Serious discussion about calorie restriction and intermittent fasting in the popular press is comparatively rare - anything that involves changes of diet will quickly be buried by idiocy as a general rule, if not by the author of the piece, then shortly thereafter. Dieting is just one of those topics in which rationality seems to flee the building whenever it comes up.
Both calorie restriction and intermittent fasting are shown to extend life and greatly improve health in mice and many other species, but they might not operate through exactly the same mechanisms. Intermittent fasting in which calorie intake is maintained at the same level as non-fasting rodents has been shown to produce some extension of life and health benefits in studies for example - equally other studies suggest that this might not be the case. For my money I'd wager the bulk of the effect is calorie based: intermittent fasting tends to result in a lower overall calorie intake, and we know that calorie intake has a large effect on health and longevity in comparison to everything else that you can try in mice.
There is a lot more research into calorie restriction than exists for intermittent fasting strategies such as alternate day fasting. You should bear that in mind when reading around the topic. Calorie restriction really is the gold standard for evidence when it comes to things you can do that will positively affect your health. Intermittent fasting is merely at the interesting and convincing level, worth the balance of risk and reward in my eyes, but nowhere near as well supported as, say, regular moderate exercise.
The present scientific consensus on calorie restriction is that it won't significantly extend life in humans. Perhaps 5-10% at most. This comes from a combination of evolutionary considerations and common sense. If calorie restriction could extend human life by 40%, as it does in mice, then we would have known all about this for centuries at least. Researchers believe that calorie restriction is an adaptation that allows better survival of periodic famine, something that tends to happen on a seasonal timescale - which is long for mice, but short for humans. So there is evolutionary pressure for mice to be able to extend lives considerably in response to a lack of food, but not so for humans. Apparently there is still evolutionary pressure for the creation of health benefits, however, as the short term effects of calorie restriction on the operation of metabolism and resistance to age-related disease are quite similar in both mice and humans.
When it comes to intermittent fasting there really isn't a consensus on life span effects. There isn't enough data and a big enough body of work for that to exist yet. It is generally believed to be a good thing for health, however. With that all said, take a look at this popular science series from the BBC, which examines a mild form of intermittent fasting - really just "intermittent eating somewhat less," a far cry from alternate day fasting in which practitioners only eat at all every other day.
Curious about the scientific research that goes into devising a new diet, I decided to volunteer as a subject in a five-month clinical trial at the University of Southern California (USC). As a human guinea pig, I signed up to test a strict diet regime and subject myself to a battery of clinical tests to evaluate its effect on my body.
It involved surviving, for five consecutive days, on a narrow range of foods that contained as little as 500 calories per day - about a quarter of the average person's consumption. There was to be no cheating, no falling off the wagon and no treats. It was an opportunity to be part of study that may help scientists unravel the complex relationship between food and the human body.
The clinical trial, which is still ongoing, is designed to investigate the feasibility, safety, potential benefits and psychological changes associated with a calorie-restricted diet. It is based on previous experiments, at a number of institutions, which have shown that mice live longer and healthier lives if their food intake is cut by up to 30%.
The limited selection of food (with no choice of flavours) means that everything has to be eaten. It's monotonous... but at least it makes meal planning easy for five days. "The reason why diets don't work is because they are very complicated and people have an interpretation problem," says Dr Valter Longo, director of the University of Southern California (USC) Longevity Institute. "The reason I think these [intermittent fasting] diets work is because you have no interpretation. You either do it or you don't do it. And if you do it you're going to get the effect."
During each five-day fasting cycle, when I ate about a quarter the average person's diet, I lost between 2kg and 4kg (4.4-8.8lbs) but before the next cycle came round, 25 days of eating normally had returned me almost to my original weight. But not all consequences of the diet faded so quickly. "What we are seeing is the maintenance of some of the effects even when normal feeding resumes," explains Dr Valter Longo, director of USC's Longevity institute, who has observed similar results in rodents.
Arguably, the most interesting changes were in the levels of a growth hormone known as IGF-1 (insulin-like growth factor). High levels of IGF-1, which is a protein produced by the liver, are believed significantly to increase the risks of colorectal, breast and prostate cancer. Low levels of IGF-1 reduce those risks.
"In animals studies we and others have shown this to be a growth factor that is very much associated with ageing and a variety of diseases, including cancer," says Longo. Studies in mice have shown that an extreme diet, similar to the one I experienced, causes IGF-1 levels to drop and to stay down for a period after a return to normal eating. My data showed exactly the same pattern. "You had a dramatic drop in IGF-1, close to 60% and then once you re-fed it went up, but was still down 20%," Longo told me.