We are told that our life expectancy is increasing all the time, but broadcaster Giles Coren, who is also restaurant critic of The Times, has been pondering a disquieting fact: in his family, the trend has been in reverse. His great-grandfather lived to a ripe old 93, his grandfather passed away at 76, and his father Alan Coren, the celebrated humorist, died at 69. Giles, who is 45 and has two small children, is understandably eager not to bow out too soon. He wants to stick around a good long time for his kids' sake.
Ever intrigued by food, he set out to discover the diet best designed to promote longevity. Naturally, this quest - the subject of an entertaining new TV documentary - took him to America, undisputed home of the world's weirdest, most extreme diets. Giles is engaging and funny, his rapid-fire quips evidence of his father's comedic legacy. He tells me that his search for longevity was a particularly Jewish enterprise, because "Jews want to live forever. Unlike Christians, they have no paradise to look forward to."
But, come on, I prod him, wouldn't he still rather have a shorter, fun life, for instance like his dad, than a longer but more staid and sensible one?
Giles insists not. "I don't believe in a short, happy life. That's what people who do heroin believe in." He admits his father ''did all the wrong things'' in terms of his eating choices, and his heavy smoking, and Giles is determined not to follow suit. He wants to live a lot longer than 69. "The aim is to create a healthy balance." And for that, the right diet was crucial. So he's been asking diet gurus the question: "how do you eat to live forever"?
He journeyed to upstate New York to see Paul McGlothin and his partner Meredith Averill, obsessive proponents of the ''calorie restriction diet". The emaciated pair live in the woods, eating small quantities of repellent stuff such as berries doused with barley-and-onion soup, and fasting for 18 hours a day. Giles bravely gave their diet a go, but it played havoc with his digestion and left him starving.
Stodgy Polish food is not healthy city living
Which reminded me of that Woody Allen joke about the two guests at a hotel in the Catskills. "The food is terrible", says one. "Yes," replies the other, "and such small portions!"
McGlothin, who is in his sixties but looks twenty years older, and the vacuous Averill are intent on extending their bizarre lives as long as possible and already planning their 125th birthday celebrations. Sometimes, in lieu of eating, they perform a ''savouring meditation'' during which they imagine they are consuming a single blueberry. And all with a straight face. Incredible.
Giles went on to consult a glamorous Manhattan gastroenterologist who carries out ''faecal transplants'' - you replace your own inferior faecal bacteria with those of a healthier donor, in the manner of an enema.
I won't go into details but watching poo being mixed in a blender is not for the squeamish. Optimising the function of your gut bacteria might boost your chances of a long life, but Giles declined the poo transplant as a step too far.
Instead, he headed out to California for a convention of Paleo Diet aficionados, who eat only what Stone Age hunter-gatherers ate - primarily animal fats - and whose leader is the self-styled ''fat-burning beast'' Mark Sisson. Muscular and self-admiring, Sisson spoke with evangelical fervour about the diet's health benefits, and looked fit on the outside, but Giles's GP warned that he could be fat on the inside, with high levels of cholesterol clogging his veins, and might drop dead at any time.
Even more extreme was a slightly scary hillbilly type in Kentucky called Derek, who eats only what he kills, and always raw. In his back yard he slit the throat of a sheep and sipped some of its blood before carving it up. He later hosted a lunch of raw, diced sheep's organs, which Giles gamely sampled whilst Derek's bemused girlfriend and father stuck firmly with their green salad.
Finally, Giles travelled to Arizona to meet one of the world's oldest people, 113-year-old Bernando LaPallo, to discover the secret of his longevity. In an echo of James Hilton's classic novel, Lost Horizon, the Brazilian-born American put his exceptionally lengthy life down to a philosophy of moderation. He doesn't eat too much, just a little fish, fruit and veg. He walks a mile each day and has never smoked or drunk alcohol.
Giles has resolved to follow this sage's example, but is prepared to sacrifice a decade or two in return for a few beers a week and the occasional burger. Instead of 113, he'll settle for 93, like his great-grandpa.
But how can he square such modest consumption with his role as a restaurant critic, who is required to indulge in endless, rich, fancy meals? He tells me it's not a problem, as he has always had self-control over his eating and drinking, unlike some of his "fat, gluttonous" counterparts. "The notion of getting pleasure from food has gone too far; we can also get pleasure from anticipating a meal, and from not being quite sated." He adds, rather moralistically: "Instant gratification is bringing this planet to its knees."
Oh, and Giles takes a dim view of the "stodgy" Jewish diet. "Just look at all the middle-aged Jewish men waddling down Stamford Hill," he says. "It was fine for my Polish Ashkenazi forebears to live on dumplings and potatoes, because they laboured in the fields. But that diet is unsuitable for an urban lifestyle.'' He certainly approves of the light, Mediterranean-style Sephardi diet, with its healthy flatbread wraps, "but challah - it's just sugar!"
And as for blintzes, bagels and latkes, well, it's safe to say you won't find him scoffing those any time soon on the Golders Green Road.
'Eat to Live Forever' will be shown on BBC 2 at 9.30pm on March 18.
