Vitamin D done right: dose, timing, blood value, myths

Why almost everyone has too little in winter

Vitamin D forms in the skin when UVB radiation hits. North of the 40th parallel, and that includes most of Europe, Canada and the northern United States, the sun sits too low from October to March to bring enough UVB through the atmosphere. Stores from summer last into December, then the level drops. Measurements show that in winter 50 to 60 percent of Central Europeans sit under 20 ng/ml, the range that counts as deficiency. Whoever works indoors, has dark skin, is older or dresses covered often sits below that in summer too.

The blood value

What is measured is 25-hydroxyvitamin D, the storage form. Under 20 ng/ml is deficiency with consequences for bone and muscle, 20 to 30 insufficient, 30 to 50 the target range in which most studies show the largest benefit, 50 to 80 harmless without added benefit, over 100 toxic with calcium excess. The test costs 20 to 30 euros or dollars and is usually only covered with suspicion of deficiency. A measurement at the end of winter, when the level is lowest, makes sense.

The dose

For maintenance in winter 1,000 to 2,000 IU daily suffice, which raises the level by about 10 to 15 ng/ml. With a deficiency under 20 it takes more at first, 4,000 IU over eight to twelve weeks with control, then maintenance. Overweight people need higher doses because vitamin D is stored in fat. Over 4,000 IU permanently only with blood value and doctor. Daily intake is equivalent to weekly high doses, monthly doses are less well supported. With a meal containing fat, because vitamin D is fat-soluble.

What it can and cannot do

Proven: with deficiency, supplementation lowers fractures in older people, improves muscle strength and lowers fall risk, reduces respiratory infections and in meta-analyses cancer mortality slightly. Not proven: in people with a normal level more vitamin D brings nothing for heart, cancer, depression or diabetes; the large studies with healthy people were negative. Vitamin D is a deficiency nutrient you top up, not a remedy you take in high doses.

K2, magnesium and the myths

Vitamin K2 is often sold as a mandatory partner so that calcium goes into bones rather than vessels. With normal doses up to 2,000 IU that is unnecessary; whoever eats vegetables and fermented foods has enough K2. Magnesium is needed to activate vitamin D but present with a normal diet. And the tanning bed makes vitamin D but with the skin cancer risk of a sunbed; a capsule is safer.