On the panel
Vitamin D — 10 mcg
Made in your skin by sunlight, and not made at all for several months of the year.
What It Is
Vitamin D is the odd entry on any panel, because it is not really a vitamin. It is a prohormone your skin manufactures when ultraviolet B light strikes a cholesterol derivative in the epidermis. The liver and then the kidney convert it into an active form that travels the body binding receptors sitting directly on DNA.
The form here is cholecalciferol, D3, the same molecule your own skin makes. It raises and holds blood levels more effectively than the plant-derived D2.
What It Actually Does
Three jobs, described without the marketing vocabulary.
Your skin is the factory, seasonally
Above roughly 37 degrees of latitude the UVB wavelength does not reach the ground with useful intensity between November and March. You can stand in full winter sunlight for an hour and manufacture none at all, which is a geography problem rather than a lifestyle one.
It arms the barrier
Vitamin D drives production of cathelicidin and other antimicrobial peptides in skin. The barrier is not a passive lid; those peptides are part of what makes it an active defence.
It has a say in maturation
Keratinocytes carry vitamin D receptors, and the active form influences how a cell matures on its way from the basal layer to the surface. That maturation is what produces a barrier that holds water rather than a loose stack of cells.
10 mcg In Every Serving
Our serving carries 10 mcg, which is 400 IU in the older unit. This is a maintenance figure rather than a corrective one. Anyone with a diagnosed deficiency will be prescribed a great deal more than this by a doctor, and should follow that prescription rather than a gummy.
The UVB wavelength that converts a cholesterol precursor into vitamin D does not reach the ground with useful intensity above roughly 37 degrees latitude between November and March. Inside skin, the active form influences how a cell matures on its way to the surface and drives the antimicrobial peptides that make the barrier an active defence.
Who Tends To Fall Short
Insufficiency is genuinely common and, unlike most items on this panel, it has a clean geographic explanation. Risk rises further for people with darker skin, who need substantially longer exposure to make the same amount, for anyone working indoors, for anyone who covers up for cultural or medical reasons, and for older adults, whose skin synthesises less efficiently.
A blood test is the only way to know where you actually stand, and it is one of the few nutrient tests worth the trouble.
Where It Turns Up In Food
| Source | Roughly | Notes |
|---|---|---|
| Salmon | 10–14 mcg per 100 g | By far the best food source |
| Tinned sardines | ~4 mcg per 100 g | Cheap, consistent, bones included |
| Fortified milk | ~2.5 mcg a cup | Fortification does most of the population's work |
| Egg yolk | ~1 mcg each | Varies with how the hens were raised |
| UV-treated mushrooms | Varies widely | The only meaningful plant source |
| Sunlight | Not a food | Ten to thirty minutes on bare skin, seasonally, and never worth a burn |
The Ceiling, And The Cautions
The adult upper limit is 100 mcg a day and ours is a tenth of that. Vitamin D is fat soluble and stored, so unlike the B vitamins it accumulates rather than being flushed away.
Genuine toxicity comes from sustained very high dosing rather than from a panel like this one, and it works by raising blood calcium: nausea, weakness, frequent urination and, if it continues, kidney damage. Anyone already on a standalone vitamin D should total the two labels before adding this one.
Where This Comes From
- Cutaneous synthesis requires UVB at wavelengths that are seasonally absent at higher latitudes.
- Vitamin D receptor signalling in keratinocytes influences differentiation and antimicrobial peptide expression.
- D3 is retained more effectively than D2 in most comparative work.
General nutritional science rather than claims about this product. Nothing on this page is medical advice; see our disclaimer.
Vitamin D Questions
What we get asked about this line of the label specifically.
Should I get tested?
If you live at northern latitude, work indoors, have darker skin or cover up, it is one of the few nutrient tests genuinely worth doing, because deficiency is common and correcting it properly needs a real number.
Is 400 IU enough?
This is a holding dose, enough to keep somebody who is already sufficient from sliding through winter. It is not a correction for a diagnosed deficiency.
Could I skip the supplement and use sunlight?
Past a certain latitude there are months when standing outside all day produces none, because the necessary wavelength never reaches the ground. Burning to make vitamin D trades a small benefit for a large risk.
Does it treat skin conditions?
Doctors prescribe vitamin D analogues as creams for psoriasis, which tells you the receptor is doing real work. That is a prescription treatment and not an argument for a supplement.