Vitamin D synthesis and sunlight through glass
Pro Research Analysisby 
Searched over 200M research papers
Vitamin D Synthesis and the Role of Sunlight
Vitamin D is mainly produced in the skin when it is exposed to sunlight, specifically ultraviolet B (UVB) radiation in the 290-315 nm wavelength range. This UVB light converts 7-dehydrocholesterol in the skin to previtamin D3, which then becomes vitamin D3 (cholecalciferol) through a natural process in the body. Factors such as skin pigmentation, age, sunscreen use, latitude, season, and time of day can all affect how much vitamin D the skin can produce from sunlight exposure Holick1995Holick2004Wacker2013+1 MORE.
Impact of Glass on Sunlight and Vitamin D Production
A key factor that limits vitamin D synthesis is exposure to sunlight through glass. Ordinary window glass absorbs UVB radiation, which means that sunlight passing through glass does not have the UVB needed to trigger vitamin D production in the skin. As a result, sitting in sunlight behind a window will not help your body make vitamin D Holick1995Holick2004Wacker2013.
Health Implications of Limited Vitamin D Synthesis
When people do not get enough direct sunlight exposure, especially if they spend most of their time indoors or live in areas with limited sunlight during certain months, they are at higher risk for vitamin D deficiency. This deficiency can lead to bone problems such as osteoporosis, osteomalacia, and rickets, and is also linked to increased risks of autoimmune diseases, some cancers, cardiovascular disease, and other health issues Holick1995Holick2004Wacker2013.
Strategies to Prevent Vitamin D Deficiency
To maintain healthy vitamin D levels, it is important to have sensible sun exposure—ideally, direct sunlight on the skin for short periods several times a week. For those who cannot get enough sun exposure, either due to lifestyle, climate, or the use of glass barriers, vitamin D supplementation and fortified foods are recommended to prevent deficiency and its associated health risks Holick1995Holick2004Wacker2013.
Conclusion
Direct exposure to sunlight is essential for vitamin D synthesis in the skin, as UVB rays do not penetrate glass. People who spend most of their time indoors or behind windows should consider alternative sources of vitamin D to maintain optimal health and prevent deficiency-related diseases Holick1995Holick2004Wacker2013.
Sources and full results
Most relevant research papers on this topic