What is vitamin D?
Vitamins are essential nutrients for our normal body function and development. Generally, they are acquired through diet, but the body can also make some vitamins from precursor molecules. Vitamin D –a fat-soluble vitamin naturally found in fatty fish such as salmon, mackerel, and sardines– can also be made in your skin thanks to sunlight. Vitamin D is key for calcium metabolism, and it is involved in promoting bone formation and absorbing calcium in the gut and kidney. It also has other functions as it helps in hormone-releasing and immune responses.
Eventually, EV McCollum discovered Vitamin D one hundred years ago when found in cod liver oil, a factor with properties against rickets that he named vitamin D. Rickets –a disorder caused by the softening and weakening of bones in children – became a public health problem during the Industrial Revolution in factory-packed cities, especially in countries with low sunshine intensity, such as England. At that time, AF Hess and LF Unger revealed that rickets could be prevented or even cured in children with sunlight, suggesting that vitamin D could be produced in the body by irradiating precursors. In addition to rickets, it has been found that vitamin D shortage can also promote bone disorders such as osteoporosis and osteomalacia as well as psoriasis. Let's explore if you can get enough Vitamin D through a window.
How can your skin produce vitamin D, and can you get Vitamin D through a window?
In the outer layer of skin, the epidermis, vitamin D is produced from its precursor dehydrocholesterol. The sunlight's ultraviolet B (UVB) component rearranges dehydrocholesterol molecules to become vitamin D. Synthesized vitamin D is then transported through the blood to the liver and kidney, where it is finally matured. Biologically active vitamin D molecules finally get into cells from tissues such as bone and gut, where they do their functions.
But what is the UVB component of sunlight, and can you get Vitamin D through a window? Sunlight is a spectrum of visible and non-visible rays with different wavelengths. Visible light comprises the rays that we see, the different rainbow colors from red to violet. Non-visible light comprises long-wavelength rays, the heating infrared rays, and short-wavelength rays –the UV rays–which are highly energetic and skin-penetrant. In addition, there are three types of UV light: A, B, and C. While UVA and UVB rays are transmitted through the atmosphere, the Earth's ozone layer absorbs all UVC and some UVB rays, so most of the sunlight you meet is made of UVA and a small amount of UVB. UVA rays can penetrate the middle layer of the skin (the dermis), and UVB rays reach the outer layer (the epidermis).
Both UVA and UVB rays can cause acute or chronic harmful health effects: from sunburn to skin aging or even skin cancer. But the small portion of sunlight UVB rays going into your skin also have some health benefits, such as vitamin D production, so sunbathing risks and benefits must be balanced carefully. In general, as little as 10 to 15 minutes a day of sun exposure is enough for your skin to produce vitamin D. However, the amount of vitamin D your skin can make depends on several factors, including daytime, season, latitude, or skin pigmentation. Vitamin D production can be decreased or absent during the winter months depending on where you live, your skin color, and your lifestyle. Sunscreens, while important to prevent skin cancer, can also reduce vitamin D production.
Can you get vitamin D through glass or a window?
The amount of UV rays going through glass or a window depends mainly on the glass's type, thickness, and color. While laminated glass offers better UVA protection than tempered glass, all commercial and automobile glass types block UVB rays. So, is it possible to get vitamin D through a window? Considering that UVB rays are needed to make vitamin D from the dehydrocholesterol of your skin cells, the answer is no. You cannot get vitamin D through a window, as you do not get enough UVB exposure sitting indoors or in a car no matter how sunny the day is or if you are trying to avoid the summer heat. And the worst thing is that much of the highly skin-penetrant UVA rays will go through the glass or window and may be harmful to your skin.
So, the best way to get your daily vitamin D dose is to go out of your house, car, or office, sunbathe a bit, and not rely on getting Vitamin D through a window. Enjoy sunlight for a few minutes but be cautious if you live near the equator, whether it is midday or summer. If you are still concerned about getting low vitamin D because you live in a country with low sunshine intensity or you cannot get regular exposure to sunlight, you can still get vitamin D through diet. Fatty fish, egg yolks, and vitamin D-supplemented food such as milk, yogurt, or cereals are good alternatives to get vitamin D. You can also try tomatoes that can produce as much vitamin D as two egg yolks, especially if you are vegan!
References/To know more about:
- Bikle DD. (2021). Vitamin D: Production, Metabolism, and Mechanisms of Action. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc
- McCollum EV, Simmonds N, Becker JE, Shipley PG. (1922). An experimental demonstration of the existence of a vitamin that promotes calcium deposition. J Biol Chem, 53:293–298
- Hess AF, Unger LF. (1921). Cure of infantile rickets by sunlight. J Am Med Assoc, 77:39
- Knuschke P. (2021). Sun Exposure and Vitamin D. Curr Probl Dermatol, 55:296-315
- Almutawa F, Vandal R, Wang SQ, Lim HW. (2013). Current status of photoprotection by window glass, automobile glass, window films, and sunglasses. Photodermatol Photoimmunol Photomed, 29(2):65-72
- Almutawa F, Buabbas H. (2014). Photoprotection: clothing and glass, Dermatol Clin, 32(3):439-48
- Li J, Scarano A, Mora Gonzalez N, et al. (2022). Biofortified tomatoes provide a new route to vitamin D sufficiency. Nat Plants, May 23. Online ahead of print