Since 85 percent of the U.S. population is deficient in vitamin D (primarily due to misleading information from conventional health "experts" that created a sun phobia, or because they are just not making enough effort to regularly get outside) this news about mushrooms becoming a new source for vitamin D would, at face value, seem like good news.

And it probably is good news for most people, as it would be the rare person that would eat too many of these vitamin D mushrooms. But it is certainly possible to overdose on vitamin D.

Cod liver oil is probably the safest form of vitamin D as it has vitamin A, which helps limit the vitamin D absorption if your levels are too high. However, it is my strong recommendation that anyone taking regular oral vitamin D, even cod liver oil, should have their blood levels checked.

You really need to be careful about all forms of oral vitamin D as it is just too easy to overdose. I had this personally confirmed this winter when I spent two months away from Chicago in a sub-tropical setting and had regular sun exposure nearly every day.

Prior to leaving, my levels were fine. When I returned, however, my vitamin D levels were in the toxic range, actually one of the highest we had ever measured at my clinic. I believe this was related to the fact that I was taking cod liver oil for two months prior to leaving.

I probably would have been fine, as I have been in past winters, if I wasn't getting so much sun exposure.

Fortunately, elevated vitamin D from sun exposure drops relatively rapidly, but I am never taking cod liver oil again as I plan on escaping Chicago winters and regularly getting sun exposure year-round.

For those who aren't able to escape to regular winter sources of sunshine, I am working on a simple sunlamp option that will safely increase your vitamin D levels and eliminate the need for expensive testing.

The bottom line here: Get your vitamin D from sun exposure, not irradiated mushrooms or cod liver oil. However, if you choose to use them regularly, then you should, at a bare minimum, have your blood levels of vitamin D checked a few times to make sure you don't overdose.