The sun gives us energy, vitality, strengthens the immune system, and triggers the synthesis of vitamin D. Perhaps you are reading this from a beach chair or another sunny place and think that this topic does not concern you. You believe that during the summer months, your vitamin D levels will be fully replenished simply by spending more time in the sun. It may even seem strange that in the middle of summer, we could have a vitamin D deficiency. After all, we assume we’ve had enough sunbathing and, as the song lyrics say, “tanned by the second of June.”
Vitamin D deficiency has recently become a major topic of interest in the scientific world, and its insufficiency is reaching pandemic proportions worldwide. It is a significant public health issue across all age groups, estimated to affect 1 billion people globally. The year-round deficiency in the Nordic population, which has an alarming prevalence, calls for necessary public health measures. Therefore, the old recommendation to take vitamin D only during months with the letter “R” is outdated and needs to change.
A group of scientists from the University of Tartu reached the same conclusion in 2010. They found that 75% of Estonians aged 25-70 suffer from vitamin D deficiency during the summer. The average summer vitamin D level was only 59.3 nmol/L, which falls below the recommended norm.
This result aligns with another significant study conducted by Estonian researchers in collaboration with the Estonian Defence Forces, who analyzed their vitamin D levels in 2018. The findings also showed a very high prevalence of vitamin D deficiency among young individuals aged 19-25 during the summer. Previously considered an issue mainly for older people, we can now confidently say that it also affects even healthy young adults. Both studies send the same message: People living in Estonia, regardless of age, gender, or health condition, need to take vitamin D even during the summer months.
But what about the fact that vitamin D is synthesized in the body through the skin when exposed to UVB radiation? This is true, but it is often overlooked that the efficiency of vitamin D synthesis depends on geographic location, time of year, and time of day. These factors significantly affect the angle at which the sun’s rays hit the Earth, which is crucial for sufficient vitamin D synthesis in the skin.
UVB radiation is adequate when the sun is at least 50 degrees above the horizon. A simple way to estimate the sun’s angle is by the length of your shadow. Stand in the sun, and if your shadow is shorter than you, your skin is synthesizing vitamin D. If your shadow is the same length as you or longer, vitamin D synthesis is no longer occurring.
The primary reason for widespread vitamin D deficiency in Estonia lies in our geographic location. It is commonly recommended that 10-15 minutes of sun exposure is sufficient for optimal vitamin D synthesis, and this advice is promoted worldwide. However, taking this seriously and looking at a world map, we see that Estonia is located on the 59th latitude in the northern hemisphere.
The equator, which divides the Earth into the northern and southern hemispheres, is where the sun shines at a 90-degree angle year-round. Moving 40 degrees north or south, the sunlight remains sufficient for adequate vitamin D synthesis. However, the further one moves from the equator, the longer one must stay in the sun to synthesize enough vitamin D. One degree of latitude equals 111.04 km, meaning Estonia is 6551.36 km from the equator.
If you spent your days in the Sahara Desert with your arms and legs uncovered, you would only need 11 minutes of sun exposure. But moving toward Estonia, the time required for vitamin D synthesis increases with every degree of latitude.
Furthermore, age and body mass index also affect your skin’s vitamin D production. In Estonia, pensioners and overweight individuals need 300 minutes of sun exposure to get a sufficient dose of vitamin D. Unfortunately, there are no days in Estonia where the sun is high enough for five continuous hours. We live too far from the equator, which is why our vitamin D levels do not normalize as expected, even during summer.
We share the same latitude as southern Greenland, and Estonia’s summer sun is not a reliable source of vitamin D.
UVB rays that produce vitamin D reach Estonia only one month before and one month after midsummer. UVB intensity increases from spring to the summer solstice and then begins to decrease along with the shortening days. Midsummer is when dusk meets dawn, marking the longest day of the year. By July 21, the length of the day has already decreased by about an hour, and from that moment on, vitamin D synthesis is no longer sufficient.
By August, UVB radiation has lost its ability to synthesize the necessary vitamin D. Thus, Estonia’s summer is an anomaly—no matter how sunny, it does not meet our expectations for vitamin D synthesis.
Vitamin D synthesis is not related to the temperature outside. Afternoon sunbathing, as recommended by doctors, no longer provides the necessary vitamin D and is not inherently safer. We are still exposed to carcinogenic UVA radiation, which primarily causes premature skin aging, and its intensity does not significantly depend on the time of day. While the warmth of a morning sunrise or an evening sunset is enjoyable and boosts serotonin, it does not produce the required vitamin D.
Despite the sun’s essential role in vitamin D synthesis, it is wise to limit direct sun exposure. Tanning became popular in the 1920s when bronzed skin became a symbol of success and fashion consciousness.
Dermatologists firmly state that any tanning damages skin health and is the leading cause of the most common cancer worldwide—skin cancer. Despite strong public awareness campaigns by dermatologists, sun enthusiasts often ignore these warnings. Just five sunburns in a lifetime increase the risk of skin cancer by 80%. Over the past decade, melanoma cases in Estonia have doubled. With Estonians’ fair and sensitive skin, sun protection is strongly recommended. However, sunscreen with SPF 8 already reduces vitamin D synthesis by 92%, meaning that while it protects against skin cancer, it also prevents an increase in vitamin D levels.
So, what is a healthy vitamin D level? In Central Europe, including Estonia, the recommended year-round vitamin D level is 75-125 nmol/L. This is sufficient to maintain good health and high energy levels. According to SYNLAB data, only 16% of tested individuals have vitamin D levels within the recommended range, while 84% suffer from deficiency. To achieve a healthy vitamin D level in Estonia, young and normal-weight individuals should regularly take 2500 IU, while older and overweight individuals should take 4000 IU.
Just as water does not boil at 80 degrees, vitamin D cannot be synthesized with insufficient UVB radiation.
Take care of yourself and your loved ones’ health!
