Summer and Vitamin D: These interfering factors you should be aware of

Figure 1: UV-B radiation from sunlight is necessary for vitamin D to be produced through the skin. However, spending a lot of time outdoors in summer does not automatically ensure adequate vitamin D status.

Vitamin D can be produced by the body with the help of sunlight. It is therefore tempting to assume that we are automatically sufficiently supplied with the so‑called “sun vitamin” in summer. But is that really the case? In this blog article you will learn why vitamin D is so important for the body and how cutaneous vitamin D production works. We also explain which factors can impair or even prevent vitamin D production in the body. Finally, you will receive seven practical tips to help maintain a healthy vitamin D level year‑round and prevent deficiency effectively.

Why is vitamin D so important for humans?

Figure 2: Vitamin D has a very broad spectrum of effects. Among other things, this nutrient can support bone health, the immune system and muscle strength and function.

Vitamin D is often mainly associated with its central role in bone metabolism. In fact, the nutrient supports the absorption of calcium and phosphate from the gut and thus makes a major contribution to the formation and maintenance of strong bones and teeth.¹ But this is by no means the only physiological function in which the “sun vitamin” is involved: vitamin D also strengthens immune defence and can influence susceptibility to infections.² It also plays an important role in muscle function, contributing to normal muscle strength and thereby helping to reduce the risk of falls.³ Last but not least, vitamin D can support the cardiovascular system by positively affecting both the heart’s contractile function and blood pressure regulation.⁴ Increasingly, studies also point to a link between vitamin D and mental health. A deficiency is frequently associated with fatigue, lack of drive and even depressive moods.⁵

How does the body produce vitamin D?

Although vitamin D is officially classed as a vitamin, in many respects it acts more like a hormone. Unlike most vitamins, it is not primarily obtained from food but is produced largely by the body itself. UV‑B radiation from sunlight is necessary for this endogenous production. When UV‑B reaches the skin, the body’s vitamin D synthesis begins: a precursor of the vitamin is first converted into previtamin D3 and then into vitamin D3 (cholecalciferol). This enters the bloodstream and is transported to the liver, where it is converted into the storage form 25‑hydroxyvitamin D. The kidneys then convert it into the biologically active form, so‑called calcitriol. Only in this active form can vitamin D exert its diverse functions in the body.

According to the Federal Institute for Risk Assessment (BfR), sun exposure in Germany from October to March is not strong enough to ensure adequate vitamin D production via the skin. As a fat‑soluble vitamin, however, vitamin D can be stored well in the body — especially in fat and muscle tissue. It is therefore all the more important to purposefully replenish the body’s stores during the sunnier months. However, this does not happen automatically: various factors can impair vitamin D production. To be well supplied during the winter months, it is therefore worth knowing and taking into account possible interfering factors.

Important potential disruptors of vitamin D uptake

Figure 3: Obesity and certain intestinal diseases can impair the formation and utilisation of vitamin D in the body, as can a very low‑fat diet or the consistent use of high‑factor sunscreens.

It is a widespread misconception that spending a lot of time outdoors in summer automatically ensures good vitamin D status. In fact, sun exposure alone does not guarantee optimal vitamin D production. Various factors — such as pre‑existing conditions, dietary habits or individual lifestyle factors — can impair the production, absorption and utilisation of vitamin D. Below we summarise the most important of these potential disruptors.

1. Certain intestinal diseases

Vitamin D is a fat‑soluble vitamin that is absorbed in the intestine together with dietary fats. Accordingly, malabsorption syndromes and various intestinal diseases can significantly impair absorption. For vitamin D to be absorbed in the small intestine, intact intestinal villi as well as sufficient bile and digestive enzymes for fat digestion are required. If these processes are disturbed, less vitamin D reaches the bloodstream and deficiency can occur — even with adequate sun exposure in summer.

The most important intestinal conditions that can impair vitamin D absorption include coeliac disease, Crohn’s disease, ulcerative colitis, exocrine pancreatic insufficiency and bile acid deficiency. The so‑called short bowel syndrome, which often develops after bariatric procedures such as gastric bypass or gastric sleeve surgery, can also lead to pronounced vitamin D deficiency due to the markedly reduced absorptive surface area in the intestine.⁶

2. Very low‑fat diet

For vitamin D to be taken up efficiently in the intestine, so‑called micelles must first be formed — small fat droplets that enable the absorption of fat‑soluble substances. On a very low‑fat diet, in malnutrition, or in diseases that impair fat digestion, this process can be disrupted. As a result, fewer micelles are formed and the absorption of vitamin D is significantly reduced. Even with apparently adequate intake of the nutrient via sunlight, food or supplements, the body may be insufficiently supplied. In general, it is advisable to consume vitamin D‑containing foods together with a small amount of fat to optimise absorption.

