Effect of production system, supermarket and purchase date on the vitamin D content of eggs

There is mounting evidence to show that vitamin D deficiency may increase the risk of many common and serious diseases, including osteoporosis, cardiovascular disease, some cancers and type 1 diabetes⁽¹⁾. Hypovitaminosis D is prevalent in the UK general population. Due to lifestyle changes most people do not endogenously synthesise sufficient vitamin D from sunlight exposure⁽²⁾. Therefore, vitamin D intakes from dietary sources have become very important. Egg yolk is known to be a useful source of vitamin D yet very few studies have investigated the effect of production method on vitamin D content of UK hens' eggs. The purpose of this study was to explore the effects of production system, supermarket and time of the year on the concentration of vitamin D₃ and 25(OH)D₃ in UK hens' eggs at retail.

Eggs (n = 259) from free range, organic (also allows outdoor access) and caged (as identified on the label) production systems were purchased from three supermarkets (Asda, Tesco and Budgens) each month from July to November in 2012. The concentrations of vitamin D₃ (n = 130) and 25(OH)D₃ (n = 129) were analysed by HPLC/Tandem MS. Statistical analysis was undertaken using general linear model of ANOVA in Minitab 16·0 and the least square means for concentration of vitamin D₃ and 25(OH)D₃ in egg yolks according to production system (PS) and effect of supermarket (SM) and month (M) are shown in the table.

^a,bdifferent superscripts indicate significantly different means (P < 0·05); NS, not significant (P > 0·05).

Overall, concentrations of vitamin D₃ were similar to those of FSA⁽³⁾ but lower than a more recent study⁽⁴⁾. The significantly higher vitamin D₃ concentrations in free range and organic eggs is presumably related to increased vitamin D synthesis by birds that have access to sunlight although why the effect on 25(OH)D₃ was only seen for organic eggs is unclear. The significant interaction effect of system and supermarket for both forms of vitamin D reflects inconsistencies in the ranking of vitamin D by production systems between supermarkets, perhaps indicating some incorrect labelling. The effect of month of purchase on 25(OH)D₃ reflected significantly higher values in July and September than in August (data not shown). Whether this is related to more sunshine hours is not known and there was no such effect on vitamin D₃.

In conclusion, these results confirm that eggs from outdoor production systems are likely to have higher vitamin D concentrations but this may not be a consistent effect in all supermarkets. Future work could expand the sampling number and research time through the whole year and match feeding schemes for the birds. Furthermore, genotype of birds that go outdoors may also be a factor.