Winter is upon us, and so are the risks of vitamin D deficiency and infections. Vitamin D, which is made in our skin after exposure to the sun and also present in fatty fish (mackerel, tuna and sardines), mushrooms and fortified dairy and non-dairy alternatives, is essential for good health. Humans need vitamin D to stay healthy and fight infections. The irony is that in winter, when people need vitamin D the most, most of us don’t get enough of it. So how much should we take? Should we take supplements? How to get more? And who needs it most?
I am a medical microbiologist and immunologist which studies the functions of vitamin D in immune cells. My lab has been interested in understanding why the immune system has vitamin D receptors that determine which cells can use vitamin D. In the immune system, vitamin D works to improve your ability to fight infection and to reduce inflammation.
Where to find your vitamin D
Vitamin D is called the sunshine vitamin because it is made in the skin after exposure to the sun. The same UVB rays that cause sunburn also produce vitamin D. Sunscreen, darker skin pigmentation, clothing, and reduced daylight in winter decrease the skin’s ability to produce vitamin D. vitamin D. People who experience the greatest seasonal variations in vitamin D levels have fair skin. people living in the northern regions of the United States and higher latitudes around the globe where there is very little daylight in winter.
But those most at risk of low vitamin D levels are people of color and people living in higher latitudes. Dark-skinned people are more likely than light-skinned people to have low vitamin D levels all year round because darker skin blocks UVB rays from producing vitamin D. However, even in dark-skinned people, vitamin D is lowest in winter.
In winter, in addition to vitamin D-rich foods, adults should take additional vitamin D from foods and/or supplements to get at least 600 IU daily of vitamin D. People who have dark skin or avoid the sun should eat more vitamin D year-round.
Vitamin D is important for bones and your microbes
Originally, doctors thought vitamin D was only important for bone health. Indeed, vitamin D deficiency caused bone diseases such as rickets in children and osteoporosis in adults.. However, in the 1980s, scientists discovered that immune cells had receptors for vitamin D.
My group’s research has shown that vitamin D plays an important role in maintain health in the gastrointestinal tract. Higher levels of vitamin D reduce susceptibility to inflammatory bowel disease and Crohn’s disease, intestine and lung infections in animals and humans.
My colleagues and I have discovered that one of the ways vitamin D works is to keep gut microbes healthy and happy. Vitamin D increases the number and diversity of microbes living in the gut, which together reduce inflammation throughout the body.
Low vitamin D levels are associated with inflammatory bowel disease in humans. The researchers found that inflammatory bowel disease patients in Japan have more symptoms in winter than in other seasons.
Why is vitamin D more important in winter?
In winter, humans are exposed to more infections and spend less time outdoors. The exact amount of vitamin D healthy adults should have is debated. Some authorities recommend 200 IU daily to 2000 IU daily. In the United States, the Institutes of medicine recommends 600-800 IU daily for adults, while the Endocrine Society states that optimal vitamin D status may require 1500-2000 IU daily. In winter, people have a reduced ability to make vitamin D when they go outside, so amounts of at least 600 IU per day of vitamin D from food or supplements would help maintain vitamin D status in summer levels.
But, like many things, too much vitamin D can be harmful. Vitamin D toxicity does not result from too much sun or food. Due to the risk of skin cancer, dermatologists and other healthcare professionals do not recommend unprotected sun exposure to boost your vitamin D levels. They suggest supplements instead. But vitamin D toxicity can occur if an individual takes too much.
The experts who established the national vitamin D intakes for the United States recommend that adults take no more than 4,000 IU per day of vitamin D to avoid toxic side effects. Vitamin D helps you absorb calcium from your diet, but when vitamin D is too high, blood calcium levels rise and this can lead to kidney disease.
By consuming more vitamin D during the winter, your gut microbes will be healthier and you will be more resistant to infection and inflammation throughout the year.
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