There are a number of things that can lower vitamin D absorption — for example, fat-blocking supplements or medications (like orlistat or mineral oil), very high doses of vitamin A, or even being low in magnesium, since magnesium is needed to activate vitamin D. But these are just examples, and everyone’s body is different. If you’re unsure, checking in with your health specialist can give you more clarity about your unique situation. Also, the deficiency test we use points out the most common deficiencies linked to certain symptoms. A simple way to see if a supplement is working for you is to notice whether your symptoms improve after taking it. For example, if cramps are linked to a magnesium deficiency, taking magnesium should ease those cramps. This makes it easier to connect the dots between what your body needs and how it responds.