Vitamin D is an essential component of vertebrate metabolism; however both Australian and global epidemiological surveys have shown a chronic vitamin D insufficiency in the human population. Vitamin D is essential for good skeletal health, healthy skin and a sound immune system and chronic insufficiencies may cause many diseases including osteoporosis.
There are few human food sources which are naturally rich in vitamin D, but oily fish, meat, milk and egg yolk contain reasonable concentrations. Interestingly, it is only oily fish which currently provide significant quantities of vitamin D in a single serve. The daily recommended dietary allowance for vitamin D by the USA Food and Nutrition Board is 400 international units (iu) (0-12 months of age), 600 iu (1-70 years of age), and 800 iu (>71 years of age). Our Australian National Health and Medical Research Council (NHMRC) have lower daily intake recommendations for vitamin D, namely 200 iu (0-50 years of age); 400 iu (51-70 years of age) and 600iu (>70 years of age).
Under the supervision of Professor Aaron Cowieson at the University of Sydney, Poultry CRC PhD student Linda Browning has been investigating the efficiency of deposition of vitamin D into egg yolks following supplementation into the feed of laying hens. Further to this, she has measured the change in vitamin D deposition into egg yolk over time, and the effect that supplementation may have on the overall performance of the hens.
Linda explains, “The modern commercial laying hen has been shown to be unique in its ability to efficiently transfer vitamin D metabolites from its diet into egg yolk. Eggs are one of the few natural food sources rich in vitamin D, and they contain both vitamin D3 (D3) and 25-hydroxyvitamin D3 (25(OH)D3) metabolites of vitamin D”. The latter is especially useful as it provides five times the relative biological activity of vitamin D in the human diet. “So my research has investigated the in-egg vitamin D concentrations resulting from supplementation”.
The results of this work look very promising indeed (see Figure 1). “Overall D3 content per kilogram of egg yolk increased as the addition of D3 increased” said Linda. “And the higher dietary levels of D3 also significantly increased the 25(OH)D3 content of egg yolk. In contrast, the addition of 25(OH)D3 in the diet of laying hens also significantly increased the 25(OH)D3 content of egg yolk, but not the D3 content of egg yolk”. Additionally, the hens showed a rapid response to vitamin D supplementation in the feed, with a plateauing of total vitamin D deposition as early as week 3 of the trial.
The research demonstrated a wide variation in the inherent vitamin D concentration of egg yolk at day one (of the trial), “which could not be explained by the diet alone because all birds had consumed the same level of dietary vitamin D prior to commencement of the trial” said Linda. The most likely cause being an inherent genetic difference between birds in their ability to transfer vitamin D metabolites across their intestinal wall and into the yolk.
Linda’s project has shown that a single egg from a laying hen, supplemented with increased levels of D3 and 25(OH)D3, has the potential to contribute significantly to the recommended daily intake of vitamin D for both adults and children without detrimental effect on hen production parameters. “I think it is a most worthwhile story for both egg producers and egg consumers alike. The cost at a full dose of vitamin D (10,000 iu D3 + 69 micrograms 25-hydroxyD3) is only about 1 cent per dozen eggs”.
Surely from a health benefit, cost of implementation and marketing point of view these results represents a major win for the egg industry.