As a chick grows inside the egg, the egg slowly releases more vitamin B2 to help the baby bird grow, and this happens because a special protein that holds the vitamin gets used up after day 14.

We analyzed the available evidence and found that riboflavin-binding protein in hen eggs appears to decrease after day 14 of incubation. This observation is based on one assertion that links the decline of this protein to the release of vitamin B2, which supports the growing chick’s development [1]. The protein normally holds vitamin B2 in place, and as the embryo matures, it seems the protein is gradually used up, allowing more of the vitamin to become available [1]. What we’ve found so far suggests that this change happens around or after day 14, a time when the chick’s metabolic needs increase significantly. There are no studies in our review that contradict this pattern. However, we note that only one assertion was analyzed, and it does not include detailed measurements, timelines, or experimental methods. While the evidence we’ve reviewed leans toward a reduction in riboflavin-binding protein after day 14, we cannot say how much it decreases, whether it drops steadily or suddenly, or if other factors might influence this process. The current data does not explain how the protein is broken down or whether its decline is consistent across different egg types or incubation conditions. In everyday terms, this means that as the chick grows inside the egg, the egg’s internal system seems to shift how it manages vitamin B2 — possibly by using up the protein that stores it — to give the baby bird more of what it needs. But more research would be needed to understand exactly how and why this happens.