UV radiation is essential for endogenous vitamin D synthesis in humans and animals [3]. Recent data show that eggs from hens exposed for 3 h to artificial UVB radiation contained sixfold
higher amounts of vitamin D3 than eggs from non-exposed animals [2]. Here, we show that free-range versus barn-rearing systems for laying hens could offer an appropriate and cheap alternative to fortify eggs with vitamin D. Eggs from free-range hens that spent 9 h/d outdoors contained on average fourfold higher amounts of vitamin D3 and 45% higher amounts of 25(OH)
D3 than barn eggs. However, we need to acknowledge that the findings are specific to the season (August 22 to September 19) and latitude (51, Central Germany) under which the experiment was conducted. It is to be expected that the efficacy of free-range systems to fortify animal-based foods such as eggs, largely depend on weather, season, and the latitude of the area where the animals are housed [4,5]. We further found that eggs from chickens with the inside/outside option contained markedly more vitamin D3 than the indoor group, although the vitamin D3 contents of the outdoor group were not fully reached. Although such eggs may not cover the daily vitamin D recommendation, the idea of vitamin D bio-addition by sunlight could be used for other animal housing systems to improve the vitamin D content of meat or milk. The fact that the vitamin D content of milk could be increased by sunlight exposure of cows [6,7] indicates the
effectiveness of the free-range system in production of other vitamin D-enriched food.