Facts About Vitamin D

Vitamin D is an important vitamin for immune system function. Everyone should ensure they get enough vitamin D, especially in winter when there is less sunlight. Aarja Health ambassador, Finnish baseball player and clinical nutrition student Janette Lepistö, has gathered some facts about vitamin D and its effect on the immune system below.

The immune system is a complex system, and its main task is to protect the body against disease-causing microorganisms. The immune system can be divided into:

• Innate immunity – the natural and hereditary protection
• Acquired immunity – developed through contact with diseases or vaccines

Adequate intake of energy and nutrients can support the normal function of the immune system, as nutrients serve as building blocks for the cells in the body's defense system. Too low energy intake increases the risk of illness and weakens the immune system, as does too little protein and essential fatty acids. Regarding diet composition, vitamin C, vitamin D, and zinc are among the nutrients most studied in relation to the immune system.

Vitamin D is a fat-soluble vitamin formed in the skin with the help of the sun's UVB rays. In summer, up to 250 micrograms of vitamin D can be formed after about twenty minutes outdoors! However, due to our northern location, there is not enough UVB radiation from October to March. Therefore, it is important to pay extra attention to vitamin D intake during the winter months.

The best sources of vitamin D in the diet are:

• Fish
• Dairy products fortified with vitamin D
• Spreads and other fortified foods

The recommended daily intake of vitamin D is:

• 10 µg per day for people between 2–74 years old
• 20 µg per day for people over 75 years old

If you do not eat foods with vitamin D daily or fish 2–3 times a week, vitamin D intake may be too low during the winter. Then a vitamin D supplement may be necessary during the dark season.

Recommended intake and effects of vitamin D

Vitamin D is stored in the body as calcidiol, and the level in the blood is used to assess whether vitamin D intake is sufficient. An optimal level for good health is between 80–100 nmol/l, but athletes may benefit from levels above 100 nmol/l.

The safety of very high levels of vitamin D is not fully clarified, so moderation is recommended when using supplements. It may be wise to take blood tests to find the correct dosage of vitamin D supplements.

The highest recommended daily intake for adults is 100 µg per day.

Vitamin D plays an important role in, among other things:

• Bone health and skeletal metabolism
• Cell renewal
• Heart function
• Hormone production
• Muscle development and muscle strength
• Recovery
• Gene function
• Maintenance of the immune system

Vitamin D has also been shown to play a key role in the immune system. Vitamin D receptors and enzymes that metabolize vitamin D are found in almost all cells of the immune system. Vitamin D is necessary for the development and function of macrophages – important immune cells that protect the body against infections.

Low vitamin D levels or insufficient intake are relatively common. In such cases, it may be beneficial to supplement the diet with vitamin D to support the body's resilience and overall health.

What is the difference between vitamin D2 and D3?

Vitamin D3, also called cholecalciferol, comes from animal foods, while vitamin D2, or ergocalciferol, comes from plant-based sources. The biological activity of vitamin D2 has been shown to be lower than that of vitamin D3.

Both vitamin D from food and vitamin D produced in the body must go through two hydroxylation processes – mainly in the liver and kidneys – to be activated.

In the liver, vitamin D is converted to 25-hydroxyvitamin D (25-(OH)-D), also called calcidiol. From there, it is released into the blood plasma and binds to transport proteins. In the kidneys, 25-(OH)-D receives another hydroxyl group and is converted to 1,25-(OH)2-vitamin D, which is the most biologically active form of vitamin D.

According to some studies, ergocalciferol (vitamin D2) increases blood levels of 25-(OH)-D 60–80% less effectively than cholecalciferol (vitamin D3). The metabolite 25-(OH)-D2 also binds less well to transport proteins in plasma. Therefore, more of the free 25-(OH)-D2 is more easily excreted in urine, and less is activated to the biologically active form of the vitamin.

More about vitamin D

Vitamin D is a fat-soluble vitamin, which means it is absorbed along with fat and stored in the body. Therefore, it is important to follow recommended dosages and preferably measure your own vitamin D levels occasionally to find the right daily dose.

The sun's UVB rays produce vitamin D in the skin during summer. About 15 minutes in the sun from May to August can be enough to produce a sufficient daily dose of vitamin D.

Recommended daily intake:

• People 2–74 years: 10 µg per day
• People over 75 years: 20 µg per day

Upper recommended limit:

• Maximum recommended daily intake is 100 µg per day

Good sources of vitamin D:

• Fish and fish products
• Egg yolks
• Fortified dairy products
• Spreads and other vitamin D-fortified foods