The Only Thing In The Produce Aisle That Makes Vitamin D

Mushrooms sit next to the lettuce, but they aren't plants at all.

They're fungi, on their own branch of life. In some ways that branch sits closer to us than to the greens around them.

And they can do one thing no plant in that aisle can do.

Leave a mushroom in real sunlight and it makes vitamin D…

The Trick They Share With Your Skin

Your skin holds a compound that turns into vitamin D when strong sunlight hits it. Mushrooms hold their own version, called ergosterol.

Same idea, same trigger. UV-B light strikes the ergosterol and it becomes vitamin D2.

The size of the change is remarkable. Two food scientists, Viraj Jasinghe and Conrad Perera, exposed shiitake to UV light and measured what happened.

Vitamin D content climbed from trace amounts into the tens of thousands of units per 100 grams of dried mushroom. Their work ran in 2005 in Food Chemistry.

They found something useful too. The gills hold about twice the ergosterol of the cap, and they convert it far faster.

Growers now do this on purpose. A UV-treated mushroom can carry 400 to 750 units per 100 grams. One grown in the dark holds about 11.

But Does It Work In People

Fair question. A mushroom making vitamin D in a lab is not the same as your blood levels rising.

Paul Urbain and colleagues at the University Medical Center Freiburg tested it. They gave adults with low vitamin D either UV-treated button mushrooms or a vitamin D2 supplement.

Blood levels rose in both groups, and the mushrooms held their own against the pill. That trial ran in 2011 in the European Journal of Clinical Nutrition.

One honest caveat. Mushrooms make D2, while your skin and most supplements make D3.

Researchers still argue about which works better. The fair summary is that D2 does raise your levels, and D3 probably does it a bit better per unit.

Now The Mistake Everyone Makes

You'll read that you should put mushrooms on a sunny windowsill for an hour.

Don't bother. Window glass blocks nearly all UV-B light, which is the exact kind that does the work.

The light streaming onto your kitchen counter is mostly UV-A. Pleasant, useless for this.

If you want to try it, the mushrooms have to go outside in direct sun, gills facing up. That's where most of the ergosterol sits.

I'll be straight with you though. Nobody has run a real trial on sunning them at home. Treat it as an experiment, not a dose.

What's In One Besides That

A hundred grams of raw white mushrooms carries just 22 calories.

You get 3.1 grams of protein, which is high for something this light. There's also 318 milligrams of potassium and 9.3 micrograms of selenium, per USDA data.

That selenium number is worth a look. Few vegetables carry much of it, and mushrooms quietly do.

They're also full of riboflavin and niacin, two B vitamins few people think about.

The Compound With Its Own Front Door

Here's the part that fascinates researchers.

Mushrooms are unusually rich in a compound called ergothioneine. Call it ergo from here on.

Your body has a transporter built for it. That's a protein whose only job is moving one thing into your cells.

Dirk Grundemann's team found it in 2005, and published in the journal PNAS. Bodies don't build a private door for something that doesn't matter.

Robert Beelman at Penn State has spent years arguing ergo may explain some of what mushrooms seem to do. Porcini carry the most, and the plain button mushroom carries less.

So far, so promising. Here's the honest part.

No human trial has shown a health benefit from ergo. Not one. The case rests on lab work and on patterns in how people eat.

Worth adding that Penn State's mushroom center takes money from the industry. That doesn't make the work wrong. It does mean you read it with your eyes open.

The Other Claims, Ranked By Evidence

Immune support comes up all the time. One real trial sits behind it.

A team at the University of Florida gave 52 adults dried shiitake daily for four weeks. Some immune markers shifted, and one marker of inflammation edged down. Published in 2015.

Fifty-two people, four weeks, funded through industry ties. Interesting, not settled.

Cancer is the bigger claim. A 2021 Penn State review pooled 17 studies. People who ate more mushrooms had lower cancer rates, breast cancer most of all.

Those studies watch people, they don't assign diets. Mushroom eaters differ in plenty of other ways.

The most useful finding is the dullest. Lawrence Cheskin at Johns Hopkins swapped mushrooms for beef in meals. People ate about 420 fewer calories a day, and felt just as full.

That was 2008, also paid for by the industry, and it's still the best thing on this list.

Cooking Them Right

Method matters more than you'd guess.

A Spanish research center in La Rioja tested four ways of cooking four kinds of mushroom. Grilling and microwaving held onto the most antioxidant power. Boiling and frying lost the most.

And about washing them. You've heard mushrooms soak up water like sponges, so you should brush them instead.

Kitchen scientists tested that. A mushroom soaked for five minutes takes on about a sixteenth of a teaspoon of water. It's already 90 percent water.

Rinse them. Your hands will thank you.

Three Things Worth Knowing

Mushrooms are strange company in the produce aisle. Not a plant, barely any calories, and quietly rich in selenium.

The vitamin D story is the real one. It all comes down to light the mushroom saw before you bought it.

Brown a pan of them this week. Check the label while you're at it.

Stay well,
The Healing Plate