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Douglas Robertson was 18 when his father, Dougal, a former merchant navy officer, decided it'd be fun for the family to sell his farm, buy a 43ft wooden schooner and sail around the world.

"Every day was an adventure - life was idyllic," says Douglas, of the 1971 voyage. Indeed, it was all going swimmingly until a pod of killer whales - near the Galapagos Islands - attacked the boat. Forced to abandon the sinking vessel, the six of them ended up adrift on the Pacific - away from any shipping lanes - onboard a 10ft dinghy.

Image copyright other Image caption The Robertson family were sailing across the Pacific, when their boat was sunk by killer whales

With minimal rations to hand, to stay alive the sea would have to provide.

On their sixth day of being shipwrecked, they managed to catch a turtle - which proved to be their salvation - and staple diet.

"A meal would be a mouthful of turtle meat, followed by drinking its blood. The blood is a little difficult to drink as it congeals - so you have to knock it back. We'd bleed the turtle into a little cup and pass it around. It's a bit like black pudding you have at breakfast," Douglas says.

As time passed drifting at sea, the Robertsons' turtle recipes became more inventive.

"We made soup out of turtle eggs - and we chewed on the bones for days on end to get the marrow."

Shipwrecked by whales: a survival story

Sustained by the reptiles - and whatever sushi they could land - the family survived for more than six weeks on the Pacific. After 38 days, they were rescued by a passing Japanese ship.

"It's amazing how civilisation is just a thin veneer - underneath it we are hunters, gatherers, innovators."

Doomsday diets

I'd been researching Douglas's innovative spirit for a new BBC podcast, All Hail Kale, which explores what would be the most nutritious way to survive the apocalypse - what would be the ideal doomsday diet.

As the Robertsons' survival story suggests, turtles aren't bad on the nutrition front, nutritionist Jo Travers says.

"Turtle meat has a lot of protein but very little fat and almost no carbohydrates," she says.

It is also a source of several micronutrients - including selenium, vitamin B12, iron, potassium, thiamine, riboflavin and zinc.

However, turtles are lacking when it comes to essential omega-3 fatty acids, vitamins A and C, and fibre. So, in the end, something like scurvy would get you, Ms Travers says.

Although turtles were once a delicacy in the US - President Taft is said to have employed a chef who specialised in turtle soup - today's doomsday "preppers" seem to be filling their bunkers with dried chicken and rice, soups, stews and oatmeal, according Sharon Kuhlman, at The Bunker store, in Leander, Texas.

So, I canvassed some other dietary experts to find out what other foods they might recommend. Of course, any nutritionist worth their salt will say you need a balanced diet, blah, blah. But, if you could pick just one food...?

Mushrooms

Edinburgh-based nutritionist Dr Laura Wyness picks mushrooms. "They are one of the few dietary sources of selenium and vitamin D - which has long been known for helping the body absorb calcium for healthy bones and teeth - but research now also suggests vitamin D is important for a strong immune system as well as regulating mood and preventing depression.

"There is even an edible mushroom called Laetiporus that apparently tastes like chicken. Also, ancient Egyptians believed mushrooms were the plant of immortality."

As to how long she could last on just mushrooms? "Probably about four weeks. Mainly due to the boredom from eating one type of food, resulting in a loss of appetite and then eventually starvation."

Spam

Summoning some retro-Blitz spirit, Nantwich nutritionist Liam Brown is turning to a comforting can of Spam. "Whilst it isn't something I would usually recommend eating daily, canned cooked meat is a good source of both fat and protein - and I like the taste.

"You could live a long time on canned meat - however, the lack of vitamins, minerals and healthy fats will soon lead to illness - and the lack of fibre may result in problems on the toilet. Tinned salmon is a healthier alternative but less energy dense. Sadly, too much of any one food will never end well."

Ginger and quinoa

Some tried to sneak two items items into the checkout. Bristol medical students - and founders of nutrition education hub Nutritank - have gone down the distinctly less Spammy route of ginger and quinoa. "We are going to need something that will jazz up our lives and a food which will definitely sustain us nutritionally. So, we have quinoa porridge for breakfast with fresh cut ginger, fried ginger sprinkled on fried quinoa for lunch and ginger-and-quinoa stew for tea - bliss."

Pizza

Flouting the one-food regulations yet further, dietician Lucy Jones is heading straight to the frozen pizza aisle. "Vitamin C from tomatoes; the base for energy, fibre and B vitamins; cheese for protein and calcium; and toppings with of course plenty of vegetables and maybe some anchovies for omega-3 fatty acids. I might not live forever but I'd enjoy a few meals."

Potatoes

Nutritionist James Collier says the spud is the "one food that contains most of the nutrients humans need, albeit some in only small amounts, but (if stored correctly) potatoes could help keep you going for some time".

"Potatoes are rich in carbs, potassium, vitamin C, folate, magnesium, protein and some essential fatty acids."

Plant-based diet proponent Dr Michael Greger, from NutritionFacts.org, also plumps for potatoes. "If you only had one food and you had to go months or a year, probably white potatoes - even though they're not the healthiest food. You'd go blind from vitamin-A deficiency and all sorts of horrible things would happen, but you'd be alive."

Download BBC Sounds to listen to All Hail Kale.