FastDiet Introduction
OVER THE LAST FEW DECADES, food fads have come and gone, but the standard medical advice on what constitutes a healthy lifestyle has stayed much the same: eat low-fat foods, exercise more . . . and never, ever skip meals. Over that same period, levels of obesity worldwide have soared.

Now many of those old certainties are being questioned.

There is nothing else you can do to your body that is as powerful as fasting.

When we first read about the benefits of intermittent fasting, we, like many, were skeptical. Fasting seemed drastic, difficult—and we both knew that dieting of any description is generally doomed to fail. But now that we’ve looked at it in depth and tried it ourselves, we are convinced of its remarkable potential. As one of the medical experts interviewed for this book puts it: “There is nothing else you can do to your body that is as powerful as fasting.”
Fasting: An Ancient Idea, a Modern Method
Fasting is nothing new. As we’ll discover in the next chapter, your body is designed to fast. We evolved at a time when food was scarce; we are the product of millennia of feast or famine. The reason we respond so well to intermittent fasting may be because it mimics, far more accurately than three meals a day, the environment in which modern humans were shaped.

Fasting, of course, remains an article of faith for many. The fasts of Lent, Yom Kippur, and Ramadan are just some of the better-known examples. Greek Orthodox Christians are encouraged to fast for 180 days of the year (according to Saint Nikolai of Zicha, “Gluttony makes a man gloomy and fearful, but fasting makes him joyful and courageous”), while Buddhist monks fast on the new moon and full moon of each lunar month.

Many more of us, however, seem to be eating most of the time. We’re rarely ever hungry. But we are dissatisfied. With our weight, our bodies, our health.

Intermittent fasting can put us back in touch with our human selves. It is a route not only to weight loss, but also to long-term health and well-being. Scientists are only just beginning to discover and prove how powerful a tool it can be.

A review article recently published in the scientific journal Cell Metabolism, “Fasting: Molecular Mechanisms and Clinical Applications,”1 which looked at some of the most recent human and animal studies, makes the point that “fasting has been practiced for millennia, but only recently, studies have shed light on its role in adaptive cellular responses that reduce oxidative damage and inflammation, optimize energy metabolism, and bolster cellular protection.”

In other words, we now know, through proper scientific studies, that fasting reduces many of the things that promote aging (“oxidative damage and inflammation”), while increasing the body’s ability to protect and repair itself (“cellular protection”).

The article concludes that fasting “helps reduce obesity, hypertension, asthma, and rheumatoid arthritis. Thus, fasting has the potential to delay aging and help prevent and treat diseases.”

This book is a product of cutting-edge scientific research and its impact on our current thinking about weight loss, disease resistance, and longevity. But it is also the result of our personal experiences.

Both are relevant here—the lab and the lifestyle—so we investigate intermittent fasting from two complementary perspectives. First, Michael, who used his body and medical training to test its potential, explains the scientific foundations of intermittent fasting (IF) and the 5:2 diet—something he brought to the world’s attention during the summer of 2012.

Then Mimi offers a practical guide on how to do it safely, effectively, and in a sustainable way, a way that will fit easily into your normal everyday life. She looks in detail at how fasting feels, what you can expect from day to day, what to eat, and when to eat, and provides a host of tips and strategies to help you gain the greatest benefit from the diet’s simple precepts.

As you’ll see below, the FastDiet has changed both of our lives. We hope it will do the same for you.
Michael’s Motivation: A Male Perspective
I am a 57-year-old male, and before I embarked on my exploration of intermittent fasting, I was mildly overweight: at five feet, eleven inches, I weighed around 187 pounds and had a body mass index of 26, which put me into the overweight category. Until my midthirties, I had been slim, but like many people I then gradually put on weight, around one pound a year. This doesn’t sound like much, but over a couple of decades it pushed me up and up. Slowly I realized that I was starting to resemble my father, a man who struggled with weight all his life and died in his early seventies of complications associated with diabetes. At his funeral many of his friends commented on how like him I had become.

While making a documentary for the BBC, I was fortunate enough to have an MRI (magnetic resonance imaging) scan done. This revealed that I am a TOFI—thin on the outside, fat inside. This visceral fat is the most dangerous sort of fat, because it wraps itself around your internal organs and puts you at risk for heart disease and diabetes. I later had blood tests that showed I was heading toward diabetes, and had a cholesterol score that was also way too high. Obviously, I was going to have to do something about this. I tried following standard advice, except it made little difference. My weight and blood profile remained stuck in the “danger ahead” zone.

I had never tried dieting before because I’d never found a diet that I thought would work. I’d watched my father try every form of diet, from Scarsdale through Atkins, from the Cambridge Diet to the Drinking Man’s Diet. He’d lost weight on each one of them, and then within a few months put it all back on, and more.

Then, at the beginning of 2012, I was approached by Aidan Laverty, editor of the BBC science series Horizon, who asked if I would like to put myself forward as a guinea pig to explore the science behind life extension. I wasn’t sure what we would find, but along with producer Kate Dart and researcher Roshan Samarasinghe, we quickly focused on calorie restriction and fasting as a fruitful area to explore.

Calorie restriction (CR) is pretty brutal; it involves eating an awful lot less than a normal person would expect to eat, and doing so every day of your (hopefully) long life. The reason people put themselves through this is because it is the only intervention that has been shown to extend lifespan, at least in animals. There are around 50,000 CRONies (Calorie Restriction with Optimum Nutrition) worldwide, and I have met quite a number of them. Despite their generally fabulous biochemical profile, I have never been seriously tempted to join their skinny ranks. I simply don’t have the willpower or desire to live permanently on an extreme low-calorie diet.

