Can extreme calorie counting make you live longer?
Bombarded with advertisements promising a longer, healthier life, the BBC's Los Angeles reporter Peter Bowes goes in search of eternal youth.
At 6ft 1in (1.85m), Martin Knight weighs 10st 4lb (65.77kg). Breakfast - the same every day - is a 170-calorie mixture of kale, shallots, sprouted oats, tomato paste and olive oil. He washes it down with green or white tea.
Knight follows a calorie-restricted diet.
He eats 10 small meals a day and survives on about 1,900 calories - more than most people who restrict their calorie intake. Each meal is weighed to ensure that his daily intake is accurate.
Adherents of this approach to health are known as Cronies - Calorie Restriction with Optimal Nutrition - and Dr Janet Tomiyama, a psychologist at the University of California, is studying them.
"The normal person needs around 2,000 calories (a day). So if you are talking about cutting 500 calories per day, every day for the rest of your life, that's a very tall order," she says.
"That's why this group is so fascinating because they've been able to do this, and they've been able to do it for over a decade."
Tomiyama says the group is mostly made up of older white men. They tend to be well-educated "connoisseurs of scientific literature".
They are mostly looking for good health and see a longer life, if it occurs, as a welcome by-product.
The results of Tomiyama's study, exploring the psychological impact on the participants, are expected to be published later this year.
Knight, 49, joined the study two years ago. He had followed a vegetarian or vegan diet for much of his adult life but in the past few years has restricted his intake of calories.
He works in the finance industry and lives in a sparsely furnished house in the coastal town of Santa Barbara, California.
End Quote Martin Knight Practitioner of calorie restriction
When you have a little bit of hunger in the background you are more aware and more alive”
He enjoys a slow, meditative existence, practising yoga, lifting weights and running in his spare time.
"It's second nature to me now, it doesn't seem difficult," he says. "It would be harder for me to live with unlimited calories, like I did before. I think I would be miserable."
It is perhaps just as well that he does not like ice cream. For a treat, he will occasionally have an avocado sandwich.
Knight says he follows his diet because it makes him happy.
"It makes you more alert. It fits in with a more healthy, slower-paced kind of lifestyle. It's very practical," he says.
"When you have a little bit of hunger in the background, you're more aware and more alive."
The science of fasting
Dr Valter Longo, director of the Longevity Institute at the University of Southern California, has established that restricting calorie intake has clear benefits for mice.
He has also identified the role of a growth hormone, insulin-like growth factor 1 (IGF-1), in causing the body to go into what he describes as "repair mode".
Studies on people who restrict their intake of calories suggest that eating less reduces their IGF-1 levels.
This seems to switch them from "growth mode" to "repair mode", and it's suggested this is good for their health.
Experiments on mice have shown that eating significantly less food can extend lifespans, so long as the food is highly nutritious.
There is still no proof that restricting calories extends human life spans, however, and two recent studies with rhesus monkeys produced conflicting results.
But both studies reported the same beneficial effects on health - all the monkeys were much healthier and suffered from fewer diseases.
"If we were able to reduce... weight in the Western world by 15 to 20%, the incidence of cardiovascular diseases, diabetes, and obesity, would go down dramatically," says Dr Raphael de Cabo, the lead author of one of the studies.
"That undoubtedly would increase the mean life span of the human population."
Perhaps the final word should go to the man many credit with pioneering the idea of restricting food intake to extend life.
Dr Roy Walford, a professor of pathology at the UCLA School of Medicine, died in 2004 due to complications from Lou Gehrig's Disease, also known as Motor Neurone Disease. He was 79. For much of his life he was a passionate believer in the power of eating less.
He consumed a mere 1,600 calories a day and believed that a lifespan of 150 was possible for human beings.
What if we could stay young forever? What if everyone had a car? What If? is a season across BBC News looking at visions of the future.
Walford was one of the first scientists to demonstrate that mice on a limited diet could double their lifespan. When I met him in 1999, at his home in Santa Monica, he offered a pragmatic view of calorie restriction and its benefits.
It is a "choice you have to make", he told me.
"You're healthier during all that time, you need less sleep, you're intellectually stimulated, you're kind of wired, there's an increased sense of well-being and vitality.
"So if you want to trade all that to eat cake, then I say - go ahead and eat cake."