3. Obesity and overweight

An increased body weight can also affect vitamin D status. People with obesity often have lower vitamin D levels than normal‑weight individuals despite comparable sun exposure or intake.⁷ Why is that? Because vitamin D is fat‑soluble, it is stored more in adipose tissue and is therefore less available in free, bioavailable form in the blood. Greater body fat thus has corresponding effects. Studies show that two to three times the usual amount of vitamin D may be required in overweight individuals to achieve an adequate serum level.⁸

4. Medication

Another important disruptor of vitamin D status is certain medications. For example, glucocorticoids (e.g. cortisone, prednisolone) can interfere with the action of vitamin D in the body. In addition to affecting vitamin D metabolism, these drugs also inhibit calcium absorption in the gut, which can promote bone loss in the long term. Certain antiepileptics can also impair vitamin D status by inducing liver enzymes that accelerate the breakdown of vitamin D. This lowers the concentration of the vitamin in the blood — even when adequate vitamin D is being supplied.⁹

5. Liver and kidney diseases

Both the liver and the kidneys play key roles in vitamin D metabolism. In the liver, vitamin D is converted into the storage form 25‑hydroxyvitamin D. In chronic liver disease, this 25‑hydroxylation can be impaired, resulting in less vitamin D being converted into its circulating form. In the kidneys, the conversion into the biologically active form calcitriol takes place. Chronic renal insufficiency can hinder this process, so that the body overall has less active vitamin D available.

6. Sunscreen

The recommendation to use sunscreen with a high sun protection factor daily, especially in summer, is widespread — and for skin cancer prevention it is absolutely sensible. However, it is often overlooked that very high sun protection factors can significantly inhibit or even almost completely block endogenous vitamin D production by blocking UV‑B radiation. Thus, vitamin D deficiency can occur even in people living in sunny regions — for example if sunscreen with SPF 50 is applied before every period spent outdoors. At the same time, short periods of sun exposure of 10 to 30 minutes are often sufficient to stimulate vitamin D production, depending on skin type. This allows a healthy balance to be achieved: supporting the body’s vitamin D production without unnecessarily increasing the risk of skin cancer and skin damage.

7. Other potential disruptors

In addition to the factors already mentioned, there are further influences that can limit endogenous production, absorption and activation of vitamin D. The skin’s ability to synthesise vitamin D naturally decreases with age. At the same time, older people often have reduced dietary intake, which overall increases the risk of deficiency. Skin pigmentation also plays an important role: dark skin has a natural UV barrier due to higher melanin content, which shields the UV‑B radiation needed for vitamin D synthesis more than light skin; consequently, longer sun exposure is required to produce comparable amounts.

Moreover, a high‑fibre diet can also have an impact, since certain fibres can bind fat‑soluble vitamins such as vitamin D in the gut and reduce their absorption. Another important factor is magnesium status. The mineral acts as a cofactor in vitamin D metabolism and is particularly involved in conversion to the active form of vitamin D. Magnesium deficiency can impair vitamin D activation and limit its effectiveness in the body. Genetic factors can also interfere with vitamin D handling. Some variants in genes involved in vitamin D metabolism (e.g. in CYP enzymes or the vitamin D receptor (VDR)) can affect individual processing and efficacy of the nutrient. Such genetic peculiarities are rare but can be relevant, particularly in therapy‑resistant deficiency states.

Well supplied year‑round: 7 tips for a healthy vitamin D level

Figure 4: Simple measures and lifestyle adjustments can effectively improve vitamin D status and help prevent deficiency in the long term.

Adequate vitamin D supply is crucial for health and wellbeing throughout the year. However, especially in summer a few things should be borne in mind so that vitamin D stores can be sufficiently replenished during the sunnier months. The following tips and recommendations can help you optimise your vitamin D intake in everyday life and effectively prevent deficiency:

• Use sunlight to produce vitamin D: Natural light — specifically the UV‑B component of sunlight — is a necessary element for producing vitamin D in the skin. In summer, you should therefore regularly expose uncovered skin (e.g. arms and legs) to sunlight. The rule of thumb is: better regular and short than rare and long. Around 10 to 30 minutes of sun exposure can — depending on skin type — be sufficient. After this short sun exposure, the skin should be protected with an appropriate sunscreen to avoid skin damage.
• Plan your time outdoors consciously: Vitamin D production depends strongly on the intensity of UV‑B radiation, which is highest around midday in this region. Ideally schedule outdoor stays between 11:00 and 16:00. At the same time, be aware that a lifestyle that takes place predominantly indoors — for example due to office work — can significantly limit vitamin D synthesis.
• Include vitamin D‑rich foods: Although diet usually makes only a limited contribution to vitamin D supply, it can still be supportive. Especially oily sea fish such as salmon, herring or mackerel, as well as egg yolk, edible mushrooms (e.g. chanterelles, button mushrooms) and Gouda contain relevant amounts of vitamin D and should therefore appear regularly on the menu.
• Always combine vitamin D with fat: The fat‑soluble vitamin D is absorbed optimally in the intestine only in combination with dietary fats. Make sure not to consume vitamin D‑containing foods fat‑free, but combine them with healthy fats — for example from nuts, avocado or vegetable oils.
• Have your vitamin D status checked regularly: To find out your individual vitamin D status, regular check‑ups with your doctor are recommended. Especially in winter months or if you have existing risk factors for vitamin D deficiency, you should consistently have blood tests carried out. If a deficiency is confirmed, targeted supplementation can be considered in consultation with medical professionals.
• Ensure adequate supply of important cofactors: To activate and utilise vitamin D optimally, the body needs various other nutrients. Above all magnesium, but also boron and calcium support vitamin D metabolism. A balanced diet provides the best basis for supplying these cofactors in sufficient amounts.
• Do not shower immediately after sun exposure: A cold shower after sunbathing is refreshing and cools you down quickly. Nevertheless, it may be sensible to wait a short time before showering after sun exposure. It is suspected that part of the vitamin D precursors formed by sunlight remains in the skin initially and only enters the bloodstream with a delay. To avoid impairing endogenous vitamin D production and “washing off” the vitamin, it is therefore recommended not to clean the skin immediately after sun contact.

