The Label
Vitamin D — 10 mcg
The one your skin manufactures, and then cannot for half the year. Here is what it is, what it does, and what our figure means.
What Vitamin D Is
Vitamin D is the strange one 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 sitting in the epidermis. The liver and then the kidney convert it into the active form, which travels the body binding receptors that sit directly on DNA.
The form on our panel is cholecalciferol, D3, the same molecule your skin makes. It raises and maintains blood levels more effectively than the plant-derived D2.
What It Does
In the barrier
Skin cells carry vitamin D receptors, and the active form influences how a keratinocyte matures on its journey from the basal layer to the surface. That maturation is what produces a barrier capable of holding water in rather than a loose pile of cells.
In immune defence at the surface
Vitamin D drives production of cathelicidin and other antimicrobial peptides in skin. Your barrier is not simply a lid; it is an active defence, and these peptides are part of what makes it one.
In the hair follicle
The vitamin D receptor is required for normal hair cycling, which is known from a rare inherited condition where a broken receptor produces near-total hair loss. That establishes the receptor matters. It does not establish that supplementing a replete person changes anything, and we will not pretend it does.
Our Amount
10 mcg In Every Serving
Our serving carries 10 mcg, which is 400 IU in the older unit. It is a maintenance amount rather than a corrective one. Anyone with a diagnosed deficiency will be prescribed considerably more than this by their doctor, and should follow that rather than a gummy.
Sunlight on skin is where vitamin D comes from, and above a certain latitude the necessary wavelength simply does not reach the ground between November and March. Inside skin it governs how cells mature on their way to the surface and drives the antimicrobial peptides that make your barrier a defence rather than only a lid.
When People Fall Short
Insufficiency is genuinely common, and unlike most items on this panel it has a clean geographic explanation. Above roughly 37 degrees of latitude the necessary UVB wavelength does not reach the ground with enough intensity between November and March. Your skin can be in full winter sunlight and still manufacture nothing.
Risk is higher for people with darker skin, who need substantially longer exposure to make the same amount, for people who work 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, and it is cheap.
Where It Occurs In Food
| Source | Roughly | Notes |
|---|---|---|
| Oily fish, salmon | 10–14 mcg per 100 g | By far the best food source |
| Sardines, tinned | About 4 mcg per 100 g | Cheap and consistent |
| Egg yolk | About 1 mcg each | Varies with how the hens were raised |
| Fortified milk | About 2.5 mcg per cup | Fortification does most of the work |
| UV-exposed 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 our 10 mcg is a tenth of it. Vitamin D is fat soluble and stored, so unlike the B vitamins it can accumulate.
Genuine toxicity comes from sustained very high dosing, not from a panel like this one, and it works by raising blood calcium: nausea, weakness, frequent urination and, if it continues, kidney damage. If you already take a separate vitamin D supplement, add the two figures together before adding ours.
Answers
Vitamin D Questions
The questions we are asked about this nutrient specifically.
Should I get a blood test?
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. Deficiency is common and correcting it properly needs a real number.
Is 400 IU enough?
It is a maintenance amount that keeps a replete person topped up through winter. It is not a correction for a diagnosed deficiency, which needs a prescribed dose.
Can I just get it from the sun?
For part of the year, above a certain latitude, no amount of time outdoors produces any, because the wavelength does not arrive. And burning to make vitamin D trades a small benefit for a large risk.
Does vitamin D help skin conditions?
Vitamin D analogues are prescribed topically for psoriasis, which tells you the receptor matters. That is a prescription treatment, not an argument for a supplement, and the two should not be confused.