What can this vitamin, which isn’t technically a vitamin, do for you? Here’s everything you need to know about this essential prohormone.
Vitamin D often causes confusion: how can it be found in food but also in sunlight? And why is it called "Vitamin" D when it’s actually a hormone? Here’s what vitamin D3 is, its role in the body, and the best sources.
At a Glance
Vitamin D3 plays a crucial role not only in bone health but also in immune function and muscle maintenance.
Foods like salmon, herring, eggs, and mushrooms are good sources of vitamin D3.
Our bodies can produce this hormone themselves, but supplements through diet and nutritional supplements can be beneficial in cases of low sunlight exposure.
First of all, vitamin D isn’t actually a vitamin, but a hormone—more precisely, a prohormone, which is a precursor to a hormone and plays a role in many biological processes.
So why is it called a vitamin? When it was discovered in the early 20th century during research for remedies, it wasn’t known that the body could produce it, unlike a typical vitamin. Initially classified as the fourth vitamin, scientists later realized that the body stores a precursor of vitamin D in the skin, which it then converts into active vitamin D3 through UVB radiation.
Vitamin D3, also known as cholecalciferol, is the form of vitamin D that our bodies produce. Since it’s still referred to as a vitamin, it’s the only one the body can synthesize. By the way, there’s also vitamin D2, a form of vitamin D synthesized in the human body from plant sterol derivatives found in food. However, this form is considered less effective as a supplement than vitamin D3.
To understand the body's production of vitamin D3, we first need to know what UV rays are. Ultraviolet rays are a type of solar radiation that can be divided into ultraviolet A (UVA) and ultraviolet B (UVB) rays. While UVA rays penetrate deep into the skin and often cause damage, UVB rays don’t penetrate as deeply and are responsible for vitamin D production. When exposed to sunlight, the cholesterol in our skin cells absorbs the UVB rays, providing the energy needed for vitamin D synthesis in the skin.
Vitamin D3 is essential because it plays a role in many biological processes, especially in bone health. Without vitamin D3, our intestines wouldn’t properly absorb calcium and phosphate during digestion, and these important minerals wouldn’t benefit your bones.
It’s worth noting that many organs and tissues in the body have vitamin D receptors, suggesting important functions beyond bone health. These include muscles, kidneys, parathyroid glands, and the skin.
It also supports the immune system, contributes to cardiovascular health, and helps maintain . Moreover, it influences our mental health and by supporting the production of neurotransmitters like serotonin and has anti-inflammatory effects.
The unique aspect of vitamin D3 is that our body produces it through UVB light exposure on the skin. However, if our skin is exposed to insufficient sunlight, the body’s production may not be enough.
The recommended daily intake is around 20 µg, but this is highly individual: for example, people with darker skin may need 10 to 50 times the UV dose compared to fair-skinned individuals, while vitamin D production in 70-year-olds can be 3 to 5 times lower than in younger people.
In northern latitudes, the sun is strong enough to reliably stimulate vitamin D production in the skin for only about 3 to 4 months a year (usually from April to September). You also need to consider your actual sun exposure—how often are you outdoors in the sun each day?
You can supplement your body’s vitamin D3 intake with the following foods:
These foods contain vitamin D3.
A Vitamin D deficiency can manifest through bone pain, muscle weakness, increased susceptibility to infections, and chronic fatigue. Mood swings, hair loss, and delayed wound healing can also indicate a deficiency. If you’re concerned about a vitamin D deficiency, your doctor can provide guidance.
When supplementing with vitamin D3 through foods or supplements, it’s important to note that vitamin D is fat-soluble, meaning it’s best absorbed with a fat source to increase bioavailability. Ideally, you should take vitamin D3 with a meal that contains healthy fats, such as olive oil, avocado, or nuts, or with a supplement that provides vitamin D3 in oil.
It’s also scientifically proven that vitamin D and vitamin K2 together support cardiovascular health, help maintain bone density, and promote normal immune function. The combination of vitamins D3 and K2 offers greater benefits together than either nutrient alone—this concept is known as nutrient synergy. For this reason, it makes sense to use a supplement that combines vitamin D3 and vitamin K2.
In theory, it is possible to take too much vitamin D3. Symptoms of an overdose may include nausea, vomiting, loss of appetite, weakness, fatigue, increased thirst, and frequent urination. However, these cases are rare, as very high amounts of the vitamin would be necessary. You can also rest assured that overproduction from too much sun exposure is unlikely.
In short: yes. Sunscreen can impair the production of vitamin D in the skin because it blocks the UVB rays needed to activate the vitamin precursor in the skin. Studies have shown that high SPF can significantly reduce vitamin D synthesis. Nonetheless, it’s important to use sunscreen to prevent skin cancer and other UV-induced skin damage. To avoid a vitamin D deficiency, you can consider alternative sources such as supplements or fortified foods.
It’s also worth noting that "supplementing" sun exposure through tanning beds isn’t necessarily helpful, as most tanning beds primarily use UVA rays. However, only UVB radiation is needed to stimulate the body’s production of vitamin D3.
