Vitamin D, and whether we’re getting enough of it, is always much discussed during the shorter winter days. “Vitamin D is a fat-soluble vitamin and is important for bone metabolism, among other things,” says Dr Anne Gürtler, a dermatologist and nutritionist based at Ludwig-Maximilian University Hospital, Munich. “Vitamin D is not primarily obtained through diet, but through formation in the skin through sunlight,” she adds, pointing out that the sun in European countries may not be strong enough between October and March to ensure sufficient vitamin D formation.
That said, the fact that the sun isn’t as strong through the winter months doesn’t automatically mean that you are vitamin D deficient. This is due to the body’s ability to store vitamin D during the sunnier months – vitamin D stores that reduce over the winter can be replenished in the spring. However, you can help to boost your vitamin D levels by incorporating your diet. “Vitamin D can also be absorbed through food, but only contributes a small proportion of the vitamin D supply,” Dr Gürtler cautions.
Which foods contain vitamin D?
Dr Gürtler points out that milk and egg yolks are particularly rich in vitamin D – more precisely vitamin D3 (colecalciferol), which can be particularly well utilised by the body. When it comes to plant-based foods, mushrooms are one of the best ingredients to stock up on as they are rich in vitamin D2 (ergocalciferol). “We can currently see that the consumption of mushrooms as a nutrient-rich food is increasing worldwide,” says Dr Gürtler.
Why are mushrooms good for boosting vitamin D?
“Ergosterol is found in the cell membrane of mushrooms and can be converted into vitamin D2 when exposed to UVB light,” says Dr Gürtler. “In particular, mushrooms with a large lamella [the gills] surface, for example oyster mushrooms, seem to be able to produce relatively large amounts of vitamin D2.” Dr Gürtler says some mushrooms are specially irradiated with UV-B light during the cultivation process, in order to promote vitamin D production. “However, most supermarket mushrooms are cultivated in the dark (especially button mushrooms) and therefore have a negligible vitamin D.”
How much vitamin D is in mushrooms?
“Theoretically, the amount of vitamin D in mushrooms can be increased through targeted exposure to sunlight or the
use of UV-B lamps. Cutting the mushrooms increases their surface area, which can further increase vitamin D accumulation,” says Dr Gürtler. Place cut mushrooms on a sunny windowsill or balcony, with gills facing up, for up to two hours. “… According to studies, this can increase the amount of vitamin D by up to 30μg (1,200IU) per 100g.” As a guide, in the UK, the recommended daily allowance of vitamin D for healthy adults is 400IU.
Why you still shouldn’t rely on mushrooms for your vitamin D intake
Dr Gürtler points out that the results of studies carried out on mushrooms around the optimal exposure duration, intensity and ambient temperature are contradictory. She also notes that it’s unclear what impact cooking mushrooms has on their vitamin D content. If you know you are vitamin D deficient, it’s important to seek out targeted vitamin D supplementation, she says.
How do you know if you have a vitamin D deficiency?
The best way to determine whether or not you have a vitamin D deficiency is with a blood test.