What does Jamie Oliver have in common with a 111-year-old Lessie Brown from Cleveland, Ohio? They both believe that eating sweet potatoes will help you live to 100. Jamie has fame on his side and a new cookery programme on Channel 4, promoting the health benefits of 14 ‘hero’ ingredients including the sweet potato.

But it’s Lessie who has the wisdom of experience, having been born in Georgia in 1904, over 70 years before Jamie. Georgia produces a lot of sweet potatoes (Ocilla, Georgia even hosts an annual Sweet Potato Festival) and Lessie would have gone through quite a few before she moved to Cleveland in 1922.

There she married, going on to have five children, 24 grandchildren, 44 great-grandchildren and 26 great-great-grandchildren. Her daughter, Bernie Wilson, says her mum used to love sweet potatoes and passed on the advice to others: “She even told a lot of people that would eat sweet potatoes. They thought that would give them longer lives too.”

But could it?

Plausibility rating: 7 out of 10. If you’re only going to eat one vegetable, you could do a lot worse than the sweet potato. Not to be confused with a yam (duels have been fought over less), a single medium-sized sweet potato has around double the recommended daily allowance of Vitamin A and is rich in Vitamin C, iron and thiamine.

Sweet potatoes are also a great source of the antioxidant beta-carotene which is associated with a range of benefits, including your immune system, protecting against free radicals, and perhaps lowering your risk of heart disease and cancer.

And it’s not just Georgia sweet potatoes that offer the benefits. Jamie’s programme focuses on the inhabitants of the Okinawa islands in Japan, who eat a purple-fleshed variety of the sweet potato as a staple part of their diet. Okinawans famously live a very long, very healthy life. And a well documented one: since 1975, the Okinawa Centenarian Study has recorded how its centenarians ‘age slowly’, delaying or sometimes even escaping the chronic diseases of aging including dementia, cardiovascular disease (coronary heart disease and stroke) and cancer.

Overall, this is about as credible as evidence gets (Jamie would perhaps call it ‘pukka’). It is not, of course, all down to sweet potatoes. The study suggests that genes combine with  lifestyle factors to produce the remarkable longevity of the Okinawans. One of the those lifestyle factors is how much you eat, which we considered briefly here, but what you eat is also critical, and high fruit and vegetable consumption is key.

So Lessie would probably have been giving good advice when she told her neighbours to eat sweet potato.