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Prevailing wisdom maintains that mushrooms can not produce Vitamin D3, and it has been a vociferously debated topic of late. But new research promises to upset conventional assumptions in favor of these fungal friends of ours.
Whether or not mushrooms provide a legitimate source of vitamin D activity has been a hotly contested topic over the past few years. Back in 2014, we were pulled into the fray after publishing the mycologist Paul Stamet's article on the topic titled, "Place Mushrooms in Sunlight to Get Your Vitamin D." Soon after, the Healthy Home Economist published an article by Sylvia P. Onusic aggressively titled "Debunking the Absurd Notion of Vitamin D in Mushrooms," wherein she calls out the presumably erroneous view of Paul that non-animal sources of vitamin D are relevant to meeting nutritional requirements for these indispensable biomolecules. Sylvia states:
"In the past few years many blogs have expounded on the "glorious" vitamin D properties of mushrooms.
"Mushrooms are not only wonderful sources of vitamin D, but they offer us many opportunities for maintaining and improving health," says mycologist Paul Stamets (5).
But that wonderful source he's talking about is vitamin D2, which is otherwise made artificially from radiating ergot and yeasts."
In her article, Sylvia points out important differences between D2 and D3, but argues so strongly for the superiority of vitamin D3 over D2 that it would seem to imply vegans or vegetarians who do not have sufficient sun exposure should reconsider their dietary/ethical stance or suffer dire health consequences.
What I find so interesting about this exchange is that Paul never argues that sun-exposed mushrooms are superior to animal derived forms of vitamin D3; nor is he denying that vitamin D3 is superior in bioactivity to vitamin D2. He is only arguing that mushrooms are a legitimate alternative, and considering the needs of sunlight-deprived vegans/vegetarians or those who are not in a position to access or manufacturer animal derived vitamin D3 themselves, they may be the only available source to them.
Decomposing The Assumption That Mushrooms Can't Produce Vitamin D3
Research exists from 2013 that demonstrates how sunlight-exposed mushrooms produce not only D2, but also D4 (with about 60% the biological activity of D3) and D3 itself – a discovery that fundamentally undermines Sylvia's argument against mushrooms.
Published back in 2013 in the journal Dermatoendocrinology and titled, "Photobiology of vitamin D in mushrooms and its bioavailability in humans," the study revealed that mushrooms indeed are a vegan source of vitamin D3 and related isomers:
"Shiitake mushrooms not only produce vitamin D2 but also produce vitamin D3 and vitamin D4."
The study also referenced research that disproves Sylvia's claims:
"A study of the bioavailability of vitamin D2 in mushrooms compared with the bioavailability of vitamin D2 or vitamin D3 in a supplement revealed that ingestion of 2000 IUs of vitaminD2 in mushrooms is as effective as ingesting 2000 IUs of vitamin D2 or vitamin D3 in a supplement in raising and maintaining blood levels of 25-hydroxyvitamin D which is a marker for a person's vitamin D status." [Note: Full study details at the end of this article]
Their conclusion clearly indicates that mushrooms constitute an excellent source of vitamin D:
"Therefore, mushrooms are a rich source of vitamin D2 that when consumed can increase and maintain blood levels of 25-hydroxyvitamin D in a healthy range. Ingestion of mushrooms may also provide the consumer with a source of vitamin D3 and vitamin D4."
Clearly, if vitamin D3 can be produced by fungi, the ongoing argument between those who believe that animal products are necessary to fulfill basic human dietary requirements and those who maintain that the vegan/vegetarian lifestyle is rendered more complex than hitherto considered. We have also recently seen other basic assumptions about human physiology and metabolic nees challenged. For instance, the discovery that chlorophyll is capable of helping the body capture energy directly from the sun, a capability formerly attributed only to autotrophs like plants. We have also seen turmeric's role in helping vegetarian's enhance DHA synthesis, perhaps undermining the Paleolithic fixation on eating much higher on the trophic scale for optimal health. There is also the fact that since we are constituted by 99% microbes, everything we once believed about our body and it's relationship to the world has basically been redefined.
