Vitamin D - how much the sunshine vitamin can do
Public interest in vitamin D has a centuries-old history. The first cases of rickets, a childhood disease leading to deformed bones and representing a severe consequence of vitamin D deficiency, were described in the 16th century. It was significantly later that the connection between rickets and vitamin D, as well as between vitamin D and sunlight, was recognised. Adolf Windaus described the chemical structure and metabolism of vitamin D in the early 20th century, for which he received the Nobel Prize in Chemistry. From then on, vitamin D was increasingly used industrially. Even a vitamin D beer called "Sunshine - Health with Enjoyment" by the Schlitz brand was intended to prevent diseases, especially during the colder months.
More recently, in the early 2000s, a veritable vitamin D hype was triggered, driven by an increasing number of study results combined with alarming blood levels in the population. Vitamin D gained enormous recognition and was attributed a key role in the treatment and prevention of various diseases. At the same time, critical voices emerged, disputing a widespread vitamin D deficiency. To this day, there are very different recommendations regarding vitamin D. Clear guidelines now come from the German Federal Institute for Risk Assessment (BfR) and the German Nutrition Society (DGE).
But first things first: What exactly is Vitamin D?
Vitamin D is the collective term for various calciferols. The two most important are plant-based vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol), found in living organisms. Vitamin D is chemically related to steroids, which is why it is sometimes referred to as a hormone. It belongs to the fat-soluble vitamins, which the body can store in fat and muscle tissue and which are best absorbed by the body with something fatty.
Daily Requirement of Vitamin D
The requirement for vitamin D is specified in either micrograms (µg) or international units (IU), where 1 µg = 40 IU.
The German Nutrition Society (DGE) recommends an estimated reference value of up to 20 µg of vitamin D per day for children aged one year and older and adults of all ages, which corresponds to 800 IU.
The European Food Safety Authority (EFSA) set new tolerable upper intake levels for vitamin D in its 2012 revision, which remained unchanged in the 2021 assessment:
- 100 µg (4,000 IU) per day for children aged 11 and over, adolescents and adults
- 50 µg (2,000 IU) for children aged 1 to 10 years
- 25-35 µg (1,000-1,400 IU) for infants under 1 year
The best way to determine individual supply is by measuring the 25-hydroxyvitamin D marker in the blood, also known as 25(OH)D. It can be expressed in units of nmol/l or ng/ml, where 1 nmol/l = 2.5 ng/ml.
If the value is 50 nmol/l, the vitamin D requirement is quite well covered. Red warning lights flash if this marker falls below 30 nmol/l. An optimal serum level would be 70 to 125 nmol/l.
Vitamin D-Rich Foods
Foods such as salmon, mackerel, egg yolk, cow's milk, and mushrooms provide vitamin D, but they would need to be consumed in kilograms to adequately cover the vitamin D requirement. In fact, only 10 to a maximum of 20% of the requirement is covered by food. Fortunately, the body is able to produce vitamin D itself; this gives vitamin D an absolutely unique position among other vitamins. At least theoretically, because vitamin D production requires a strong supporter: sunlight.
Summer, Sun, Vitamin D
For the formation of vitamin D in the body, cholesterol from the liver is always the starting point. Under the influence of UVB radiation and heat, cholecalciferol is first formed, and via calcidiol and through the liver and kidneys, finally the hormonally active form of the vitamin, calcitriol, also known as dihydroxyvitamin or 1,25-OH.
The extent of vitamin D production is influenced, among other things, by clothing, skin type, and age, but naturally also by the duration, time of day, and manner of sun exposure. While in southern regions, a 10- to 12-minute stay in the midday sun with uncovered arms, legs, and face can cover the daily requirement, we in the higher latitudes of Germany need proportionally longer. From October to March, the sun is generally too low for us; the angle of incidence of the sun's rays is then insufficient to stimulate vitamin D production in the skin cells. Thus, in Germany, we only have the opportunity to replenish our vitamin D stores during the summer months, and ideally between 10 AM and 4 PM. Without sunscreen, of course, because it blocks UVB rays. And this already points to a dilemma. Because foregoing sunscreen is a direct invitation to sunburn or worse.
The Right Way to Handle the Sun
How does one find the right balance between sun exposure and vitamin D formation on one hand, and adequate sun protection against long-term damage on the other? What you can definitely do is accustom your skin to the sun. Because when bare skin is exposed to UVB radiation, our clever body begins to adapt to the environment and store melanin in the epidermis as a self-protection mechanism. Melanin is a pigment formed from L-tyrosine that gives skin and hair their colour. The darker the skin, the more melanin is stored and the less sensitive it is to radiation. The flip side of the coin: the lower the vitamin D production. If the skin is regularly exposed to small doses of UVB radiation, you can benefit from the onset of vitamin D production before pigmentation sets in and inhibits this mechanism. You should definitely start this with the first spring sun rays, where UVB radiation is comparatively less aggressive. But – and this is very, very important – when in doubt: apply sunscreen sooner rather than later. If the skin starts to redden, skin damage is already present that can lead to serious long-term consequences. To put it simply: for vitamin D deficiency, there are tablets and drops; for skin cancer, there aren't.
