Vitamin D is vital in activating human defenses, and low levels suffered by about half the world's population may mean their immune systems' killer T cells are poor at fighting infection, say scientists.
The findings by Danish researchers could help the fight against infectious diseases and global epidemics, they said.
The researchers found that immune systems' killer cells, known as T cells, rely on vitamin D to become active. If vitamin D is lacking in the blood, they remain dormant and unaware of threats from an infection or pathogen.
"When a T cell is exposed to a foreign pathogen, it extends a signaling device or 'antenna' known as a vitamin D receptor, with which it searches for vitamin D," said Carsten Geisler of Copenhagen University's department of international health, immunology and microbiology, who led the study.
"This means the T cell must have vitamin D or activation of the cell will cease. If the T cells cannot find enough vitamin D in the blood, they won't even begin to mobilize."
Scientists have known for a long time that vitamin D is important for calcium absorption, and that there is a link between levels of the vitamin and diseases, such as cancer and multiple sclerosis.
"What we didn't realize is how crucial vitamin D is for actually activating the immune system — which we know now," Geisler wrote in the study in the journal Nature Immunology.
Most vitamin D is made by the body as a natural by-product of the skin's exposure to sunlight. It can also be found in fish liver oil, eggs, and fatty fish such as salmon, herring, and mackerel, or taken as a supplement.
Almost half of the world's population has lower than optimal levels of vitamin D, and scientists say the problem is getting worse as people spend more time indoors.
Currently, the Institute of Medicine recommends 200 international units (IU) of vitamin D daily for children and adults up to the age of 50. Adults over the age of 50 should get 400 to 600 IU daily. However, many experts believe these recommendations to be too low, and it is likely they will be revised upward. For example, the American Academy of Pediatrics now recommends that children get 400 IU of vitamin D daily.
Vitamin toxicity is rare even in those who take supplements, and the main consequence is a buildup of calcium in the blood.
According to the Mayo Clinic symptoms of vitamin D toxicity include:
• Nausea
• Vomiting
• Weakness
• Confusion
• Kidney stones
• Heart rhythm abnormalities
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