Vitamin D is an important vitamin for the functioning of the immune system. Everyone should ensure they are getting enough of it, especially during the winter when there is very little sunlight. Aarja’s ambassador, Finnish baseball player and clinical nutrition student Janette Lepistö gathered us a few facts about vitamin D and its effects on immune function in this blog post.



Immunity is a complex system, and its main task is to protect humans against pathogens. The immune system can be divided into innate, i.e., innate and inherited, and acquired, i.e., by encountering pathogens or vaccines. Adequate intake of energy and nutrients can support the normal functioning of the immune system, as nutrition also provides building blocks for cells in the defence system. Restricting energy intake increases the risk of disease and weakens the immune system, as does too little intake of protein and essential fatty acids. Of the dietary composition, the effects of vitamins C, D and zinc on susceptibility to infection have been studied the most.

Vitamin D is a fat-soluble vitamin that is formed on the skin by the sun’s UVB radiation. In the summer, up to 250 micrograms of vitamin D can be formed during twenty minutes of outdoor activities! Due to Finland's northern location, there will not be enough UVB radiation from October to March, so it is good to pay attention to sufficient vitamin D intake during the winter months. The best sources of vitamin D in the diet are fish foods as well as dairy products and spreads that have vitamin Dadded. The recommended intake of vitamin D for people aged 2 to 74 is 10 µg / day and for people over 75 years of age 20 µg / day. If you do not consume food products daily containing vitamin D or fish 2-3 times a week, the intake of vitamin D from your diet may be insufficient during the winter and the use of a vitamin D supplement may be necessary during the dark season.



The storage form of vitamin D is calcidiol, the blood content of which describes the adequacy of vitamin D intake. The optimal serum vitamin D level for good health is 80-100 nmol / l, but athletes may benefit from vitamin D levels above 100 nmol / l. The safety of very high vitamin D levels is not entirely certain, so it is advisable to be moderate in the administration of vitamin D supplementation. Taking tests to find out your vitamin D levels is recommended to be sure of a suitable dose of vitamin D supplementation. The maximum acceptable daily intake for adults is 100 /g / day. Vitamin D plays an important role in for example, skeletal metabolism, in addition to which it is required for cell regeneration, cardiac function, hormone formation, muscle development and maintenance of muscle strength, recovery, gene function, and maintenance of resistance.

Vitamin D has been reported to play a key role in the immune system. Vitamin D receptors, as well as vitamin D metabolizing enzymes, are present in almost all cells of the immune system. Vitamin D is required for the differentiation and function of macrophage cells important in the immune defence. Low vitamin D levels or insufficient vitamin D intake are relatively common. In that case, adding vitamin D to your diet may be beneficial, for example in terms of resistance.



Vitamin D3, or cholecalciferol, is obtained from foods of animal origin and vitamin D2, or ergocalciferol, from plant products. The biological activity of vitamin D2 is shown to be lower than that of vitamin D3. Vitamin D from food and Vitamin D formed in the body requires two hydroxylation reactions, mainly in the liver and kidneys, to be activated. In the liver, vitamin D is formed to be 25-hydroxy-vitamin D (25-(OH)-D) which is called calcidiol. From there it is released into plasma and bound to its transporter protein. In the kidney, vitamin 25-(OH)-D is associated with another hydroxyl group to form 1,25-(OH)2 vitamin D, which is the most biologically active metabolite of vitamin D. According to some estimates, ergocalciferol (vitamin D2) increases plasma levels of vitamin 25-(OH)-D by 60-80% less than cholecalciferol. The metabolite of ergocalciferol, 25-(OH)-D2, is poorly bound to the plasma transporter protein, when free 25-(OH)-D2 being readily excreted in the urine and there is no final activation of the vitamin.



  1. Vitamin D is a fat-soluble vitamin, meaning it is absorbed with fats and stored in the body. Therefore, it is a good idea to follow the safe intake recommendations and measure your own vitamin D levels from time to time so that you know your daily dose. 

  2. The sun’s UVB radiation forms vitamin D on the skin during the summer. The 15 minutes spent in the sun from May to August can form an adequate daily dose of vitamin D.

  3. The recommended intake of vitamin D for people aged 2 to 74 is 10 µg / day and for people over 75 years of age 20 µg / day.

  4. The highest recommended daily intake of vitamin D is 100 µg / day. 

  5. Sources of vitamin D from food include e.g., fish dishes, egg yolks, and fortified dairy products and spreads.

Add to your basket Aarja’s Calcium, D3+K2 with bilberry, which contains 50 micrograms of vegan Vitamin D3: