The text is discussing the different sources and methods of obtaining vitamin D, a hormone that is important for human health. It explains that solar ultraviolet radiation and vitamin D supplementation are the two main sources of vitamin D for those living in northern latitudes. Sunlight is necessary for the conversion of 7-dehydrocholesterol in the skin into previtamin D3, which then isomerizes into vitamin D3. Studies have shown that exposure to UVB radiation is equally or more effective in increasing vitamin D levels compared to taking vitamin D supplements. Though vitamin D can be obtained from diet and supplements, sunlight is still considered the primary source. There are also claims that regular sun exposure is necessary for maintaining healthy vitamin D levels. Overall, the text highlights the importance of sunlight for obtaining vitamin D and the various ways in which it can be obtained.