Extreme Cuisine: Haydn Shaughnessy examines the life-prolonging properties of the sweet potato
There are parts of the world where sane people make a huge virtue of the potato, but the sweet rather than the starchy variety. Age researcher Brad Willcox, who has spent much of his academic career studying the long-lived Japanese in Okinawa, attributes some of that longevity, and their remarkable good health, to sweet potatoes.
The reason?
The sirtuin pathway, a protein signalling device that, some scientists believe, switches the body from its customary reproductive mode to preservation mode. Yes, the potato can do that, but what does it mean?
Scientists have long speculated that much of our bodies' instinctive, or organic, reactions to the world around them, are fashioned by an overwhelming motivation to reproduce, something that every teenager looks forward to.
Of course, we have to survive to do that. Nonetheless, the impulse bequeathed to us all, whether a virus, a bacterium, an animal, or human, or indeed a gene, by evolution, is, theoretically, to reproduce ourselves and prolong the life of the species.
That simplistic view of how we are programmed has recently been joined by a second evolutionary string to the human bow, one more selfish than Cupid's: preservation of self. It seems to be as prevalent as the reproductive impulse but not in the way we might expect.
Since last year's devastating tsunami, the areas affected by the freak Christmas waves have experienced a baby boom. Reproduction has reassumed its customary importance and women are busy compensating for their losses and restocking the species should further disaster strike.
Just after and during the latter part of the second World War, western society witnessed a similar phenomenon: we called that the baby boom too. Disasters encourage reproduction.
Something quite different motivates people to switch from reproduction to preservation.
In areas like Okinawa, which has the highest proven proportion of its population over 100 years of age, a small number of foods play a role in determining people's evolutionary status, according to Willcox.
Sweet potatoes, fermented foods, soy derivatives. All these foods are relevant to our sirtuin switching mechanism.
The reason seems to be that these foods are characterised by their own struggles with nature. Sweet potatoes in particular grow in difficult terrain and, like sun-damaged grapes that make the best wines, carry the proteins that signal to our bodies that the threat of scarcity is nearly upon us. Time to switch to preservation mode.
Does that mean that people in long-lived areas like Okinawa are less fertile? There's some evidence of it says Willcox. But that's not wholly the point.
Some small corners of civilisation happen upon diets that allow them longer and healthier lives and one of the reasons appears to be that when the foodstuff is stressed, it can pass on survival signals to those who eat it.
Is this just another of those peculiar discoveries of the spiritual East, who again seem to have the drop on those of us in the material West?
Not really.
Portugal is the European home of sweet potato cuisine. In the Alentejo region in south west Portugal, they hold sweet potato festivals each autumn. What's more they share with the Japanese a preference for using naturally sweet foods as desserts, based around the potato, rather than creating a sweet course artificially.
Desserts that for us are predominantly sugar-based, are for the Okinawans and Alentejanos predominantly based around sweet potatoes and nuts.
As with their Mediterranean counterparts nearby they also enjoy a slow pace of life and year-round sunshine, two good reasons to stay alive.
To date, though, nobody has cross-checked Okinawan and Alentejano longevity or health patterns but the wisdom of using foods that are naturally sweet instead of sweetening foods is surely a way of having the right kind of cake and eating it.
Indeed, it chimes well with our own growing food wisdom.
Sweet potatoes are, in western parlance, low GI foods; they clearly count as healthy options. They are potentially a low GI sweet.
While there are parts of the world that may by chance, and through spiritual direction, have sought longer, healthier lives, there are many of us in the West consciously pursuing the same objective, but checking the instinctive relationship between food and health that others are blessed with against what we know from science and tradition.
Call us faddist and fussy, but it seems to me we're just being sensible. The trick with humble foods is to use them as luxuries, a difficult concept when confronted by the potato but one that the following recipes will help with.
• Sweet potato fries and sweet potato with orange:
In the deep south of the United States they use sweet potato French fries as a dessert. The purist never takes a knife to a sweet potato (apparently the metal accelerates the oxidisation process). Instead, the purist roasts the sweet one in a fiery oven (200 centigrade) for about an hour and then scoops out the flesh or strips off the skin.
For sweet potato fries, take the latter course and then cut into strip or chunks and shallow fry on a medium heat. For sweet potato with orange, scoop out the flesh after baking and then puree with a little orange juice and butter, and season with pepper and ginger.
• Sweet potato pie:
Pierce the skin of four sweet potatoes and bake them for 45 minutes and when cooled, skin and puree along with a little milk, butter and honey, to taste. Spread into the bottom of a small baking tray. Mix a cup of roasted almonds or pecan nuts, crushed roughly, with a cup of desiccated coconut, and spread onto sweet potato. Drizzled two tablespoons of honey, on top. Drizzle a little melted butter, bake for 30-40 minutes.