‘Free-range’ eggs contain up to 30 per cent more vitamin D than those from factory farms, according to research at the University of Reading.
The study also found that eggs from organic farms had higher levels of calcifediol, which has been shown to increase the body’s ability to absorb calcium.
During the study, published in the journal Food Chemistry, the researchers analysed the contents of 270 eggs sold by UK supermarkets.
A typical barn egg contains 1.7 microgrammes of vitamin D, while free-range eggs contain 2mcg. Organic free-range eggs contain 2.2mcg. The recommended daily allowance for an adult is 10mcg.
The researchers concluded that the higher vitamin D content found in the yolks of free-range organic eggs can be explained by greater sun exposure. They wrote: ‘The vitamin D nutrition of birds is similar to that of humans; vitamin D is either synthesised by ultraviolet radiation from sunlight or consumed in the diet.
‘Unlike the conventional indoor egg production system, free range and organic birds have more opportunity to be exposed to sunlight as they can access pasture continuously during the day.
‘The key finding of the current study is that both vitamin D3 and calcifediol were significantly different according to production system.
‘It is probable that the main reason for greater concentrations of vitamin D3 and calcifediol in eggs from free range and/or organic systems is higher sun exposure of the laying birds.’
Eggs from supermarkets were observed over the course of five months from five different retail outlets to observe how the production system affected the level of vitamin D in each of the eggs. The main sources for vitamin D in humans are from sunlight and dietary intake. Vitamin D deficiency is still being linked to a lot of disease, although how causation works is still under debate. Most people within our latitudes do not have sufficient vitamin D levels in the winter and autumn months and therefore it is argued that we should all be taking supplements.
On the whole, the trend was for an increasing level of vitamin D from caged to free range to organic.
Ultimately, I think this is obvious — you would expect that animals treated well, with good levels of husbandry, would invariably result in a more nutritious product. This has been shown with omega-6 and omega-3 oils in chickens, for example.
Unfortunately, this is where the study stops being relevant. The average egg might provide about 2mcg of vitamin D, but the recommended daily allowance is 10mcg; a five-a-day consumption of eggs would be required to achieve this. To make things more tricky it is suggested that the vitamin D could be damaged or destroyed in some way by cooking.
Second, the difference in quantity of vitamin D between the eggs is not huge.
So, while it gives me great comfort to know that Clarence Court eggs are indeed the best, the difference in vitamin D is likely to be negligible.