The periods of eating very little or nothing may be the key to controlling chemicals produced by the body linked to the development of disease and the ageing process.

This backs up recent studies on animals fed very low-calorie diets which found the thinnest (without being medically underweight or malnourished) are the healthiest and live the longest.

The key, say researchers at the University of Southern California’s Longevity Institute, is the hormone Insulin-Like Growth Factor 1 (IGF-1).

IGF-1 and other growth factors keep our cells constantly active. It’s like driving with your foot on the accelerator pedal, which is fine when your body is shiny and new, but keep doing this all the time and it will break down.

According to Professor Valter Longo, director of the Longevity Institute, one way to take the foot off the accelerator, and reduce IGF-1 levels dramatically – as well as cholesterol, and blood pressure – is by fasting.

“You need adequate levels of IGF-1 and other growth factors when you are growing, but high levels later in life appear to lead to accelerated ageing,” he says.

“The evidence comes from animals such as the Laron mice we have bred which have been genetically engineered so they don’t respond to IGF-1. They are small but extraordinarily long-lived.”

The periods of eating very little or nothing may be the key to controlling chemicals produced by the body linked to the development of disease and the ageing process

The average mouse has a life span of two years – but the Laron typically live 40% longer. The oldest has lived to the human equivalent of 160. They are immune to heart disease and cancer and when they die, as Prof. Valter Longo puts it: “They simply drop dead.”

Trying various fasts, for three days straight, and for two days a week, for six weeks, you can see dramatic results. Not only weight loss, but your cholesterol levels and blood pressure improve. These findings chime with recent reports that reaching a “healthy” Body Mass Index (BMI) may not be enough – we need to be as slim as possible to reduce our risk of illness.

The reason experts haven’t emphasized this is that they don’t want to trigger eating disorders or demotivate the overweight trying to get into the healthy weight range. There is only so long, however, we can shy away from this because the evidence keeps mounting.

Matthew Piper, of the Institute of Healthy Ageing, University College London, says: “Studies on monkeys show if we restrict the diet there is a delay in the onset of cancer, coronary heart disease and diabetes in later life as well as staving off dementia.”

Reducing our food intake over months or years could boost lifespan by 15 to 30%, experts believe.

Although the Scarsdale Medical Diet was a hit for the Seventies audience relatively new to slimming, it is brutal physically and mentally. But Dr. Rachel Thompson, of the World Cancer Research Fund, says: “Whatever your BMI, if it goes up so does your cancer risk. It’s better to be at the lower end of the healthy BMI range if possible.”

For every two points you jump up the scale, your risk of postmenopausal breast cancer goes up 3%.

The Scarsdale Diet is a high-protein, low-carbohydrate mix with a fixation on grapefruit.

Unlike other high-protein diets that allow you to stuff yourself with fatty bacon and cheese, this diet imposes strict limits.

Breakfast is always half a grapefruit and a piece of toast with no butter or jam. Lunch on day one is cold cuts of meat with all fat removed and a tomato.

Supper is fish with salad and a piece of bread followed by more grapefruit. You must also drink lots of water and, thankfully, black tea and coffee are allowed.

The first days are a blur of dry toast, fruit and sliced tomatoes and meat.