Primary source of vitamin D for human beings is

The primary source of vitamin D for human beings is the sun. Vitamin D3 (cholecalciferol) is synthesized endogenously in the skin when 7-dehydrocholesterol absorbs ultraviolet B (UVB) radiation from sunlight [t1][t4][t5]. This process is so efficient that as long as a person has regular access to sunlight, there is often no strict dietary requirement for the vitamin [t2]. While vitamin D can be obtained from animal-based foods like oily fish and egg yolks, or through fortified foods and supplements, sunlight remains the most natural and significant source [t3][t6]. In contrast, citrus fruits are primary sources of vitamin C, and green vegetables generally do not contain significant amounts of vitamin D. Yeast contains ergosterol, which can produce vitamin D2 when exposed to UV radiation, but it is not the primary source for humans [t4][t7].

Sources

1. [1] https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/secosteroid
2. [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC3897598/