Built on the same mechanism as this month’s supplement profile, as the best months of this series are. One tray, thirty-five minutes, and the two foods that genuinely carry vitamin D — plus an honest account of how far a plate of food gets you once the Scottish sun stops working.
This month’s supplement is vitamin D, and the argument there is that almost nobody measures it before deciding what to do about it. The obvious follow-up question is the one clients actually ask: can I just eat it?
Partly. And the honest answer has numbers attached, which is why this recipe comes with a paragraph of arithmetic before the ingredients.
Almost nothing in an ordinary diet carries meaningful vitamin D. The two exceptions are oily fish, which carries D3 (cholecalciferol, the same form your skin makes), and mushrooms — but only mushrooms that have been exposed to ultraviolet light, and what they carry is D2 (ergocalciferol).
That distinction is not pedantry, and there is a meta-analysis about it.
A systematic review and meta-analysis of randomised trials directly comparing the two forms found vitamin D3 significantly more effective than D2 at raising serum 25(OH)D (p=0.001).
But the difference was driven entirely by bolus dosing (p=0.0002), and disappeared with daily supplementation. Which is the same daily-versus-bolus distinction that separates the two big vitamin D trials in this month’s supplement piece — arriving from a completely different direction.
So a mushroom is not a fish. On a plate eaten regularly, the gap narrows considerably.
“Mushrooms are full of vitamin D” is half of a true statement. Ordinary supermarket mushrooms contain less than 1 µg per 100 g — effectively none. What they do contain is ergosterol, which converts to D2 when UV light hits it.
There is one randomised trial in humans, and it is worth reading closely rather than quoting.
What they did: irradiated fresh button mushrooms at 1.5 J/cm² of UV-B, which raised D2 content from under 1 µg/100 g to 491 µg/100 g, and made them into a soup. 26 young adults with 25(OH)D at or below 50 nmol/L were randomised in winter to the mushroom soup, a D2 supplement, or placebo — 28,000 IU (700 µg) of vitamin D2 per week in each active arm, for five weeks.
What happened: 25(OH)D rose by 3.9 nmol/L per week (95% CI 2.9–4.8) in the mushroom group and 4.7 nmol/L per week (3.8–5.7) on the supplement. Both beat placebo. The mushrooms were not different from the supplement.
Grade: a genuine randomised result, and a small one. n=26, single-blind, five weeks, young adults, one mushroom species.
Now the arithmetic the headlines skip. 28,000 IU a week is 4,000 IU a day — the whole of the NHS adult upper limit, every day, for five weeks. And it moved 25(OH)D by about 20 nmol/L in total.
That is a real effect and a modest one. It is also not achievable by eating mushrooms from a shop, because 491 µg/100 g is a laboratory number. It came from a controlled UV-B dose. Retail “vitamin D mushrooms” are irradiated commercially and should declare a figure on the pack; ordinary ones will not, because they have none.
What I am not going to do is give you a vitamin D figure for mackerel. The numbers in circulation vary several-fold depending on species, season, and whether the fish was farmed or wild — and quoting one to a decimal place would be exactly the behaviour this site spends its time objecting to. Oily fish is the best dietary source of D3 in the UK diet. That much is not in dispute. The precise number for the fillet in front of you is not knowable from a table.
It is a very good dinner that contributes to your vitamin D intake. It is not a substitute for supplementing through a Scottish winter, and anyone telling you a plate of food replaces 10 µg a day from October to March is selling something. Eat it because mackerel is cheap, sustainable, quick and delicious, and because it happens to carry the thing you are short of. Not instead of the capsule.
Serves 2. Ten minutes of work, twenty-five in the oven, one tray.
The fennel is not decoration. It roasts down sweet and cuts the oiliness of the mackerel, which is the thing that puts people off oily fish in the first place. If you have been avoiding mackerel because you find it too rich, this is the version to try.
You will read that leaving mushrooms gill-side up in the sun converts their ergosterol to D2. The mechanism is real — it is the same photochemistry the trial used.
What has not been established is what a windowsill in Edinburgh achieves. Urbain used 1.5 J/cm² of controlled UV-B and measured the result. A kitchen windowsill filters out most UV-B through the glass, and by October there is very little UV-B reaching Scotland to begin with — which is the entire reason this month’s supplement piece exists.
So: try it outdoors on a bright day if you like, and do not count on it. Commercially UV-treated mushrooms declare a figure on the pack. That is the version with a number attached.
Flake the cold mackerel through the roasted vegetables with a spoon of crème fraîche, a little horseradish and more lemon. That is lunch, and it is better than it was hot.
September’s set runs on one mechanism, which is how this series works best. Supplement of the Month is vitamin D — what the big trials actually found, why two of them disagree, and a correction to a range this site’s own AI was using until August. Fungi of the Month is turkey tail, which is also about the gap between a studied thing and a sold thing. And the recipe archive holds the earlier ones.
If you’re about to start supplementing for the winter and have never seen a 25(OH)D result, you don’t know whether you’re correcting a deficiency or topping up someone already fine. The free call takes twenty minutes.
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Urbain P, Singler F, Ihorst G, Biesalski H-K, Bertz H. Bioavailability of vitamin D2 from UV-B-irradiated button mushrooms in healthy adults deficient in serum 25-hydroxyvitamin D: a randomized controlled trial. Eur J Clin Nutr 2011;65(8):965–971. doi:10.1038/ejcn.2011.53 · PMID 21540874
Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr 2012;95(6):1357–1364. doi:10.3945/ajcn.111.031070 · PMID 22552031
Scientific Advisory Committee on Nutrition. Vitamin D and Health. London: TSO, 2016. RNI 10 µg (400 IU) per day for everyone aged 4 and over.
NHS. Vitamins and minerals — Vitamin D. Adult upper limit 100 µg (4,000 IU) per day.
Educational content, not medical advice. Oily fish is not suitable for everyone: if you are pregnant, UK guidance limits oily fish to two portions a week. If you take anticoagulants, have a fish allergy, or have been told to restrict purines, take the relevant advice before making this a weekly habit.