An Alternate Way to Acquire Vitamin D
With the colder months approaching in the Northern hemisphere, or for those indoors during the warmer months in the Southern hemisphere, getting enough vitamin D could be challenging. For various reasons, some cannot take vitamin D supplements or prefer not to, so there is now some evidence of a “new” food based way to boost your vitamin D levels. I first heard of this a few years ago, however the data was lacking then to furnish anything of measurable value.
How We Normally Get It
The way it usually works is, your skin produces vitamin D3 when ultraviolet light hits the skin and interacts with cholesterol. From that point, the kidneys convert the it into an active form. It should be noted that some individuals have difficulty converting vitamin D into its fully active form. If that’s not worrisome enough, many areas of the globe are devoid of UVB rays during the winter months.
Adding in Food Based Vitamin D
Button Mushrooms (Agaricus bisporus) appears to be the food based solution to the problem. Not only is this a food sourced form of vitamin D, it is also a fully active form, which means it is not dependent upon kidney health. In other words, eating white mushrooms could kick up your active vitamin D into high gear.
When researchers randomized 25 volunteers to take either 2,000 international units of supplemental vitamin D2, vitamin D3 or the equivalent in a mushroom extract. Blood levels of vitamin D in all three groups increased equally after 12 weeks. 
A Whole “New” Food Based Way to Get your Vitamin D
Those who shy away from animal based products now have a viable way to acquire vitamin D during the winter without having to resort to taking lanolin based vitamin D supplements or for those who prefer to avoid unconverted vitamin D. While it’s been known that vitamin D2 from ergosterol in mushrooms is produced upon exposure to ultraviolet (UV) irradiation, the existence of non-vitamin D2 products in mushrooms derived UV products, lumisterol(2) and tachysterol(2) were identified and quantified in white button mushrooms. It is yet unknown what sort of effects these have, however in wild type mushrooms, it is a normal phenomenon. It is likely there are potential health benefits from these newly identified features. 
Stimulated by Ultraviolet Light
Wild mushrooms themselves are an excellent source of vitamin D. It is however reliant upon UV stimulation, which catalyzes the conversion of fungal ergosterol to vitamin D2 via a series of photochemical and/or thermal reactions. In light of the new findings, growers of mushrooms now utilize ultraviolet light treatments ensuring that the vitamin D levels will be comparable to those cultivated in the wild. Moreover, mushrooms “raised” in this fashion have been found to be safe for human consumption.  Prior to this intervention, mushrooms “raised” in the dark without UV stimulation have deprived consumers of wild crafted vitamin D2 until now.
Immune System Response via Mushroom derived Vitamin D2
Unless you have been kept in the dark about the immune modulating effects of Vitamin D against the risk of developing the flu during the winter months, recent research set out to establish proof that this food will function as we would expect it to. Well, we now know if works in rats. Researchers examining 100 rats, fed varying degrees of vitamin D, including from mushroom sources, the finding was those fed ultraviolet exposed mushrooms had significantly higher blood vitamin D levels and anti-inflammatory effects than those without.  In another study, researchers found that white button mushrooms boosted memory in mice. Not just any mice, as the study used both wild type and transgenic mice. Specifically, a transgenic model that easily develop Alzheimer’s disease widely used in experimental studies. When compared with mice on the control diet, those fed UV stimulated mushrooms, both the wild type and Alzheimer’s transgenic mice displayed improved learning and memory, had significantly reduced amyloid plaque load and glial fibrillary acidic protein, and elevated interleukin-10 in the brain. 
. Food Chem. 2012 Nov 15;135(2):396-401.
. Food Chem Toxicol. 2013 Jun;56:278-89.
. Mol Nutr Food Res. 2013 Aug 28.
. PLoS One. 2013 Oct 18;8(10):e76362.