There have been quite a few articles over the past few years suggesting you can create vitamin D in mushrooms by putting them in the sun. I'm always sceptical about super food claims and thought I would do some online research to find the scientific basis of this claim.

My basic findings are this:

Yes, mushrooms do create vitamin D in the sun
The amount in a portion of mushrooms (100g) is about the amount suggested as a daily supplement (10 mcg or micrograms)
You need to expose your mushrooms to direct sunlight or place them under special acrylic plastic that allows UV through
Your mushrooms should be in the sun for about an hour but no more than two hours
Cut your mushrooms thin to get the maximum surface area and maximum vitamin D

If you want to try, then these points are all you really need to know, however if you would like to see what I did, then keep reading :-)

Mushrooms contain a chemical called Ergosterol. When exposed to ultraviolet light, a photo-chemical reaction converts the ergosol into Ergocalciferol also known as vitamin D2. Leave the mushrooms in the ultraviolet light too long and the vitamin D2 is reduced as it breaks down.

Sunlight is a natural source of ultraviolet, so exposing your mushrooms to direct sunlight will increase their vitamin D significantly, however placing your mushrooms on an indoor windowsill or in a greenhouse will not work as glass and glass alternatives like plexiglass blocks UV.

There have been some news reports about vitamin D reducing the risk of coronavirus. The published scientific papers I have found contradict each other and the NHS in the UK say there is currently not enough evidence to support this, however they suggest taking 10 micro grams vitamin D a day as it wont hurt and has other health benefits.

I've put a full set of the source documents I used for this instructable at the end, but here are a few of the media articles on the subject:

BBC: Increasing vitamin D in mushrooms

Metro UK: Supercharge your mushrooms

Mushroom Council: Vitamin D in Mushrooms

Insight Plus Australia: Tan your mushrooms

Supplies

At least 100g of mushrooms
A sharp knife
A tray
Sunshine

Step 1: Prepare Your Mushrooms

The first step is to prepare your mushrooms. A portion is normally considered to be about 100g.

I used supermarket button mushrooms and five weighed in at 109g

Step 2: Slice Your Mushrooms

To get the highest amount of vitamin D in your mushrooms the scientific papers suggest you need to maximise the surface area exposed to the sun, so slice them as thin as you can.

Step 3: Prepare Your Box

Your sliced mushrooms will need to be spread out on a tray for exposure to the sun. If you want to keep off any flies etc. you will need to cover them, but the cover must allow UV light through. Glass, perspex and most clear acrylic will block UV so you can not use these. There are a couple of specialist acrylics designed to allow UV through that are used in sun beds. Alternatively you can use clingfilm as polyethylene is also transparent to UV :-)

I have designed and made a box specifically for leaving mushrooms in the sun and also to dry them for storage, but you can use any box that can hold your tray of mushrooms.

My box uses sunbed grade acrylic in the lid and is laser cut, has a lid that be placed on it when the optimum amount of sun has been achieved so the mushrooms can be dried and has two holes to allow air flow covered with a mesh to stop insects. I have included the design files as attachments above if you have access to a laser cutter, I also sell them here.

Step 4: Lay Out Your Mushrooms

Spread out your sliced mushrooms ready to be placed in the sun.

Step 5: Put Your Mushrooms in the Sun

Your mushrooms need to be exposed to full sunlight for about an hour and no longer than two hours. I set an alarm on my phone to remind me. This should give you about 30 micro grams of vitamin D per 100g.

Tilt the box or tray so it points directly at the sun.

The graph above shows data taken from a scientific paper (see information source 3) and shows the amount of vitamin D that is produced in 100g of sliced mushrooms.