Vitamin D

Vitamin D is really a hormone your body makes itself. When sunlight falls on your skin with enough strength, that skin makes vitamin D from a cholesterol-like substance, which your liver converts into the form the lab measures and your kidneys into the active form. That active form lets your gut absorb calcium and phosphate, keeps your bones firm and your muscles strong, and takes part in your immune system. Food, oily fish, eggs and fortified products, delivers only a small part; the sun is the source. That is the problem for anyone living in the Netherlands. Above the fiftieth parallel the sun stands too low from October to March for the skin to make anything, and the store you build in summer runs out over the course of winter. The blood value is therefore lowest in March and highest in September, and that difference is larger than the difference between two labs. Anyone who spends little time outdoors, has a darker skin, wears covering clothing or applies sunscreen makes less even in summer; anyone with excess weight stores the vitamin in fat where it does not come out. Your report shows vitamin D in nmol/L, as 25-hydroxyvitamin D. Normal is about 50 to 130 nmol/L, where the labs overlap; below 30 nmol/L there is a real deficiency, and the national target is above 30 nmol/L under seventy and above 50 nmol/L above. Above 250 nmol/L it becomes too much, and you only reach that with supplements.

A vitamin D deficiency is the most common of all deficiencies in the Netherlands, and it gives no symptoms for a long time. With a severe deficiency the bones soften, muscles and bones hurt and you become weak in the upper legs; with a mild deficiency nothing happens on the outside, but your body takes calcium from the bones to keep the blood value up, and that costs bone density over the years. For older people muscle strength counts too: anyone with enough vitamin D falls less and breaks less. That is why the advice in the Netherlands is simple, and it applies without a blood test: women from fifty, everyone from seventy, anyone with a darker skin, covering clothing or little time outdoors, and pregnant women take ten micrograms daily, and above seventy twenty. Anyone not in those groups who is regularly outside with bare arms from April to October usually gets through winter without a deficiency, though a large part of the population sits below 50 nmol/L at the end of winter. For anyone who does sport or works indoors a lot, the blood value is mainly a check on winter. A value in September says how large the store is you go into the dark months with, and a value in March whether it worked. More is not better: above 100 no extra gain has been shown, and high doses of supplement can get the calcium in your blood and your kidneys into trouble.

A low value means a store that has run out, and the question is above all when you were tested. From common to rare: little time outdoors, because above the fiftieth parallel the skin makes nothing from October to March and the rest of the year only in the middle of the day; a darker skin, in which more pigment holds back the light and production goes much slower; and covering clothing or sunscreen, which block exactly the light that starts production. After that come excess weight, in which the vitamin is held in fat, poor absorption in the gut with coeliac disease, Crohn's disease or after stomach surgery, and medicines such as anti-epileptics and long-term prednisone. Below 30 nmol/L there is a real deficiency that affects the bones, and you top that up. A high value comes almost always from too much supplement, because your skin stops making it by itself when the store is full and a pill does not. Above 250 nmol/L the calcium in your blood can rise, with thirst, passing a lot of urine, nausea and kidney stones as a result, and then you stop the supplement and have calcium measured. Rare is sarcoidosis or another disease with granulomas, in which inflamed tissue activates vitamin D outside the kidney and calcium rises with an ordinary blood value. Always read vitamin D with the month of the draw alongside, beside calcium and PTH, because a deficiency lets PTH rise and take calcium from the bones, and beside ALP if you have bone pain. With a high value calcium belongs with it.