The study shows that the vitamin D3 content of egg yolk was three- to fourfold higher in the groups that were exposed to sunlight (outdoor and indoor/outdoor groups) compared with the indoor group (P < 0.001). Egg yolk from the outdoor group revealed the highest vitamin D3 content, which averaged 14.3 mg/100 g dry matter (DM), followed by that from the indoor/outdoor group (11.3 mg/100 g DM).