The Fast Diet: the science

The facts, stats and compelling science behind Michael Mosley's sensational Fast Diet

Once upon a time parents told their children not to eat between meals. Those times are long gone. Recent research in the US, which compared the eating habits of 28,000 children and 36,000 adults over the past 30 years, found that the amount of time spent between what the researchers coyly described as “eating occasions” has fallen by an average of an hour. In other words, over the past few decades the amount of time we spend “not eating” has dropped dramatically. In the 1970s, people like my mother would go around four and a half hours without eating, while children like me would be expected to last about four hours between meals. Now it’s down to three and a half hours for adults and three hours for children, and that doesn’t include all the drinks and nibbles.


In the study I quoted above, they found that compared to 30 years ago, we not only eat around 180 calories a day more in snacks – much of it in the form of milky and fizzy drinks and smoothies – but we also eat more when it comes to our regular meals, up by an average of 120 calories a day.

In other words, snacking doesn’t seem to mean that we eat less at meal times; it just whets the appetite. Eating throughout the day is now so normal, so much the expected thing to do, that it is almost shocking to suggest there is value in doing the absolute opposite.

Fast now… Live Longer

Professor Valter Longo is director of the University of Southern California’s Longevity Institute. His research is mainly into the study of why we age. Valter has access to his own supply of genetically engineered mice,
known as dwarf or Laron mice. These mice hold the record for longevity extension in a mammal.

The average mouse doesn’t live that long, perhaps two years. Laron mice can live twice that, many for over four years when they are also calorie-restricted. In a human, that would be the equivalent of reaching almost 170.

The fascinating thing about Laron mice is not just their longevity, but the fact that they stay healthy for most of their very long lives. They simply don’t seem to be prone to diabetes or cancer, and when they die, more often than not, it is of natural causes.

The reason these mice are so small and so long-lived is that they are genetically engineered so that their bodies do not respond to a hormone called IGF-1, Insulin-like Growth Factor 1. IGF-1, as its name implies, has growth-promoting effects on almost every cell in your body. It keeps your cells constantly active. You need adequate levels of IGF-1 and other growth factors when you are young and growing, but high levels later in life appear to lead to accelerated ageing and cancer. As Valter put it, it’s like driving along with your foot flat down on the accelerator, pushing the car to continue to perform all the time. “Imagine, instead of occasionally taking your car to the garage and changing parts and pieces, you simply kept on driving it and driving it and driving it. Well, the car, of course, is going to break down.”

Valter’s work is focused on trying to figure out how you can go on driving as much as possible, and as fast as possible, while enjoying life. He thinks the answer is periodic fasting. Because one of the ways fasting works is by making your body reduce the amount of IGF-1 it produces.

Switch into repair mode

As well as reducing circulating levels of IGF-1, fasting also appears to switch on a number of repair genes. The reason this happens is not fully understood, but the evolutionary argument goes something like this. As long as we have plenty of food, our bodies are mainly interested in growing, having sex and reproducing. Nature has no long-term plans for us. She does not invest in our old age. Once we have reproduced we become disposable.

So what happens if you decide to fast? Well, the body’s initial reaction is one of shock. Signals go to the brain reminding you that you are hungry, urging you to go out and find something to eat. But you resist. The body now decides that the reason you are not eating as much and as frequently as you usually do must be because you are now in a famine situation.

In a famine situation there is no point in expending energy on growth or sex. Instead the wisest thing the body can do is spend its precious store of energy on repair, trying to keep you in reasonable shape until the good times return. The result is that, as well as removing its foot from the accelerator, your body takes itself along to the cellular equivalent of a garage. There, all the little gene mechanics are ordered to start doing some of the urgent maintenance tasks that have been put off until now.

It’s not just the calories

If you eat 500 or 600 calories two days a week and don’t significantly overcompensate during the rest of the week, then you will lose weight in a steady fashion. But is there any evidence that Intermittent Fasting does more than that?

In one particularly fascinating study, scientists from the Salk Institute for Biological Studies took two groups of mice and fed them a high-fat diet. The mice got exactly the same amount of food to eat, the only difference being that one group of mice was allowed to eat whenever they wanted, while the other group of mice had to
eat their food in an eight-hour time period. This meant that there were 16 hours of the day in which they were, involuntarily, fasting.

After 100 days, there were some truly dramatic differences between the two groups of mice. The mice who nibbled away at their fatty food had developed high cholesterol, high blood glucose and had liver damage. The mice that had been forced to fast for 16 hours a day put on far less weight (28 per cent less) and suffered much less liver damage, despite having eaten exactly the same amount and quality of food. They also had lower levels of chronic inflammation, which suggests they had reduced risk of a number of diseases, including heart disease, cancer, strokes and Alzheimer’s.

The Salk researchers’ explanation for this is that all the time you are eating, your insulin levels are elevated and your body is stuck in fat-storing mode. Only after a few hours of fasting is your body able to turn off the “fat-storing” and turn on the “fat-burning” mechanisms.

Why you won’t overeat

Dr Krista Varady of the University of Illinois at Chicago has done a number of studies on Alternate Day Fasting (ADF) using human subjects, and what surprised her is that, even when they are allowed to, people don’t go crazy on their feed days. “I thought when I started running these trials that people would eat 175 per cent the next day; they’d just fully compensate and wouldn’t lose any weight. But most people eat around 110 per cent, just slightly over what they usually eat.

Krista wanted to know whether ADF would also work if her subjects were allowed to eat a typical American high-fat diet. So she asked 33 obese volunteers, most of them women, to go on ADF for eight weeks. The volunteers were divided into two groups. One group was put on a low-fat diet, eating low-fat cheeses and dairies, very lean meats and a lot of fruit and vegetables. The other group was allowed to eat high-fat lasagnes, pizza, the sort of diet a typical American might consume.

The results were unexpected. The volunteers on the high-fat diet lost an average of 5.6kg, while those on the low-fat diet lost 4.2kg. They both lost about 7cm around their waists.

Krista thinks the main reason this happened was compliance. The volunteers on the high-fat diet were more likely to stick to it than those on the low-fat diet, simply because they found it a lot more palatable.

Find out more about how to fast and the benefits of The Fast Diet.

See seven fasting day menu examples.


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