Vitamin D in summer: levels don’t rise automatically

19. August 2026 ca. 16 Minuten

What to expect in this article

1. The assumption that falls apart in June
2. What happens in the skin
3. Four conditions compared
4. Why the season is only one factor
5. The cited source
6. What a low value in summer could mean
7. Three statements fact-checked
8. The key point in one sentence
9. Which number appears on the test result
10. Three substantiated points
11. Why the date should be included in the test result
12. Who might benefit from a measurement in summer
13. What a capillary blood test can show here
14. Limits: what the value doesn’t reveal
15. What matters about this value
Frequently asked questions
Sources

The assumption that falls apart in June

Vitamin D is seen as a winter issue. People write about it in autumn, test for it in spring, and by May the question has slipped from their minds.

The assumption behind it is that summer takes care of everything by itself. If you spend time outdoors, you get enough, and a test result in July would be a mere formality.

There is some truth to this assumption, and that is exactly why it persists. The IQWiG notes that levels rise again in summer because of production in the skin (as of November 15, 2023).

This article does not contradict that. It shows what other conditions come into play, and why a season does not tell you a number.

What happens in the skin

It starts with something unusual about this vitamin. It is the only one the body produces itself in significant amounts.

The IQWiG writes: The body can produce vitamin D itself, in the skin with the help of the sun’s UV-B radiation (as of November 15, 2023). The MSD Manual puts the same point more technically: vitamin D3 is synthesized in the skin through sunlight (as of April 2025).

Because it is produced in the skin, its production depends on everything between the sun and the skin, and on anything that changes the skin itself. There are more factors than most people expect.

The IQWiG gives an approximate figure: exposing the face, hands and arms uncovered and without sun protection to the sun for a few minutes about 2 to 3 times a week (as of November 15, 2023). This describes a habit, not a dosage, and this article does not turn it into a recommendation.

Four conditions compared

The table compares four conditions that affect production in the skin. It provides context and does not constitute a diagnosis.

Three of the four rows don’t change with the calendar. The third even moves in the opposite direction in summer.

Why the season is just one of the factors

The difference between a possibility and an outcome is the whole point here.

In winter, vitamin D production isn’t possible in our latitudes, according to the cited source, because the sun is too low in the sky. Summer lifts this barrier. What happens after that depends on the other conditions.

Here’s an image to illustrate it: Winter is a closed door, summer an open one. The open door doesn’t tell you whether anyone walks through it, or how often.

That’s why this article makes a narrower claim than the widely circulated headline. It doesn’t say that levels remain low in summer. It says that the month alone doesn’t determine a number.

The documented source

The sentence that establishes the direction appears in a public source and is unambiguous.

This quote is included here deliberately, even though it contradicts the widely circulated headline on this topic. We’re not arguing against a source, but alongside it.

What the sentence says: There is a seasonal movement, and it goes up in summer. What it doesn’t say: how far up, for whom, and from what starting point.

Only a measurement can answer these three questions. And because levels vary seasonally, every measurement needs a date.

What a low summer level may mean

Someone who gets tested in July and gets a low result is in a different situation from someone with the same result in February.

In February, an obvious explanation is right there: the sun has been too low in the sky for months. In July, that explanation no longer applies, leaving the factors in the table above.

This makes a summer measurement more informative than a winter measurement, and that’s the point this article makes. It measures the level when the season explains it the least.

This article doesn’t say what follows from that. IQWiG notes: Low blood levels alone do not mean that someone has a vitamin D deficiency (as of November 15, 2023). Interpretation belongs in a medical practice.

Three statements fact-checked

These three statements come up every year between May and September.

Checked against the evidence

The bottom line in one sentence

This is where the topic can be brought together.

That’s the opposite of the common logic. Not because everything is fine in summer, but because less of the picture is obscured by the season.

A winter reading alongside a summer reading is even more informative because it makes the seasonal shift described by the source visible.

Which number appears on the test result

A test result doesn’t say vitamin D, but a particular form of it, and there’s a reason for that.

The MSD Manual describes 25(OH)D as the main form in the bloodstream and notes that serum levels reflect the body’s vitamin D stores and correlate better with symptoms than levels of other vitamin D metabolites (as of April 2025).

