The Fungi's Surprising Secret
Unlike plants, fungi are biologically more similar to animals in some ways. Just as our skin produces vitamin D when exposed to sunlight, mushrooms have a similar, though distinct, mechanism. Most commercially grown mushrooms are cultivated in the dark
and contain very little vitamin D. However, they are rich in a compound called ergosterol, which is a precursor to vitamin D2. This ergosterol is found in the mushroom's cell walls and is ready to be transformed. When ultraviolet (UV) light—either from the sun or a specialized lamp—hits the mushroom, it triggers a photochemical reaction that converts this ergosterol into vitamin D2, also known as ergocalciferol.
Harnessing the Power of Light
This conversion process can happen both before and after harvesting. Recognizing this potential, some commercial mushroom growers now deliberately expose their mushrooms to UV light post-harvest. This is a highly efficient process. In some cases, brief pulses of high-intensity UV light for just a few seconds can dramatically increase the vitamin D2 content, taking it from negligible amounts to well over the daily recommended intake in a single serving. The most effective wavelength for this conversion is UV-B light. This controlled exposure allows growers to create a vitamin D-enriched food without negatively affecting the mushroom's appearance or taste. You can even try this at home: placing store-bought mushrooms in direct sunlight for a period can also boost their vitamin D levels.
From the Store to Your Plate
Once mushrooms have generated vitamin D2, the next question is how well it holds up during storage and cooking. Studies show that the vitamin D2 in UV-exposed mushrooms is fairly stable. When refrigerated, they retain a significant amount of the nutrient for about a week or more. The effect of cooking depends on the method. Because vitamin D is fat-soluble, cooking mushrooms in oil can cause some of the vitamin to leach out. However, cooking methods like boiling, baking, or pan-frying without oil have been shown to preserve a high percentage of the vitamin D2. Some studies have found that frying for five minutes can retain as much as 88% of the original vitamin D2 content.
D2 vs. D3: A Food-Focused Comparison
It's important to note the type of vitamin D mushrooms produce. The form found in fungi is vitamin D2 (ergocalciferol). The type our bodies produce from sun exposure and that is found in animal products like fatty fish and egg yolks is vitamin D3 (cholecalciferol). From a nutritional standpoint, both forms are used by the body. However, many studies suggest that vitamin D3 is more effective at raising and maintaining overall vitamin D levels in the blood. This is because D3 appears to have a higher bioavailability and a longer duration of action in the body. While D2 is still beneficial and contributes to your total intake, D3 is generally considered the more potent form. Nonetheless, for those following plant-based diets, UV-exposed mushrooms are one of the only significant, unfortified food sources of vitamin D available.
