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Common at our latitudes, especially in winter — important for bones and muscles, and easy to clarify with a simple test.
Also known as: vitamin D deficiency · low 25-OH vitamin D
Open in the tool — with search and all the links →This compilation is for information and replaces neither medical diagnosis nor treatment. If you have symptoms, see a doctor.
Vitamin D is really a hormone, and the body makes it itself — in the skin, under ultraviolet light. North of the Alps the sun is not enough for this from about October to March: the radiation comes in too flat. So the store empties over the winter, and the lowest values are found in late spring. What is measured is 25-OH vitamin D, because it reflects the store. What counts as a good value is handled differently in different places; there is agreement about the lower limit, below which bones and muscles suffer. The benefit is genuinely established where the shortage is large: in soft bones, in the tendency to fall in older age, in children to prevent rickets. For many further promises the evidence is thinner than the reputation suggests — large studies in people with normal starting levels have shown little. That does not speak against topping up a real deficiency, but against the idea that more is always better. Very high doses over long periods can do harm, because they drive the calcium level. Hence the simple principle: measure first, then dose, and re-check once with longer use.
Skin makes vitamin D from UVB. Works well in summer and at midday — but in Swiss winter the sun is too low.
The most reliable way to raise levels — especially in winter. Dose to your baseline; K2 can sensibly accompany it.
Very high doses over long periods can harm (calcium). Establish your baseline and match the dose — ideally with a clinician.
Very high amounts over long periods drive the calcium level and can harm the kidney. More is not better here, and at some point worse.
Vitamin D is fat-soluble and taken up much better this way. On an empty stomach part of it is lost.
The store fills slowly. A check says whether the chosen amount fits — too little for some, long since enough for others.
Short spells outdoors with face and arms uncovered are usually enough in summer. In winter that does not work — supplementing is then the simpler route.
With a marked shortage the parathyroid hormone rises to hold calcium — at the bone’s expense. The two together show how serious the shortage is.
Between October and March the sun stands too low at our latitudes for the skin to make vitamin D — no matter how long one stays outside. The level falls over the winter and rises again from April; refilling the store takes eight to twelve weeks.
Vitamin D fills a store that builds over weeks: with daily intake the blood level reaches its new plateau in eight to twelve weeks. It acts on bone and calcium — there the benefit is clear when a deficiency is documented. In people with sufficient levels, large trials showed no added benefit for colds, heart or cancer. Too much over months (well above 4000 units a day without checks) drives calcium up.
A mild shortage is not felt. So the value says more here than the feeling — and without measuring it is easy to take too much or too little.
Out of a chair, going upstairs. With a marked shortage, an earlier sign than bone pain.
Unlike joint pain: deeper, more spread out, often pelvis, ribs or shin, and tender to pressure. A sign of a marked, long shortage.
All three lower what the skin can make. Sun protection does too — which does not make it bad, only explains why little forms despite summer.
Vitamin D is taken up with fat. Where fat absorption suffers, so does it — the usual dose is then not enough.
What a deficiency makes visible on the body — skin, nails, tongue, eyes and nerves. The signs are unspecific: they show where to look, they prove nothing.
More a dragging in thighs and upper arms than pain in one place, often together with difficulty getting up and climbing stairs.
In winter the sun does not reach the necessary angle at our latitudes — which is why the season is decisive here.
Often supplemented together; the studies on the need for this combination are thin.