Here in Wisconsin, we joke that we only have two seasons, construction and winter. Most years, that seems true. Even those who can't wait for the gloves and winter coats to come out have a big problem with winter. That problem is our lack of sun exposer.
I know the sun is terrible for you, lots of sunscreen and a big hat are supposed to keep us youthful and blemish-free, but the truth is that not all of the sun is bad for you. The sun emits a spectrum of UV light part of that spectrum is UVB, and it's not bad for you. In fact, it's what helps your body produce Vitamin D. First, your bare skin is exposed to light; melanin is in those skin cells and helps protect your body from UVA radiation, as you get more sun exposure, it gets darker to reflect the sun, but the cholesterol in the same skin cells, is reacting to the UVB radiation and making vitamin D. The vitamin D is stored in adipose tissue (fat) and is metabolized as you go about your day, passing through your liver and assisting your intestines in the absorption calcium thus increasing your bone density.
But that's not the whole story. Vitamin D is known to enhance the function of immune cells, including T cells and macrophages, that protect your body against pathogens. In fact, the vitamin is so essential for immune function that low vitamin D levels have been associated with increased susceptibility to infection, disease, and immune-related disorders. For example, low vitamin D levels are associated with an increased risk of respiratory diseases, including tuberculosis, asthma, and chronic obstructive pulmonary disease (COPD), as well as viral and bacterial respiratory infections. What's more, vitamin D deficiency has been linked to decreased lung function, which may affect your body's ability to fight respiratory infections.