In his promotional activities for his new book, How to Eat Better, James Wong has been tweeting little bursts of advice on how to increase the nutritional benefits of the food that you eat. One such piece of advice, tweeted at the end of February, caught my eye. It said that putting mushrooms on a windowsill increases their vitamin D content:
Putting 🍄 on a windowsill can trigger the same reaction, making them 1 of nature’s richest Vit D sources. Heres how:https://t.co/aiqgPT1AYh
— James Wong (@Botanygeek) February 24, 2017
Why did it catch my eye? Because it’s almost certainly not true. It sent me off on a little bit of a journey, so if you bear with me, I will recap it for you here. (Impatient? You can skip to the end, if you want.)
James has a page on his website (linked in the above tweet) which explains his mushroom theory. It links to one source: Vitamin D2 enrichment in fresh mushrooms using pulsed UV light, published by scientists at Penn State University. It appears to be written by a grad student and a professor, and – as far as I can tell – it hasn’t been published in a peer reviewed journal. It’s freely available as a PDF download from the department’s website.
The paper talks about two different sorts of vitamin D (D2 is produced in mushrooms exposed to UV (ultraviolet) light, and D3 is the one made in the human body when skin is exposed to UV light). It talks a little bit about whether D2 is as useful to the human body as D3, and then cites previous work that shows small amounts of D2 are found in wild mushrooms.
It then goes on to talk about previous experiments that showed large increases in D2 in cultivated mushrooms (which are usually grown in the dark) that have been exposed to intense pulses of UV-C light for 5 minutes. The new experiments that this particular paper is about showed that similar results could be obtained in cultivated mushrooms exposed to short, intense pulses of UV-B light.
Again, this research is aimed at a commercial setting, where growers/packers could quickly irradiate mushrooms before they were sent off for sale. Using UV-B for this purpose would be safer for the workers than UV-C, as UV-C can generate ozone (which is a toxic pollutant at our level of the atmosphere, though the ozone layer – much higher in the atmosphere – protects us from UV exposure).
So… exposure to UV-B or UV-C light (the difference being one of wavelength) does increase the vitamin D content of mushrooms. Does sunlight have the same effect?
One paper I found – Vitamin D Mushrooms: Comparison of the Composition of Button Mushrooms (Agaricus bisporus) Treated Postharvest with UVB Light or Sunlight, published in the Journal of Agricultural and Food Chemistry – compared the effects of sunlight with UV-B exposure. There were some differences – sunlight seems to have a wider effect on the chemicals in mushrooms, whereas the UV-B light just affects the vitamin D content. But it confirms that that sunlight can increase vitamin D in cultivated mushrooms. So what’s my problem? The windowsill.
Although, architecturally speaking, a sill can be indoors or out, in common parlance most people would think of a windowsill being inside (and a window ledge being outside). And if you put your mushrooms on the windowsill indoors, there’s a sheet of glass between them and the sun. And that’s a problem.
You see, window glass blocks UV-B and UV-C light (although, fortunately for us, the atmosphere blocks UV-C before it gets as far as the window). So mushrooms indoors aren’t being dosed at all. For you to increase their vitamin D content, you’d have to leave them outside in the sunshine for a couple of hours.
If we go back to the original paper that James quotes, the senior author is Professor Robert Beelman, Professor Emeritus of Food Science and Director of the Center for Plant and Mushroom Foods for Health. One of his areas of expertise is Vitamin D-enrichment of mushrooms. I wrote to Prof. Beelman and asked him what he thought about putting mushrooms on the windowsill for vitamin D enrichment.
When he got back to me (and scientists are often really great about answering questions on their work, if you ask nicely), he said:
So an expert on the topic of vitamin D enrichment is basically saying that he doesn’t know whether it would work indoors – because (a) no one has done any experiments on it and (b) he’s not an expert on glass.
I asked James whether he had any other sources on this topic, and he replied that he had lots. But when I asked him to share them with me… it went quiet.
@Botanygeek I can haz, please?
— Emma Cooper (@emmathegardener) March 7, 2017
When I checked this morning, it turned out that I am not the only person to have taken issue with James’ advice:
@Botanygeek re mushrooms & Vit D – maybe clarify they need to go on OUTSIDE windowsill? UVB needed to produce Vit D is blocked by glass!
— Emma Hoyle (@emmahoyle) April 7, 2017
— Susie Bailey (@susiebaileyuk) April 8, 2017
So… to bring a long story to a close, if you want to increase the vitamin D level of your mushrooms, they need to go sunbathing outside. Oh, and it may help if you slice them first.
If you want more information on vitamin D and vitamin D deficiencies, then take a look at Debunking the Absurd Notion of Vitamin D in Mushrooms, which is written by “a board certified and licensed nutritionist with a PhD in Public Health Education”, and lists 30 references.
However, if you’re worried that you may have a vitamin D deficiency, the best advice would be to speak to your doctor.
This blog post was written by Emma Cooper and was published on The Unconventional Gardener website. If you're reading it elsewhere you may want to navigate away from plagiarised content.