future (the eminent biologist Lewis Wolpert recently declared his belief in the predictive power of the genome; others think that a mere listing of our genetic make-up is next to useless). And so Dorian’s doctors can “read” what lies in store for him in terms of his likelihood of suffering from certain diseases.

Imagine that his genome predicts a risk of heart disease; Dorian may choose to start taking a statin and lead a blameless lifestyle (a vigorous but not punishing exercise regimen, the occasional glass of claret). Unlike his literary namesake, he marries well (married men live longer than single men).

Dorian would not need to worry about any genetic diseases; gene therapy eliminated them at birth. To stretch the genome-as-book analogy, gene therapy can be thought of as a copy-editing process, correcting bad grammar and inserting punctuation marks and missing letters where needed. Scientists usually achieve this by sending a virus into the body. The virus, which contains the new genes to be inserted, invades each cell — every cell of your body contains a copy of your genome — and manipulates the genome from within.

At the same time as Dorian’s genome was fixed, it was upgraded: a suite of genes thought to code for longevity was inserted (the biggest predictor of how long you will live is how long your parents lived, suggesting that many old people use a genetic advantage to cheat death). Neither would he need to fret about physical degeneration of his vital organs: his umbilical cord blood was banked at birth, providing a ready source of stem cells that could be used to grow a new liver, kidneys or even a new heart (or scientists could clone a skin cell, as they did with Dolly, creating an embryonic Dorian whose stem cells could be harvested for similar purposes).

Now to Dorian’s diet, which would involve him consuming a third fewer calories than normal. There is much evidence to suggest that eating less (while not becoming malnourished) extends life; a 20-year study of rhesus monkeys at the Wisconsin National Primate Research Centre put one group of animals on a normal diet and a matched group of monkeys on a diet containing 30 per cent fewer calories. When the study ended this summer, 37 per cent of the monkeys on the normal diet had died from age-related diseases (such as diabetes, cancer and heart diseases) compared with only 13 per cent in the restricted-calorie group.

There is tentative evidence that what applies to worms, flies and monkeys may apply also to people. Some of the longest-lived people on the planet reside on the Japanese island of Okinawa, which has a history of low food supply because of crop failures. A 2007 study reported in the Annals of the New York Academy of Science suggested that Okinawans lived, on average, five years longer than Americans because they ate a tenth fewer calories.

A hard core of longevity enthusiasts have already bet on the low-calorie route to long life by joining the Calorie Restriction Society. This society, which offers advice on how to eat fewer calories while staying healthy (avoid second helpings; fill up on vegetables; skip the occasional meal; “pre-eat” before parties) sounds less daft than it did a decade ago and will even be taking part in a Harvard University conference on healthy ageing this month.

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