With a combined age of 1,075 years and 68 days, the 13 Donnelly siblings of County Armagh in Northern Ireland can proudly lay claim to being the world's oldest family.

Indeed their ability to defy the passing years has seen them achieve a Guinness world record. But what can now be revealed is the secret behind the family's longevity - traditional porridge oats.

Having grown up on a farm, their diet has always been made up of locally sourced ingredients; free-range pigs and chickens for bacon and eggs and vegetables grown in their backyard.

However, the Donnelly clan are convinced that what has really guaranteed their long life is their habit of eating porridge, not just for breakfast everyday but for supper, too.

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"The key is that you need to get your oats at night," said Leo Donnelly, who at 72 is the youngest of the siblings.

"We've always followed Daddy's habit of that nice warm bite before sleep. Porridge at around 10pm, then porridge again for breakfast at 7am. "Cooked oats, milk, perhaps a spot of jam on top. It has always stood us well, porridge before sleep and after sleep. People thought it was unusual, but now the proof is there for all to see."

The family's reliance on oats and their apparently beneficial effect on their health and years have prompted scientists at the International Longevity Centre (ILC) to launch a study of the Donnellys' diet.

Porridge has been shown to lower blood pressure, combat diabetes, reduce cholesterol and protect the body against heart disease.

Oats are naturally high in vitamin B-12, a crucial component in regulating the body's melatonin levels and sleeping patterns. The grain is also naturally loaded with beta-glucan, a "magic genie" that lowers "bad" LDL cholesterol and reduces heart disease.

Porridge was labelled a superfood when it was discovered that avenanthramides, antioxidants that are only found in oats, minimise plaque build-up in artery walls and reduce the hardening of arteries.

But for the Donnellys, tucking into a bowl of porridge is just part of their fundamentally wholesome diet and lifestyle, which also sees them steer clear of alcohol.

"The farm oats in our porridge were always local and of the finest quality," said Leo. "Our diet has never been from processed or polluted foods.

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"We worked with nature, everything eaten was either grown or reared by us. Mummy would say go outside and wring a chicken's neck, that would be for dinner.

"The vegetables we ate we had grown, the fruit we ate we picked, the eggs laid were from our chickens, the bacon came from our pigs, the bread we made by hand. Farm oats did not arrive from foreign lands.

"There was never a fat Donnelly raised."

RECIPE FOR LONGEVITY: THE DONNELLY'S DAILY MEALS

Breakfast at 7am

Porridge with homemade jam

Lunch at 1pm
Cooked seasonal vegetables from the farm, occasional eggs, homemade bread and tea

Dinner at 6pm
Eggs, soup, seasonal vegetables from the farm, homemade bread and fruits
Meat from their own animals as a weekend treat

Supper at 10pm
Porridge with the option of apple jam

Snacks
Apples, bread and tea

- The Sunday Telegraph