Vitamin D regulates the amount of calcium and phosphate in the body; these substances are needed to keep bones, teeth, and muscles healthy. Below is a breakdown of the headlines: what the sun provides, what food provides, who lacks it, and how it is measured.
What vitamin D does
Vitamin D deficiency in children leads to rickets with bone deformity, and in adults to osteomalacia with bone pain. Biochemically, it is not quite a vitamin but a prohormone: the skin converts 7-dehydrocholesterol into cholecalciferol under ultraviolet light, the liver produces 25-hydroxyvitamin D, and the kidneys produce active calcitriol. The forms of vitamin D are calciferols: ergocalciferol (D2) comes from yeast, and cholecalciferol (D3) is obtained from animal tissues. About 80% of the requirement is provided by skin synthesis, and the rest by food.
Sun, season, and food
From late March to late September, most people get all the vitamin D they need from direct sunlight. From October to early March, the body does not produce enough vitamin D from sunlight, and this period is covered by food and supplements. It is impossible to overdose on vitamin D from sunlight.
The number of foods containing vitamin D is small:
- oily fish — salmon, sardines, trout, herring, mackerel;
- red meat and egg yolks;
- fortified products — some fat spreads and breakfast cereals;
- liver, which is not recommended during pregnancy.
Cow's milk is not fortified with vitamin D in many countries, so it is not a good source.
Who lacks it
People who get little or no sun exposure include those confined to the home, residents of long-term care facilities, and those who wear clothing that covers most of the skin. People with dark skin — of African or South Asian origin — synthesize less vitamin under the same sun.
Physiological groups include children in the first years of life and pregnant women who get little sun exposure. Medical causes include chronic kidney failure, liver disease, malabsorption syndrome, and long-term use of anticonvulsant medications. Low vitamin D levels are associated with obesity and overweight.
Deficiency can also develop in children over three years of age — during periods of intensive growth, with insufficient protein nutrition, and with abrupt climate changes. In adults, especially pregnant women who get little sun exposure and eat an insufficiently complete diet, bone tissue loses calcium.
Signs of deficiency and associated conditions
The classic signs are bone pain and muscle weakness; in children, mostly in the first three years of life, signs of rickets appear. Low vitamin D levels are associated with osteopenia and osteoporosis, frequent falls in the elderly, chronic pain, fibromyalgia, certain heart diseases, arterial hypertension, type 1 and type 2 diabetes, obesity, several types of cancer, multiple sclerosis, psoriasis, muscle spasms and weakness, reduced immunity, fatigue, sleep disorders, depression, irritability, and anxiety.
How the level is tested
The laboratory marker is 25(OH)D in serum. A concentration below 50 nmol/L corresponds to deficiency, and below 30 nmol/L to a severely deficient state; in European populations this is 40.4% and 13.0% of people, and worldwide deficiency is recorded in more than 1 billion.
Excess vitamin D and consulting a doctor
Prolonged excess of supplements raises blood calcium (hypercalcemia), which weakens bones and harms the kidneys and heart. In people with certain medical conditions, the safe amount of vitamin D may be lower, so when in doubt it is advised to consult a doctor.
Vitamin D deficiency on its own is not a reason to call an ambulance; 103/112 — for a life-threatening condition, in particular for signs of stroke or heart attack.
This material is for reference and does not replace consultation with a doctor.
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This content is for reference and does not replace a doctor's consultation. Medical disclaimer