Disclaimer

This article does not replace treatment by a qualified therapist. The basis of this contribution is studies and current literature. It must not be used for self‑diagnosis or self‑treatment. Discuss any inspirations from this article with a therapist you trust.

Biography

Katharina Korbach regularly writes blog posts on medicinal plants and natural active substances for the Narayana Verlag. She developed an interest in language early on and began writing her own literary texts. A serious illness during her final school year prompted an intensive engagement with health and nutrition topics that continues to this day. After repeated failures of conventional medical treatments, she decided on a more self‑empowering, naturopathic therapeutic approach. A plant‑based diet was a key element of her healing journey.

Katharina studied Cultural Studies (B.A.) and Applied Literary Studies (M.A.). In 2022 she published her debut novel “Sperling” with Berlin Verlag. Today she works as a freelance author, medical editor and lecturer in Berlin. She spends her free time preferably with friends or at barre training. She also loves to travel and try out new vegan recipes.

1. Voulgaridou G, Papadopoulou SK, Detopoulou P, Tsoumana D, Giaginis C, Kondyli FS, Lymperaki E, Pritsa A. Vitamin D and Calcium in Osteoporosis, and the Role of Bone Turnover Markers: A Narrative Review of Recent Data from RCTs. Diseases. 2023 Feb 8. https://pmc.ncbi.nlm.nih.gov/articles/PMC9944083/.
2. Johnson CR, Thacher TD. Vitamin D: immune function, inflammation, infections and auto-immunity. Paediatr Int Child Health. 2023 Nov. https://pubmed.ncbi.nlm.nih.gov/36857810/.
3. Stockton KA, Mengersen K, Paratz JD, Kandiah D, Bennell KL. Effect of vitamin D supplementation on muscle strength: a systematic review and meta-analysis. Osteoporos Int. 2011 Mar. https://pubmed.ncbi.nlm.nih.gov/20924748/.
4. Han L, Xu XJ, Zhang JS, Liu HM. Association between Vitamin D Deficiency and Levels of Renin and Angiotensin in Essential Hypertension. Int J Clin Pract. 2022 Jun 10. https://pubmed.ncbi.nlm.nih.gov/35814306/.
5. Dionisie V, Gaman MA, Anghele C, Manea MC, Puiu MG, Stanescu-Spinu II, Baiu OI, Antonescu F, Manea M, Ciobanu AM. Vitamin D and depression in adults: A systematic review. Biomol Biomed. 2025 Apr 30. https://pubmed.ncbi.nlm.nih.gov/40322928/.
6. Peterson LA. Bariatric surgery and vitamin D: key messages for surgeons and clinicians before and after bariatric surgery. Minerva Chir. 2016 Oct. https://pubmed.ncbi.nlm.nih.gov/27280871/.
7. Park CY, Han SN. Vitamin D and obesity. Adv Food Nutr Res. 2024. https://pubmed.ncbi.nlm.nih.gov/38777414/.
8. Ekwaru JP, Zwicker JD, Holick MF, Giovannucci E, Veugelers PJ. The importance of body weight for the dose response relationship of oral vitamin D supplementation and serum 25-hydroxyvitamin D in healthy volunteers. PLoS One. 2014 Nov 5. https://pubmed.ncbi.nlm.nih.gov/25372709/.
9. LoPinto-Khoury C, Brennan L, Mintzer S. Impact of carbamazepine on vitamin D levels: A meta-analysis. Epilepsy Res. 2021 Dec. https://pubmed.ncbi.nlm.nih.gov/34847425/.

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Figure 1: Nicoleta Ionescu/shutterstock.com ; Figure 2: onimate/shutterstock.com ; Figure 3: Dina Lukoianova/shutterstock.com ; Figure 4: Tatjana Baibakova/shutterstock.com

23.04.2026