There are several kinds of vitamin D with the two most important being:
Vitamin D2, ergocalciferol, that is found naturally in the plant kingdom.
Vitamin D3, cholecalciferol, that is found naturally in the animal kingdom.
Humans synthesise vitamin D (cholecalciferol) from sunlight (UV-B rays) and a cholesterol compound in the skin, which is converted to active vitamins by means of processes in the liver and kidneys. We humans are only able to synthesise vitamin D during the summer season when the sun is high in the sky. Excess amounts of the nutrient are stored in the liver for future use. Light skin produces substantially larger amounts of vitamin D than dark skin. In contrast, dark skin protects against vitamin D overproduction. As we grow older, our vitamin D production decreases. Vitamin D is also called a provitamin or hormone, simply because we are easily able to make it ourselves, and all cells in the body have vitamin D receptors.
Vitamin D is fat-soluble and is stored in the body's fat tissue when ingested in large amounts. Vitamin D is destroyed by light and heat from cooking.
Functions and importance for
- Physical and psychological stress
- The body's uptake and utilisation of calcium and phosphor
- The blood's calcium balance together with the thyroid hormone calcitonin
- Strong bones and teeth (especially together with calcium, magnesium, and vitamin K2)
- Muscle function
- Immune system
- Counteracts inflammation that is also seen with autoimmune diseases
- Normal cell division
- Energy levels and mood
- Cancer prevention (according to some studies)
Vitamin D3 is 56-87% more effective than vitamin D2 in terms of increasing blood levels of the nutrient. Also, it is stored more effectively in the liver.
Deficiencies and poor utilisation may be caused by
- Too little sun exposure during the summer season
- Veiling and dark skin
- Impaired lipid absorption
- Low-fat diets without sources such as oily fish and eggs
- Old age, especially in combination with an unbalanced diet, lack of sunlight, and thin skin
- Kidney failure or liver failure (these organs activate vitamin D)
- Genetic defects. Identified in people who are unable to synthesise vitamin D in their skin
- Sunscreen with more than factor 8 blocks the synthesis of vitamin D in the skin
- Long-term use of medical drugs such as:
Drugs against epilepsy
- Long-term use of paraffin oil against constipation
- Weight loss products such as Orlistat/Xenical that inhibit the uptake of fat and fat-soluble vitamins
- Osteoporosis and bone deformities
- Bone fractures
- Tooth decay
- Loss of muscle strength and muscle cramps
- Cardiovascular disease
- Impaired immunity
- Autoimmune diseases and perhaps type 1 diabetes and sclerosis
- Fatigue and mood swings
- Psychological and neurological diseases such as depression, schizophrenia, and Alzheimer's disease (according to several newer studies)
Children: The classic deficiency disease rickets (rachitis) with its characteristic symptoms that include soft and deformed bones and underdeveloped tooth enamel.
Many vitamin D deficiency symptoms are a result of the accompanying calcium deficit.
- Primarily sunlight. Other sources are cod liver oil, cod liver, cod roe, and (oily) fish. Smaller amounts of the vitamin are found in eggs, meat, avocado, oils, dairy products, and breastmilk. Vitamin D is bound to lipids and oils.
Please note: Plants also need sunlight in order to produce vitamin D (D2). For that reason, a sun-ripened avocados contain far more vitamin D than avocados that have not been sun-ripened.
Vitamin D content in micrograms per 100 grams
|Cod liver oil||250|
|Fresh salmon and eel||30|
(whole eggs 1.75)
Vitamin D from sunlight
During the summer season we are able to get enough vitamin D simply by being outdoors and exposing our face, forearms, and lower legs to sunlight for 5-30 minutes a few times per week. Sun exposure should never result in erythema, as this may increase the risk of skin cancer. If you prolong your stay in the sun it is advisable to apply protective sunscreen once the skin has been exposed to sufficient amounts of sunlight for being able to produce vitamin D.
The amount of vitamin D synthesised either from sunlight or artificial UV radiation depends on our skin colour, the sun's position on the sky, and how soon after sun exposure we bathe. It may take as long as 48 hours for newly synthesised vitamin D precursors in the epidermis to reach the bloodstream. Before these precursors effectively penetrate through the skin layers and into to the blood, a large amount of them may vanish when we apply soap and water to our skin. It is factors like these that make it difficult to assess how much vitamin D we get from sun exposure.
Recommended daily allowance (RDA)
Adults: 11 years of age and older: 5 micrograms
Children: 1-10 years of age: 10 micrograms
Infants: Breastmilk alone is not enough to ensure optimal amounts of vitamin D. According to the Danish Health and Medicines Authority, infants with light skin must be supplemented with 10 micrograms of vitamin D daily from the age of two weeks until the end of their first year. Dark-skinned infants are advised to take supplements until they are three years of age.
Increased need and recommendation of supplements
- The above mentioned deficiency symptoms
- The health authorities recommend a daily supplement to:
- Children in the age group 0-2 years
- Pregnant women
- Children and adults with dark skin
- Children and adults who wear fully covering clothing during the summer period
- Individuals who normally don't go outdoors on a daily basis or who avoid sunlight
- Nursing home residents
- People older than 70 years of age
- Vegans and vegetarians are advised to follow the official guidelines concerning sun exposure and possibly discontinue vitamin D supplementation during the winter period.
Detection of vitamin D deficiency
In order to determine the body's vitamin D status, blood levels of the natural 25-hydroxyvitamin 3 are measured. The content is measured in nmol/l and graduated in the following way:
|Less than 12 nmol/l||Severely deficient|
|Above 50 nmol/l:||Sufficient|
|75-150: nmol/l:||Optimal level in people with osteoporosis and in kidney patients|
|Over 200 nmol/l:||Risk of toxicity|
According to the Danish Health and Medicines Authority, the following people should have their vitamin D status checked:
- People with clinical signs of vitamin D deficiency
- People whose lifestyle may give reason to suspect a vitamin D deficiency
Overdosing - side effects
It is not possible to overdose on vitamin D through excessive sun exposure or by eating too much fish.
Very large vitamin D levels may lead to elevated blood calcium levels, and elevated calcium levels may cause:
Extreme dosages (e.g. 100 more than the RDA) over longer periods of time may cause calcium to deposit in the kidneys and bladder and may also lead to kidney stones.