Vitamin D Food Supplements
Food supplements are a sensible way to complement insufficient dietary and endogenous vitamin D production. The active ingredient is often of animal origin and is extracted from the sebaceous glands of sheep (called lanolin or wool wax). But vegan alternatives are also possible; for these, lichens and fungi are used as raw materials. Tablets and drops are easy to dose and supply us with the required amount of vitamin D all year round.
Vitamin D serum levels are subject to strong seasonal fluctuations. To know if everything is in the healthy range, it is advisable to have your vitamin D level professionally tested once or twice a year and to optimally adjust the amount of supplements. High-dose vitamin D intake over a longer period can be medically prescribed but should not be undertaken independently.
What Role Does Vitamin D Play in Bone Health?
The sun hormone is involved in a number of processes in the body. Particularly through its interaction with calcium and phosphorus, it manages the entire bone metabolism. In the case of vitamin D deficiency, the body cannot properly absorb and utilise the calcium ingested through food, which often simultaneously leads to a calcium deficiency. Because the body tries to counteract this, it releases calcium from the bones (osteomalacia), which in turn can lead to a decrease in bone density and subsequently to osteoporosis in adults or rickets in children. A suboptimal vitamin D status demonstrably reduces bone formation in children and adolescents and accelerates bone breakdown in adults and older people. Thus, vitamin D is indispensable for normal mineralisation of bones and teeth.
Further Functions of Vitamin D
- Cell Division: Vitamin D regulates cell proliferation and differentiation in many tissues, maintaining the growth of pathological and normal cells at a healthy level.
- Muscle Function: Vitamin D stimulates protein synthesis, which is the basis for the formation of muscle cells and muscle fibres. In clinical vitamin D deficiency, muscle weakness is typical, which can manifest, for example, in gait disturbances (so-called waddling gait). Independently, vitamin D stimulates calcium influx into muscle fibres.
- Immune Response: Vitamin D plays an important role in the functioning of the immune system by promoting the formation of T-lymphocytes (also killer cells/helper T-cells), which are crucial for the immune response. It primarily supports the innate immune system in fighting viruses and bacteria.
Risk Groups Regarding Vitamin D Supply
According to the German Federal Institute for Risk Assessment (BfR), 60% of Germans have insufficient vitamin D levels. This is often due to the fact that we spend a lot of time indoors even in the summer months, for example, due to our jobs in buildings, but also due to illness and frailty. Elderly people, children, and infants particularly often have a vitamin D deficiency; overweight individuals also belong to the risk group, as their fat tissue can retain a lot of vitamin D.
A British cross-sectional study concluded that pre-tanned individuals and those with darker skin colours sometimes benefit more from food supplements than from UV radiation (a reminder: their skin contains more of the skin pigment melanin, which inhibits vitamin D formation). Furthermore, vitamin D synthesis in summer must be high enough to sufficiently fill the body's own vitamin D stores for supply during the winter months. This is quite a challenging matter, and for this reason alone, almost all individuals in our latitudes are potentially at risk, at least during the darker season.
Over- and Under-dosing
A deficiency can manifest non-specifically as fatigue, limb and muscle pain, cramps, as well as circulatory and concentration problems. On the other hand, an overdose is also possible, for example, through long-term intake of high-dose food supplements without medical advice. Since fat-soluble vitamin D is stored in the body, a gradual overdose is possible in addition to an acute one. Therefore, supplementation, especially in higher doses, should always be preceded by a professional check of the 25-hydroxyvitamin D level.
Vitamin D plus K - a sensible combination?
At this point, we want to focus on vitamin D. However, this is hardly possible without also taking a look at vitamin K. So, here's a small spoiler: among other things, vitamin K2 is responsible for incorporating the calcium absorbed with the help of vitamin D into the required places in the body. Both vitamins work synergistically.
You can find out exactly how this works and more here.
Conclusion:
A complete supply of vitamin D through diet is almost impossible. Factors such as age, ability for self-production, duration of time spent outdoors, number of sun hours, application of sunscreen, and the season determine how much vitamin D your skin produces. In many cases, supplementation via food supplements is recommended to consistently achieve target blood serum levels. Vitamin K represents a sensible addition.
