct cause and effect, but it’s certainly an association that’s enough of a reason to be thinking seriously about knowing what your vitamin D level is.