# Vitamin D in Switzerland: Why October to March Is Critical > From October the skin in Switzerland barely produces vitamin D. What the numbers say, how to measure and how to dose sensibly until March. The sun still shines in November. It simply sits too low. For your skin, that is the difference between producing vitamin D and producing none. This article looks at how well people in Switzerland are supplied, when it gets tight, how to measure your level and how to supplement sensibly. ## What vitamin D does in the body Strictly speaking, vitamin D is not a vitamin but the precursor of a hormone. The body makes it itself when UVB radiation hits the skin. The liver and kidneys then convert it into its active form. It regulates the absorption of calcium from the gut and contributes to the maintenance of normal bones, teeth and muscle function. It also plays a role in the normal function of the immune system. These are the functions regarded as established in Europe. Much else written about vitamin D is still the subject of ongoing research. ## The numbers for Switzerland The Federal Office of Public Health has looked into the supply of the population and paints a clear picture: in the winter months a large share of the population sits below the level considered sufficient. The threshold is a blood level of 50 nmol/l 25-hydroxyvitamin D. Below 25 nmol/l is classed as severe deficiency. In summer the distribution looks better, in winter it tips over. Particularly affected are older people, people with darker skin, people who work indoors during the day and everyone who wears sunscreen consistently in summer. The FOPH recommendation for adults is 600 IU (15 micrograms) per day, from age 60 it is 800 IU (20 micrograms). That is hard to achieve through food. Oily fish, egg yolk and some mushrooms contain vitamin D but typically cover only a small part of the requirement. The rest has to come from the sun or from supplementation. ## Why October to March is critical The skin only produces vitamin D when UVB radiation arrives. That depends on the angle of the sun. When it sits low, the atmosphere filters out the UVB share before it reaches the ground. For the latitude of Switzerland, roughly 46 to 47 degrees north, this means: from about October to March, your own production is practically zero, even under a blue sky and even at 1500 metres. A simple rule of thumb: if your shadow is longer than you are, your skin is not producing vitamin D. In October that is the case in Switzerland even at noon. The body stores vitamin D in fatty tissue. A full store from summer carries you for a few weeks. After that the blood level falls steadily until the sun climbs high enough again in spring. If you only react in January, you have already reached the low point. The right time to act is October. ## Measure instead of guess The level can be determined in the blood: 25-OH vitamin D, also called calcidiol. Your family doctor takes the sample and the lab delivers the result within a few days. Depending on the lab, the cost is around 50 to 60 francs. Without a medical reason you pay for the test yourself, with an indication basic insurance covers it. How to read the values: - below 25 nmol/l: severe deficiency, see a doctor - 25 to 50 nmol/l: insufficient - 50 to 75 nmol/l: sufficient - 75 to 125 nmol/l: named by many experts as the target range - above 250 nmol/l: too high Two measurements a year give you the full picture: one at the end of September at the summer high, one at the end of February at the winter low. The gap between the two shows how strongly your body depends on sunlight. ## Dosage: what makes sense The basic rule: dose according to the measured value, not according to feeling. Without a test, the FOPH recommendation remains the safe frame, meaning 600 to 800 IU daily. If the value is clearly below 50 nmol/l, the doctor will usually dose higher for a few weeks and then switch to a maintenance dose. The European Food Safety Authority names 4000 IU per day as the upper limit for adults that should not be exceeded without medical supervision. Vitamin D is fat-soluble. An excess is not excreted but stored. Very high doses over months can throw the calcium level out of balance. Practical points: 1. Form: vitamin D3 (cholecalciferol) is the form the body also produces itself. 2. Intake: with a meal that contains fat. That improves absorption. 3. Rhythm: daily is easier to remember than weekly. If you prefer a weekly dose, always take it on the same day. 4. Duration: October to the end of March, then measure again. 5. Partners: vitamin K2 and magnesium are often mentioned together with vitamin D because they are involved in calcium metabolism. If you take blood thinners, talk to your doctor first. ## Using the summer sun properly The best preparation for winter begins in April. From April to September, 10 to 20 minutes of midday sun on face, arms and legs, three to four times a week, is enough for the skin to build a store. After that, sunscreen, because the goal is not sunburn. Nothing happens behind glass, window panes do not let UVB through. Your store does not fill up in the car or at the office window either. ## Your plan until March - October: measure the blood level, discuss the result with your doctor, set the dose. - October to March: take it daily, ideally tied to breakfast. - February: second measurement, adjust the dose. - April: reduce or stop the supplement, use the sun. Vitamin D is not a topic for January. It is a topic for today. ## Frequently asked questions **Is a winter walk enough for vitamin D?** For mood and sleep rhythm yes, for vitamin D no. From October to March the sun in Switzerland sits too low. The skin produces practically no vitamin D, even under a clear sky. **Can I overdose on vitamin D?** Yes. Vitamin D is stored in fatty tissue. Permanently more than 4000 IU per day without medical supervision is not recommended. Anyone dosing high should have their blood level checked regularly. **Do I really need to measure my level?** Not necessarily, but it pays off. Without a measurement you are dosing blind. Two tests a year show you how your body responds and whether the dose is right.