You spend more time outside in summer. The sun feels strong, and by August you probably have a bit of a tan. So your vitamin D must be sorted, right? For a surprising number of adults in Germany, it is not, and that is not because they did anything wrong.
The most representative survey of German adults found that 61.6 percent had serum vitamin D below the threshold considered adequate, with 30 percent in clear deficiency. Even at peak sunshine, only about half the adult population reaches a level researchers call sufficient (Rabenberg et al., 2015). Summer helps. It just does not finish the job for everyone.
Vitamin D status across the year in German adults
Share of adults reaching a sufficient level (25-OH-D at or above 20 ng/ml), by season, from the DEGS1 national survey
Adults at or above 20 ng/ml50% of population benchmark
Illustrative chart based on DEGS1 seasonal data. Not patient data.
Why summer often falls short
Vitamin D in the body comes from two routes: a small amount from food, and a much larger amount produced in the skin when UVB radiation hits a precursor molecule. The summer assumption is that the skin route takes care of itself. Several things in modern life quietly interfere with that.
Indoor lives. Most adults spend their daylight hours inside. Office, gym, cafe, home. The sun is strongest between roughly 11am and 3pm, which is precisely when most people are not in it. A walk after work in late afternoon does very little for vitamin D synthesis because the UVB component has already faded.
Latitude. Germany sits between 47 and 55 degrees north. Even at the height of summer, the sun is only at a useful angle for a few hours around midday. From October to March, atmospheric scattering blocks most UVB regardless of how much time you spend outside. The DEGS1 data showed a clear north-to-south gradient inside Germany, with people in northern latitudes consistently lower than those in the south (Rabenberg et al., 2015).
Body composition. Vitamin D is fat-soluble. The more body fat you carry, the more of it gets held in fat tissue and kept away from circulation. The same German survey identified higher body mass index as one of the strongest independent predictors of low vitamin D, even after accounting for sun exposure and supplement use.
Skin tone and age. Melanin slows synthesis. Darker skin types need significantly longer sun exposure to produce the same amount of vitamin D as fairer skin. Synthesis efficiency also declines with age, so a 70-year-old produces a fraction of the vitamin D from the same sun as someone in their twenties.
Magnesium status. Even when your skin produces vitamin D, your body needs magnesium to activate it. Every enzyme that converts vitamin D into its active form depends on magnesium (Uwitonze and Razzaque, 2018). If magnesium is low, vitamin D output can look fine on paper while the activation step is held back.
Sunscreen is often cited, but the evidence that it drives widespread deficiency is weak. Wearing it is absolutely the right call for skin protection. The reality is that most people in Germany simply do not get the right kind of midday sun exposure consistently enough, regardless of what they apply.
Take-away. Sunshine alone is an unreliable supply line. Where you live, how you spend your days, your body, and even your magnesium status all shape the result.
Where each factor intervenes in vitamin D synthesis
From sunlight on skin to the active hormone, and the points where common factors reduce the output
Illustrative diagram of the vitamin D pathway and common modifiers. Not to scale.
What the numbers actually mean
The standard test measures a molecule called calcidiol, also written as 25-OH-D. This is the storage form of vitamin D that your liver makes from the vitamin D reaching it through the blood. It is what laboratories report when they give you a vitamin D level, and it reflects your status over the past few weeks. When the body needs vitamin D to act, the kidneys convert calcidiol into its active hormonal form. Most international guidelines split calcidiol levels into the ranges below.
Below 12 ng/ml
Deficient
A clear shortfall, associated with bone, immune and muscle effects in research.
Low
12 to 20 ng/ml
Suboptimal
Below the German Nutrition Society adequacy threshold but above clinical deficiency.
Watch
20 to 30 ng/ml
Sufficient
The standard adequacy target used in most German and European guidelines.
In range
30 to 50 ng/ml
Extended target
A target endorsed by European expert consensus for the broader, non-bone effects of vitamin D (Pludowski et al., 2022).
Optional
The 20 ng/ml mark is where most German and international guidelines define adequacy for bone health. A European expert panel has placed the optimal target for the broader effects of vitamin D higher, at 30 to 50 ng/ml (Pludowski et al., 2022). Without testing, you cannot know which side of these thresholds you sit on.
Take-away. One number tells you whether you are comfortably in range or sitting just below a line you did not know existed.
Why it matters beyond bones
Vitamin D is best known for calcium absorption and bone health, but receptors for it sit in almost every tissue in the body, including immune cells, skeletal muscle, and the brain. That helps explain why low status keeps showing up in research on outcomes well outside the skeleton.
