Eat less, live longer?

Dreams of eternal youth feature in many cultures throughout history, but it was only in the 20th century that research into longevity really began. Much about ageing is still mysterious - we don't even know the underlying reasons why we journey into old age. There are many lines of enquiry into how to live longer, though, with one of the most intriguing being calorie restriction: in effect, going on a lifelong diet.