By Rebecca Morelle
Science reporter, BBC News
Scientists believe they are a step closer to working out why an extremely restrictive diet boosts longevity.
Bob Cavanaugh has restricted his diet for six years
This well-documented calorie-cutting phenomenon has been seen in many species, from yeast to mice to dogs.
Although the effects of "calorie restriction" in humans are as yet unknown, some are undertaking this in a bid to live longer.
Here, Bob Cavanaugh, managing director of the Calorie Restriction Society, tells the BBC News website about his diet.
I've been doing calorie restriction for six years.
When I was 53, I had a physical and found out my blood cholesterol level was very high. My doctor wanted to try to control it with diet before resorting to drugs, and I was all in favour of that.
I had read a book about calorie restriction and ageing back in 1988, and was convinced by the science, so I decided to try it.
I use software to work out what to eat every day. I believe without it, it is impossible to have proper nutrition.
You weigh your food, and it works out the calories, the ratio of fat, carbohydrate and protein and the breakdown of vitamins, minerals and amino acids, then runs a nutritional profile of what you eat throughout the day.
On an average day, I eat 1,800 calories. Younger people can restrict their calories more severely, but I've been told that, based on lab animal evidence, I have already accumulated years of damage to my mitochondria (the powerhouses of the cells), so I utilise my food less efficiently than a younger person.
On a typical day, I will eat an oatmeal-based recipe for breakfast, which is about 455 calories and it gives me about half of my daily nutrients.
I don't eat lunch - after this breakfast I just don't feel hungry - so that leaves me about 1,350 calories for my evening meal, which is a lot.
If you are smart, by eating small portions of meat and small quantities of starchy things, that leaves an enormous amount of room for fruit and vegetables. You wind up eating quite a large meal and it is very filling, nutritious and satisfying.
Contrary to popular belief, you are not hungry on this diet, and I feel excellent. When I started the diet when I was 53, I felt like I was starting to get on in years and didn't quite have the vim and vigour I used to have.
But starting calorie restriction, that exhilaration that I used to experience in youth returned and my whole sense of well-being returned to levels I experienced as a child.
It really made me feel like I got my life back.
In terms of health, my cholesterol level has really dropped; I now weigh 150lb, and I haven't had any illnesses at all - not even a cold.
My motivation for doing calorie restriction was two-fold. One was to reduce my risk of age-related diseases such as heart disease - with the cholesterol level I had. I was in line for this.
And the prospect of extending your lifespan is very appealing - although I guess you have to be somewhat narcissistic to think you are worth living a lot longer than anyone else.