A pooled analysis of 25 randomised trials with more than 11,000 participants found that vitamin D supplementation reduced the risk of acute respiratory infections, with the strongest effect in people who started clearly deficient (Martineau et al., 2017). For mood, a meta-analysis of randomised trials suggests vitamin D supplementation may produce a modest improvement in depressive symptoms in adults, particularly in those with a confirmed shortfall (Mikola et al., 2023). For physical performance, a meta-analysis in athletes found that supplementation improved lower-limb strength, with the largest effect in those training mostly indoors (Zhang et al., 2019).
None of this is a guarantee that lifting your level fixes a specific symptom. What the research does suggest is that vitamin D matters in more places than the skeleton alone, and that the difference between deficient and sufficient is not trivial.
A blood test does not diagnose anything on its own. It tells you whether you are working from a strong starting point or a quiet shortfall.
How to know where you stand
Vitamin D is one of the few markers where intuition is genuinely unreliable. In a randomised trial of 644 adults, dose and starting level together explained only about a quarter of the variability in how much serum vitamin D actually rose with supplementation. Body mass, baseline status, sun exposure and genetics accounted for the rest (Waterhouse et al., 2014). Two people on the same routine can sit at very different levels. The only way to know is to measure.
Which test fits which question
1
Just want a focused answer. A Vitamin D add-on measures calcidiol as a single marker, with results in up to 2 working days. Best if your other panels already look fine and you only want to close this one gap.
2
Curious about your immune capacity more broadly. The Immune Health package covers vitamin D alongside a white blood cell differential, IgG, hs-CRP, zinc, selenium, and B12. Best if you catch every cold and want the wider picture.
3
Suspect a broader nutritional gap. The Nutrition package covers vitamin D alongside iron, ferritin, transferrin saturation, and the B-vitamins B2, B6, B9 and B12. Best if you also feel persistently low on energy.
Aware Add-On
Vitamin D, single biomarker
One marker, one venous blood draw, results in your Aware app within 2 working days. The fastest way to find out where your level sits today.
Want a wider picture? The Immune Health package covers vitamin D alongside a full immune panel including IgG, hs-CRP, zinc and selenium. The Nutrition package pairs vitamin D with iron, ferritin and the key B-vitamins. Both return results in up to 4 working days.
Vitamin D in summer, frequently asked
What is a sufficient vitamin D level?
Most international guidelines consider serum calcidiol below 12 ng/ml deficient, 12 to 20 ng/ml suboptimal, and 20 ng/ml or above adequate. A European expert panel places the target for the broader effects of vitamin D higher, between 30 and 50 ng/ml. There is no single official optimal value, and your doctor can interpret your number in the context of your overall picture.
Can I get enough vitamin D from food alone?
It is difficult. The richest natural sources are oily fish, egg yolks, and a few mushrooms exposed to UV light. Most German adults take in less vitamin D from diet alone than the recommended intake, which is one reason status drops so steeply in winter when sun exposure also disappears.
Does sunscreen completely block vitamin D synthesis?
No. Laboratory studies show that high SPF can reduce vitamin D synthesis under controlled conditions, but in real-world use most people apply less than the recommended amount, so some UVB still reaches the skin. Sunscreen contributes to suboptimal status rather than acting as the main driver, and it remains the right choice for protecting your skin.
Why does the same vitamin D dose work better for some people than others?
Individual response to supplementation varies widely. In a randomised trial of more than 600 adults, dose and starting level together explained only about a quarter of the variability in how much serum vitamin D actually rose. Body composition, baseline status, sun exposure and genetics account for the rest. Without a blood test, it is difficult to know whether your own routine is delivering what you think it is.
How often should I test my vitamin D level?
Once a year is a reasonable starting point for most adults, ideally measured at the end of winter to capture your low point and again at the end of summer to see your peak. If you are supplementing or making lifestyle changes, retesting after 8 to 12 weeks shows whether your level has actually moved.
EN. This article is for informational and educational purposes only. It does not constitute medical advice, a medical opinion, or a diagnosis, and must not be used as a substitute for professional medical consultation, diagnosis, or treatment. Aware's blood testing services are designed to provide health data, not to diagnose or treat medical conditions. Always consult a qualified healthcare professional before making changes to your health routine or if you have concerns about any symptoms.
DE. Dieser Artikel dient ausschliesslich zu Informations- und Bildungszwecken. Er stellt keine medizinische Beratung, kein medizinisches Gutachten und keine Diagnose dar und darf nicht als Ersatz für eine professionelle medizinische Beratung, Diagnose oder Behandlung verwendet werden. Die Bluttestdienste von Aware sind dazu bestimmt, Gesundheitsdaten bereitzustellen, und dienen nicht der Diagnose oder Behandlung von Krankheiten. Bitte wende dich immer an eine qualifizierte Ärztin oder einen qualifizierten Arzt, bevor du Änderungen an deiner Gesundheitsroutine vornimmst oder wenn du Bedenken hinsichtlich deiner Symptome hast.
