Vitamin D25

Vitamin D is essential to people. Almost all tissue needs it, in addition to which vitamin D is required for the absorption of calcium and phosphate. Therefore, vitamin D has a significant impact on the health of the bones. Vitamin D prevents osteoporosis and associated fractures.

Vitamin D is also involved in the defence of the body, and studies have indicated that low vitamin D levels are associated with an increased risk of upper respiratory tract infections. In addition, vitamin D deficiency has been linked to diabetes, cancer, vascular diseases, and degenerative nerve diseases.

Vitamin D is mainly obtained from sunlight (80–90% of the natural intake of vitamin D). Because there is too little sunlight in Finland in the winter for the production of vitamin D, food and dietary supplements are an important source of vitamin D. The vitamin D stock accumulated in the summer runs out in approximately 1–2 months, and therefore oral vitamin D supplements are needed in the winter.

It is difficult to get enough vitamin D from food. Normally, only 10–20% of vitamin D comes from food. Fish is the only food with a significant amount of natural vitamin D3. Therefore, vitamin D is added to dairy products and fats in Finland. Vegans should note that a strictly plant-based diet provides relatively little vitamin D, meaning that without vitamin supplements, they might not get a sufficient amount of vitamin D.

Other risk groups include newborn infants, children, the elderly, those spending a lot of time indoors, and people of colour. In the elderly, production of vitamin D in the skin following exposure to sunlight is decreased. The production of vitamin D is estimated to be only 30% of the corresponding production by a young person in those aged 70.

When should vitamin D levels be measured?

Finnish studies have found that approximately one in four men and one in five women suffer from vitamin D deficiency. It can be difficult to notice, because insufficient levels of vitamin D only cause symptoms after a long time. Severe deficiency causes rickets in children and osteomalacia, or softening of the bones, in adults.

Vitamin D is not usually tested due to symptoms, but when vitamin D deficiency is suspected for some other reason. A test is usually also taken when investigating excessive or insufficient calcium or phosphate levels in the body.

What does a vitamin D test measure?

Vitamin D exists in several forms in the body, and its form also changes. The test measures vitamin D25, which is the storage form of vitamin D and is the most accurate of the vitamin tests to measure whether someone has too little or too much vitamin D in their body. The body produces vitamin D25 in the liver from a vitamin received from plant-based food (D2), and in the skin from a vitamin produced from animal-based food (D3). Vitamin D25 is also known as calcifediol.

The fourth form of the vitamin, calcitriol (1,25-dihydroxycholecalciferol), is the active form of vitamin D25. The body produces calcitriol in the kidneys. It is not usually tested unless certain hereditary diseases are suspected.