Vitamin D has raised its profile recently, particularly as it has been discovered that many of us in the UK are lacking this essential vitamin due to poor sunlight during the winter. Many breakfast cereals and milk substitute drinks are fortified with vitamin D and some supermarkets chains have started fortifying bread too. Vitamin D can also be obtained naturally from our diet through consuming animal-based products such as dairy foods, fish and eggs. However, you might have noticed that supermarkets have started marketing their mushrooms as “high in vitamin D”, which makes them one of the few naturally occurring vegan sources of this “sunshine vitamin”, so-called because our main source of it comes from the sun.
However, during the months of October through to March, the sunlight in the U.K. isn’t sufficient enough to give many of us the amount of vitamin D we need. This is particularly true for those of us who are house-bound, who cover their skin up, who are dark-skinned or don’t eat foods rich in vitamin D.
Mushrooms and Vitamin D
Mushrooms grown in sunlight or under ultraviolet (UV) light can accumulate vitamin D in a similar fashion to our skin, because they are naturally rich in ergosterol which, like us, converts to vitamin D. Vitamin D as we know it has two main subtypes: vitamin D2 and vitamin D3. Mushrooms provide us with vitamin D2, as do most food sources of vitamin D, which isn’t as biologically active as the type we get from the sun. The important question we have to ask when we look at nutrients in food is, do they make it past the digestive tract and into our blood stream? A process we call “bioavailability” –asking how bioavailable vitamin D in mushrooms is means we need to understand whether it gets to where it needs to get to.
A recent research project undertaken by Irish scientists suggests that the evidence to back the claim that mushrooms are a good source of vitamin D is surprisingly lacking. Few clinical trials have been conducted that test whether eating mushrooms increases our vitamin D status. when the researchers looked at those trials available, they combined the results into a pool known as a meta analysis and found that there was no significant change in vitamin D status despite consuming mushrooms purported to be enriched when the participants’ vitamin D status was already adequate, but if they had low vitamin D, then it showed some modest benefit. However, some studies suggest that cooking mushrooms might deplete some of their vitamin D content.
So although this is interesting, we can’t draw too many conclusions from this without more research. Overall it appears that consuming raw mushrooms during the winter months, when our vitamin D levels are lower, might have some benefit – particularly if you don’t like or cannot eat other food sources. Not all mushrooms will be marketed as vitamin D because they may not be grown under UV lamps, but simply placing your mushrooms outside in the sun for an hour or so should do the trick for you!
Mushrooms and diet
There are plenty of sources out there that say you should not eat mushrooms raw because they contain the toxin, agaritine. This is noted to cause cancer, but breaks down during cooking and therefore the general advice is to cook them first. But do they contain enough to harm us?
Most of the agaritine is stored in the skin of the mushroom and within the gills, so skinning them can reduce this to an extent. The stem has the least. However, research suggests that agaritine is highly unstable and so degrades during storage (particularly if you keep them in the fridge), so by the time mushrooms get to your salad bowl, they probably have very low levels, and the levels present anyway are insufficient to cause any ill-effects. So it seems, mushrooms are safe to eat raw, whether you skin them or not.
Cashman et al. (2016) Effect of ultraviolet light-exposed mushrooms on vitamin D status: liquid chromatography-tandem mass spectometry reanalysis of biobanked sera from a randomized controlled trial and a systematic review plus meta-analysis. The Journal of Nutrition. 146(3);565-575. doi: 10.3945/jn.115.223784
Roupas et al (2010) Mushrooms and agaritine: a mini-review. Journal of Functional Foods. 2(2);91-98
Schulzová et al (2002) Influence of storage and household processing on the agaritine content of the cultivated Agaricus mushroom. Food Additives and Contaminants. 19(9);853-862
Urbain et al. (2016) Impact on Vitamin D2, Vitamin D4 and Agaritine in Agaricus bisporus Mushrooms after Artificial and Natural Solar UV Light Exposure. Plants Foods for Human Nutrition. 71(3);314-321
Seb is a writer and blogger of food and nutrition. He holds a bachelors and a masters degree in nutrition science, and has studied sports and exercise nutrition at postgraduate level. He specialises in plant-based nutrition and believes passionately that we can all live with a little less meat. He writes for www.veggieandspice.com and www.itsaboutnutrition.com