So I was delighted to discover intermittent fasting (IF), which involves eating fewer calories, but only some of the time. If the science was right, it offered the benefits of CR but without the pain.

I set off around the United States, meeting leading scientists who generously shared their research and ideas with me. It became clear that IF was no fad. But it wouldn’t be as easy as I’d originally hoped. As you’ll see later in the book, there are many different forms of intermittent fasting. Some involve eating nothing for twenty-four hours or longer. Others involve eating a single, low-calorie meal once a day, every other day. I tried both but couldn’t imagine doing either on a regular basis. I found it was simply too hard.

Instead I decided to create and test my own modified version. Five days a week, I would eat normally; on the remaining two I would eat a quarter of my usual calorie intake (that is, 600 calories).

I split the 600 calories in two—around 250 calories for breakfast and 350 calories for supper—effectively fasting for around twelve hours at a stretch. I also decided to split my fasting days: I would fast on Mondays and Thursdays. I became my own experiment.

The program, Eat, Fast, Live Longer, which detailed my adventures with what we were now calling the 5:2 diet, appeared on the BBC during the London Olympics in August 2012. I expected it to be lost in the media frenzy that surrounded the Games, but instead it generated a frenzy of its own. The program was watched by more than 2.5 million people—a huge audience for Horizon—and hundreds of thousands more on YouTube. My Twitter account, @DrMichaelMosley, went into overdrive, my followers tripled; everyone wanted to try my version of intermittent fasting, and they were all asking me what they should do.

The newspapers took up the story. Articles appeared in The Times (London), the Daily Telegraph, the Daily Mail, and the Mail on Sunday. Before long, it was picked up by newspapers all over the world—in New York, Los Angeles, Paris, Madrid, Montreal, Islamabad, and New Delhi. Online groups were created, menus and experiences swapped, chat rooms started buzzing about fasting. People began to stop me on the street and tell me how well they were doing on the 5:2 diet. They also e-mailed details of their experiences. Among those e-mails, a surprisingly large number were from doctors. Like me, they had initially been skeptical, but they had tried it for themselves, found that it worked, and had begun suggesting it to their patients. They wanted information, menus, details of the scientific research to scrutinize. They wanted me to write a book. I hedged, procrastinated, then finally found a collaborator, Mimi Spencer, whom I liked and trusted and who has an in-depth knowledge of food. Which is how what you are reading came about.
Mimi’s Motivation: A Female Perspective
I started intermittent fasting on the day I was commissioned to write a feature for The Times about Michael’s Horizon program. It was the first I’d heard of intermittent fasting, and the idea appealed immediately, even to a cynical soul who has spent two decades examining the curious acrobatics of the fashion industry, the beauty business, and the diet trade.

I’d dabbled in diets before—show me a fortysomething woman who hasn’t—losing weight, then losing faith within weeks and piling it all back on. Though never overweight, I’d long been interested in dropping that reluctant seven to ten pounds—the pounds I picked up in pregnancy and somehow never lost. The diets I tried were always too hard to follow, too complicated to implement, too boring, too tough, too single-strand, too invasive, sucking the juice out of life and leaving you with the scraps. There was nothing I found that I could adopt and thread into the context of my life—as a mother, a working woman, a wife.

I’ve argued for years that dieting is a fool’s game, doomed to fail because of the restrictions and deprivations imposed on an otherwise happy life, but this felt immediately different. The scientific evidence was extensive and compelling, and (crucially for me) the medical community was positive. The effects, for Michael and others, were impressive, startling even. In his Horizon documentary, Michael called it the “beginning of something huge . . . which could radically transform the nation’s health.” I couldn’t resist. Nor could I conceive of a reason to wait.

The scientific evidence was extensive and compelling, and (crucially for me) the medical community was positive.

In the two and a half years since I wrote the Times feature, I have remained a convert. An evangelist, actually. I’m still “on” the FastDiet now, following a 6:1 pattern, but I barely notice it. At the outset, I weighed 132 pounds. At five feet, seven inches, my BMI was an okay 21.4. Today, as I write, I weigh 119 pounds, with a BMI of 19.4. That’s a weight off. I feel light, lean, and alive. Fasting has become part of my weekly life, something I do automatically without stressing about it.

I feel light, lean, and alive.

These days, I have more energy, more bounce, clearer skin, a greater zest for life. And—it has to be said—new jeans (27-inch waist) and none of my annual bikini dread as summer approaches. But perhaps more important, I know that there’s a long-term gain. I’m doing the best for my body and my brain. It’s an intimate revelation, but one worth sharing.
The FastDiet: The Potential, the Promise
We know that for many people, the standard diet advice simply does not work. The FastDiet is a radical alternative. It has the potential to change the way we think about eating and weight loss.

• The FastDiet demands that we think about not just what we eat, but when we eat it.

• There are no complicated rules to follow; the strategy is flexible, comprehensible, and user-friendly.

• There is no daily slog of calorie control—none of the boredom, frustration, or serial deprivation that characterizes conventional diet plans.

• Yes, it involves fasting, but not as you know it; you won’t “starve” on any given day.

• You can still enjoy the foods you love—most of the time.

• Once the weight is off, sticking to the basic program will mean that it stays off.

• Weight loss is only one benefit of the FastDiet. The real dividend is the potential long-term health gains—cutting your risk of a range of diseases, including diabetes, heart disease, and cancer.

• You will soon come to understand that it is not a diet. It is much more than that: it is a sustainable strategy for a healthy, long life.

Now you’ll want to understand exactly how we can make these dramatic assertions. In the next chapter, Michael explains the science that makes the FastDiet tick.