That is why the value on a lab report is usually called 25-OH vitamin D or 25-OH vitamin D3. It is the value for which comparative figures are available.

For target levels, the same source states: For optimal bone health, 25(OH)D target levels should be above 20 to 24 ng/ml, or about 50 to 60 nmol/l (as of April 2025). IQWiG cites the Robert Koch Institute’s threshold for inadequate levels as below 12 ng/ml (as of November 15, 2023). Both figures are shown here side by side with their sources because they answer different questions.

Three documented figures

Three figures from the reviewed sources. They provide context and are not a diagnosis.

Documented figures

The first two figures are far apart, yet they do not contradict each other. One marks a lower limit, the other a target range for a specific question.

And as with any lab result, the reference range shown on your own report is what matters, because reference values can vary from lab to lab (IQWiG, as of April 2, 2025).

Why the date belongs on the lab report

For most lab results, the date is a formality. For this one, it is part of the result.

If levels naturally decrease in winter and rise again in summer (IQWiG, as of November 15, 2023), then a number without a month is incomplete. The same number means something different in February and in August.

This leads to a practical rule for your records: note the month for each vitamin D result, and later compare results from the same season whenever possible.

Having two results from different seasons doesn’t mean you did anything wrong. It gives you an additional comparison that makes seasonal changes visible instead of hiding them.

Who should consider a summer measurement

This comparison addresses whether a measurement in the warmer half of the year would be useful to you.

What a capillary blood test can show in this case

There are two different ways to measure this value, and they answer different questions. Neither provides a diagnosis.

The mybody®x (MYBODY Lab GmbH) self-test is a rapid test that gives a result within a few minutes. The laboratory option provides a value with a reference range and additional results from the same sample.

If you want to see the result alongside other values and compare it later, you need the laboratory option.

Limitations: what the level cannot tell you

The first limitation is interpretation. IQWiG notes: Low blood levels alone do not yet indicate a vitamin D deficiency (as of 15 November 2023).

The second limitation is the cause. A level does not tell you which of the factors in the table is the most significant for you. Age, sun protection, pigmentation and time spent outdoors are not shown separately.

The third limitation is the implications. A medical practice decides whether a finding has any implications and what they are. The same source notes that taking it without a specific reason usually offers no benefits (as of 15 November 2023).

The fourth limitation is diagnosis. All the classifications in this article provide context but do not make a diagnosis. A diagnosis is made in a medical practice based on several factors, and no self-test can make one in advance.

What matters for this level

If you take away just one thing from this article, let it be this: Summer is a condition, not a guarantee.

The direction is established. Levels naturally decrease in winter and increase again in summer due to production in the skin (IQWiG, as of 15 November 2023). It is also established that age, sunscreen and dark skin pigmentation reduce synthesis (MSD Manual, as of April 2025), and none of these three factors depends on the calendar.

This leads to an unexpected conclusion: A measurement in summer tells you more than one in winter, because the season explains less then. A medical practice determines what follows from the number, and according to the cited source, a low level alone is not a deficiency.

Frequently asked questions

Sources

1. Institute for Quality and Efficiency in Health Care (IQWiG): Vitamin D deficiency, as of 15.11.2023 – gesundheitsinformation.de
2. MSD Manual, Professional Edition: Vitamin D deficiency and dependency, Johnson LE (full review August 2024, last updated April 2025) – msdmanuals.com
3. IQWiG: Understanding laboratory results correctly, as of 02.04.2025 – gesundheitsinformation.de

The verbatim quote about the season, the statements about production in the skin and the lack of production in autumn and winter, the reference to the face, hands, and arms, the threshold of 12 ng/ml cited by the Robert Koch Institute, the sentence about low blood levels, and the sentence about taking supplements without a specific reason are from source [1]. The statements about synthesis through sunlight, age, sunscreens, and skin pigmentation, the description of 25(OH)D as the most important form in the bloodstream, and the target values above 20 to 24 ng/ml are from [2]. The explanation of the reference range is from [3]. Information on price, values, sample type, turnaround time, and laboratory comes from mybody®x product pages, accessed on 18.08.2026; the processing times for the laboratory test follow the central policy for blood tests. All sources were accessed and reviewed on 18.08.2026.