Eggs from free-range hens are healthier to eat, a new study has found.
Researchers discovered that yolks from birds that are allowed to wander outdoors contain up to 30 per cent more vitamin D than those from birds kept in sheds or cages.
The vital hormone has been shown to help keep at bay a range of diseases including cancer, diabetes and heart disease, but is found naturally in only a handful of foods.
And so millions of Britons become deficient in vitamin D over the winter as it’s mainly generated when the sun’s rays hit the skin.
Scientists from Reading University analysed the contents of 270 eggs on sale in UK supermarkets.
They found that hens with access to outdoor runs – including those that meet 'free range' standards – produce yolks containing significantly more of the 'sunshine vitamin'.
But eggs from organic farms - which allow hens the greatest freedom of all - also have elevated levels of a form called 25-hydroxy D3, which is especially protective of human health.
The study, published in the journal Food Chemistry, states: ‘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 op-portunity 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 25(OH) D3 were significantly different according to production system.
‘It is probable that the main reason for greater concentrations of vitamin D3 and 25 (OH) D3 in eggs from free range and/or organic systems is higher sun exposure of the laying birds.’
Nearly half of all British eggs are produced intensively in giant barns in which thousands of hens are kept, often in so-called colony cages, which allow for perching and nesting - but the birds never go outside.
‘Free-range’ - which accounts for 50 per cent of production - gives chickens access to daytime runs with vegetation.
Meanwhile, ‘organic’ guarantees less cramped conditions, more pop-holes for the hens to use and a guarantee of no genetically-modified foodstuffs in their feed.
The research team found that a barn egg typically contains 1.7 microgrammes of vitamin D while a free range one delivers 2mcg and there’s 2.2mcg in an organic one.
Government guidelines recommend a daily intake of 10mcg for an adult and 7-8.5mcg for a child.
Robert Brown, of the nutritional think tank the McCarrison Society, said: ‘Vitamin D is essential for promoting and preserving good health – that has never changed.
‘What has changed is the industrialisation of food production since the Second World War.
‘We shouldn’t see these results as organic eggs being abnormally advantageous; rather, we should see factory farmed eggs as being abnormally deficient.
‘Hens do not live naturally in cramped conditions in a shed 24 hours a day; they, too, need the sun on their backs and proper nutrition to prosper.
‘Intensive farming techniques may produce cheap food but at what underlying cost to the nation’s well-being?
‘The answer to this question must lie at the heart of future government policy in farming and agricul-ture.’
Jon Walton, standards project manager for the Soil Association – which certifies organic food – added: ‘These results make sense - organic systems put animal welfare first and allow hens a truly free range life.
‘[Being outdoors] provides hens with many behavioural opportunities by letting them explore, forage for insects, scratch around in the ground, dust bathe - all the sorts of things they would naturally do in the wild and which are crucial for a good life.’
Vitamin D is also found in oily fish and liver.
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