Total Vitamin D

Total vitamin D measures 25-hydroxyvitamin D, the circulating form your body makes from sunlight and absorbs from food. It has a half-life of two to three weeks, so it reflects your status over the previous month or two rather than yesterday.

Vitamin D acts more like a hormone than a vitamin. Receptors for it appear in tissue throughout the body, and it governs calcium absorption and bone mineralisation as well as taking part in immune signalling and muscle function.

Below 20 ng/mL is under the reference range. At latitudes above roughly 37 degrees, skin synthesis effectively stops between autumn and spring because the sun never gets high enough in the sky, so a low winter result is the norm rather than the exception in northern Europe.

Darker skin needs substantially longer sun exposure for the same synthesis, and the capacity to make vitamin D in skin declines with age. Body fat sequesters vitamin D, so a higher body fat percentage lowers the circulating result at the same intake.

Supplementation is the practical route back into range for most people through winter, since diet alone rarely covers the gap.

Between 20 and 50 ng/mL your result sits inside the reference range for this panel.

Expect seasonal movement. A result taken in late summer and the same person tested in February can differ substantially, which is worth knowing before reading too much into a single number.

Above 50 ng/mL almost always reflects supplementation rather than sun exposure. Skin synthesis is self-limiting, so you cannot reach a high result from sunlight alone.

Vitamin D is fat soluble and accumulates. Sustained high intake raises calcium absorption, and the consequences of that show up as raised blood calcium rather than as anything you would notice early.

If your result is above the range, review your total intake including multivitamins, fortified foods and cod liver oil, and discuss it with your practitioner before continuing at the same dose.

Indoor work, glass between skin and sun, and sunscreen use during the hours when synthesis is possible have made low vitamin D a default state rather than a deficiency in the older sense of the word.

The dietary sources that historically covered the winter gap at northern latitudes, oily fish and cod liver oil, have largely dropped out of the Western diet at the same time.

Oily fish is the main food source: salmon, mackerel, sardines and herring. Cod liver oil is the traditional concentrated form.

Egg yolks and UV-exposed mushrooms contribute smaller amounts. Mushrooms supply D2, which raises the total but is less effective at doing so than D3.

Fortified foods vary widely by country, so what counts as a reliable source depends on where you shop.

Vitamin D3 raises and holds the result better than D2 at the same dose.

Take it with the largest fat-containing meal of the day. Absorption is considerably lower without dietary fat.

Vitamin K2 is often taken alongside D3 for its role in directing calcium to bone. Magnesium is required as a cofactor in vitamin D metabolism, so a magnesium result at the low end is worth addressing at the same time.