Vitamin D and sunshine: how much daylight you really need

Strictly speaking, vitamin D is not a classic vitamin at all, but a hormone precursor. What makes it special is that your body can produce it itself with the help of sunshine. That is exactly why it is so relevant if you want to stay fit and healthy over the long term.

Vitamin D helps regulate how well your body absorbs calcium, making it essential for strong bones and teeth. It also plays a role in muscle strength and supports the immune system. A persistent deficiency often causes non-specific symptoms, such as tiredness, muscle weakness or increased susceptibility to infections.

Foods such as oily fish, eggs and fortified products provide only small amounts. The rest depends on daylight. Next, let’s look at exactly how your skin makes vitamin D from sunlight.

The key factor is the sun’s UV-B radiation. When it reaches your skin, it converts a cholesterol precursor into vitamin D. Your liver and kidneys then turn this into the active form your body can use.

Whether this works depends mainly on the angle of the sun. When it is high in the sky, enough UV-B reaches your skin. When it is low, the atmosphere filters out the radiation, so hardly anything happens. This is why the time of day matters: during the summer half of the year, production is highest between roughly 11 am and 3 pm.

It also matters what blocks the radiation. Glass, clothing and high-factor sun cream let hardly any UV-B through. Behind a window or when fully covered, your body produces virtually no vitamin D, even if it feels warm and sunny.

The good news is that it is less than many people think. In high summer, short spells are often enough to top up your stores. As a rule of thumb, expose around a quarter of your body surface, such as your face, hands and parts of your arms and legs, to the sun several times a week.

The exact duration depends on your skin type and the season. Fair skin produces vitamin D more quickly, while darker skin needs considerably longer. Balance is important: you want enough sunshine to produce vitamin D, but no sunburn. Staying out longer than necessary offers no added benefit. On the contrary, it raises your risk of skin cancer.

The table below offers a rough guide for unprotected skin around midday. It does not replace an exact measurement, but it helps you get a feel for it.

In longevity circles, vitamin D is often promoted as a key to a longer life. The evidence is more sober, and that is exactly why it is worth taking a closer look.

Its role in bones and muscles is clearly proven: good vitamin D levels reduce the risk of falls and brittle bones in older people. There are also plausible indications of benefits for the immune system. That is no small thing when it comes to staying active and independent as you age.

For the big promise of living longer, with less cancer and fewer heart attacks, however, the evidence is thin. The large VITAL study from 2019 found that additional vitamin D reduced neither cancer nor serious cardiovascular events. Individual meta-analyses suggest that long-term use may slightly reduce overall mortality, but the findings are inconsistent.

The bottom line: vitamin D is an important building block, but not a miracle cure. Correcting a deficiency is good for you. If your levels are already adequate, taking more tablets offers little additional benefit.

By winter at the latest, the question of drops or capsules comes up. They are particularly useful if you produce little vitamin D through sunlight or belong to a risk group: older people, people with darker skin, those who rarely spend time outdoors, or those who usually keep their body covered.

The DGE gives 20 micrograms (800 IU) per day as guidance when the body produces no vitamin D itself. This matches the maximum daily amount for food supplements stated by the BfR. A low daily dose is better than infrequent high-dose boluses.

Before taking a high dose long term, it is worth checking your actual level. A simple blood test with your doctor will show your 25-OH level. This lets you supplement based on your needs rather than guesswork, while avoiding taking unnecessarily large amounts.

You do not need to track your sunlight exposure meticulously. A few simple habits will see you through the year: vitamin D through your skin in summer, with a little support in winter. Our Longevity Snapshot helps you put your routines into perspective, while Outdoor Weather helps you find the best time window for getting outside.