The lesson here may be that in the field of human nutrition and physiology we must consider the presumption of certainty utterly premature at this stage of our learning, with open-mindedness always a more appropriate intellectual stance.
Full Vitamin D Bioavailability Study Details: We conducted a clinical study to determine if ingestion of vitamin D2 in a dried white button mushroom extract (Monterey Mushrooms, Inc.) was as effective at increasing and maintaining vitamin D status as supplemental vitamin D3 and vitamin D2. Thirty healthy adults were enrolled in the study (6 male, 19 female, mean age 35.2 y) and were randomized to ingest capsules containing 2000 IU vitamin D2, 2000 IU vitamin D3 or 2000 IU mushroom vitamin D2 once a day for three months during the winter. Vitamin D concentrations were verified to be within 10% by HPLC. Twenty-five subjects completed the study. Fourteen subjects were randomized to the mushroom vitamin D2 group, eight subjects to the supplemental vitamin D2 group and 3 subjects to the supplemental vitamin D3 group. Serum concentrations of 25(OH)D2, 25(OH)D3 and 25(OH)D were measured once a week for 12 weeks by liquid chromatography tandem mass spectroscopy (LCMS/MS) as previously described.28
Subjects in the mushroom vitamin D2 group had a mean baseline serum 25(OH)D2 of 0.6 ± 0.3 ng/mL that increased significantly to 18.6 ± 1.4 ng/mL at the end of 12 weeks (p < 0.0001). Total serum 25(OH)D levels increased from 20.6 ± 2.4 ng/mL to 30.1 ± 2.6 ng/mL (p < 0.001) (Fig. 8A).
Figure 8. Mean (± SEM) 25-hydroxyvitamin D2 (♦), 25-hydroxyvitamin D3 (■) and total 25-hydroxyvitamin D (●) concentrations over time after oral administration of (A) 2000 IU of mushroom vitamin D2 in capsules (n = 14), ...
Subjects in the supplemental vitamin D2 group had a mean baseline serum 25(OH)D2 of 1.5 ± 1.2 ng/mL that increased significantly to 13.3 ± 2.0 ng/mL at the end of 12 weeks (p < 0.001). Total serum 25(OH)D levels significantly increased from 19.4 ± 2.3 ng/mL to 29.2 ± 2.0 ng/mL (p < 0.01) (Fig. 8B).
Subjects in the supplemental vitamin D3 group had a mean baseline serum of 25(OH)D3 of 16.3 ± 0.6 ng/mL with a final baseline serum level of 34.4 ± 1.3 ng/mL (Fig. 8C). Total 25(OH)D increased from 17.1 ± 1.4 ng/mL to 34.4 ± 1.3 ng/mL (p < 0.05). The discrepancy in mean baseline serum levels is due to some detectable 25(OH)D2 of 0.8 ng/mL (Fig. 8C).
Baseline serum total 25(OH)D levels were not significantly different between the groups; 17.1 ± 1.2, 19.4 ± 2.3 and 20.6 ± 2.4 ng/mL for the supplemental D3 and vitamin D2, and mushroom vitamin D2 groups respectively. Serum 25(OH)D levels gradually increased and plateaued at seven weeks and were maintained for the following five weeks. At the end of 12 weeks, serum total 25(OH)D levels were not statistically significantly different in all three groups: 34.4 ± 1.1, 29.2 ± 2.0 and 30.1 ± 2.6 ng/mL for supplemental vitamin D3, D2 and mushroom D2 respectively.
 De Luca HF, Weller M, Blunt JW, Neville PF. Synthesis, biological activity., and metabolism of 22,23-3H vitamin D4. Arch Biochem Biophys. 1968;124:122–8. doi: 10.1016/0003-9861(68)90310-X. [PubMed]