AUSTRALIANS spend $100 million on vitamin D supplements every year, yet we have extremely high rates of vitamin D deficiency, with almost one in four individuals being deficient (serum 25(OH)D < 50 nmol/L) and a further two in five insufficient (serum 25‐hydroxyvitamin D [25(OH)D] 50 to < 75 nmol/L). Low vitamin D levels are now a global public health issue.
Vitamin D is so unique that it’s technically not a vitamin and acts more like a hormone. This is because unlike other vitamins, humans can make our own vitamin D when our skin is exposed to ultraviolet (UV) light. We obtain 90% of the vitamin D we need this way, with the remaining 10% from foods and supplements.
Vitamin D’s primary function is to regulate how the body absorbs calcium, so it is essential for bone health. But nearly every cell and tissue in our bodies interacts with vitamin D, and it even influences hundreds of our genes. It is unsurprising then that there is growing research showing that low levels of vitamin D might be linked to cardiovascular disease, cancer, gastrointestinal health, immune function (here and here), mental health, metabolic health, fertility and diabetes.
Sun exposure or supplements: only one source is currently recommended
The prevalence of vitamin D deficiency in Australia is even higher among office workers, people who have immigrated from Africa, and in patients undergoing chemotherapy. Less exposure to sunlight and having a darker skin colour both mean that you make less vitamin D. Other reasons for less sunlight exposure include winter, living in southern states (less UV light available even if you get some sunlight exposure), or avoiding the sun altogether due to skin cancer risk, illness or limited mobility (eg, aged care). Even if people get enough sunlight exposure, other lifestyle factors may also affect your risk, such as not being physically active, being obese and smoking. Sun exposure is so important that the recommended targets for vitamin D blood levels are higher at the end of summer to provide a buffer and account for the fact that in winter most people won’t be getting enough vitamin D and their stores will dramatically drop.
What are the current guidelines to address deficiency and what are their limitations?
The Royal Australian College of General Practitioners has guidelines for vitamin D and musculoskeletal health. They recommend increased sun exposure in cases of mild deficiency (serum 25(OH)D 30–49 nmol/L), and supplementation in cases of moderate to severe deficiency (serum 25(OH)D < 30 nmol/L). The guidelines do not consider diet at all. Given that Australia has one of the highest rates of skin cancer globally, research shows that it is near impossible for Australians to meet vitamin D requirements in winter while adhering to sun safe messages (here and here). Up to half an hour of sunlight exposure at midday is required in some states during winter, and that’s with your face, arms and hands exposed (a very unrealistic scenario for most). In summer, however, just 10 minutes of mid-morning or late afternoon sunlight exposure, with face, arms and hands exposed, will provide enough sunlight for most people, no matter where you live in Australia. For those who can get a little sun exposure, there are other lifestyle benefits. For example, going for a walk during a working break with a colleague may lead to increased social connection, physical activity, and stress management.
Although supplementation is an effective way to increase blood levels of vitamin D, many people may be taking supplements when they don’t need to (here, here and here). Supplements also cost money, may be forgotten about or not used regularly enough to increase vitamin D in the body, and also don’t provide benefit beyond increasing vitamin D levels (unlike foods, which have other nutrients, or sunlight, which may encourage physical activity). For individuals who are vitamin D deficient and have limited sunlight exposure, such as those who have darker skin, who cover up, or those who live in the southern states, supplements may be effective and the only way to meet vitamin D targets.
What can diet do for vitamin D?
While sunlight exposure and supplementation are recommended to address vitamin D deficiency, there is a strong case to consider dietary sources of the vitamin. As a bonus, when consuming foods and beverages with vitamin D, it may naturally lead to increased intakes of nutritious foods that contain other essential nutrients and bioactive compounds such as polyphenols. In contrast to sunlight, foods rich in vitamin D do not increase skin cancer risk.
Unfortunately, Australians don’t yet consume enough vitamin D through foods and beverages. Low dietary vitamin D intake in Australia is likely due to the fact that few foods are rich in vitamin D, and foods that are good sources, such as oily fish, egg yolks and other animal foods such as chicken skin, are not consumed in large enough amounts. Foods fortified with vitamin D can also play a role, but unlike other countries, few foods require mandatory fortification with vitamin D in Australia and this includes oils and spreads (eg, margarine) but excludes milk. What most people don’t know is that another significant dietary source of vitamin D is fungi.
“Tanned” mushrooms: the star vitamin D food?
If you regard mushrooms as just another vegetable alongside carrots, green beans or broccoli, you would be mistaken. Mushrooms are not actually a vegetable at all, but rather fungi, and have unique properties that are generally only found in animals. This means that, unlike plants, fungi have a high concentration of ergosterol (pre-vitamin D), which is extremely similar in structure and function to the type naturally found in the human skin. The only difference between the two is a double bond between carbon 22 and 23 and an extra methyl group at carbon 24. Like in our skin, ergosterol is also converted to vitamin D when exposed to UV light.
Exposing mushrooms to UV (from sunlight or in a laboratory) increases the amount of vitamin D in mushrooms by nearly eightfold. Putting five store-bought button mushrooms in the sun, or just one portobello mushroom, produces 24 µg of vitamin D, which translates to nearly 1000 international units, providing the amount of vitamin D one needs in an entire day, and the equivalent found in most vitamin D supplements.
If you’re wondering if the vitamin D from mushrooms actually makes it into your bloodstream, it does. A recent meta-analysis of randomized controlled trials showed that tanned (UV-exposed) mushrooms may be effective in increasing active vitamin D levels in adults with low levels of vitamin D, and studies (randomised controlled trials) have shown that it may be just as effective as supplements at increasing vitamin D levels in the blood (here, and here).
Some research is very positive, saying that putting your mushrooms in direct sunlight for 10–15 minutes may provide you with 100% of your daily vitamin D needs, and the vitamin D content in sunlight-exposed mushrooms may be retained with refrigeration for up to 8 days. The production of vitamin D may be increased by a further 30% by placing them in the sun with the underside, or gills, facing up, or by 60% if you slice them. Important to consider is that the same factors that affect vitamin D production in humans also affect mushrooms (where they grow and the season).
For those who enjoy mushrooms, they may present a prevention and treatment strategy to help address vitamin D deficiency, especially during winter or when sunlight exposure is limited. Consuming UV exposed mushrooms may also offer other health benefits that are related to the consumption of whole foods, while potentially replacing the consumption of less nutritious foods too.
While vitamin D is unique to the other vitamins in that we can make it ourselves, it is worth remembering we can also obtain it from food. Current guidelines and recommendations may have forgotten this. Combining evidence-based dietary strategies with sunlight or supplementation may significantly improve the vitamin D status of Australians as well as provide other important lifestyle benefits.
We must realise the potential of every source of vitamin D and make recommendations to suit the needs of the individual. Next time you’re thinking about a patient that is at risk of vitamin D deficiency, don’t forget to ask if they eat mushrooms.
Dr Flavia Fayet-Moore is a nutrition scientist, Accredited Practising Dietitian, board-certified Lifestyle Medicine Professional, and Honorary Associate of the University of Sydney. She is the CEO of Nutrition Research Australia, where she leads a team of highly skilled researchers to conduct end-to-end nutrition and communications projects.
Funding statement: The research used to inform this article was funded by Hort Innovation, using the mushroom research and development levy and funds from the Australian Government.
The statements or opinions expressed in this article reflect the views of the authors and do not represent the official policy of the AMA, the MJA or InSight+ unless so stated.