The Personalized Diet

The Pioneering Program to Lose Weight and Prevent Disease


By Eran Segal, PhD

By Eran Elinav, MD, PhD

With Eve Adamson

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A paradigm-shifting diet book that explains why one-size-fits-all diets don’t work and helps readers customize their diet to lose weight and improve health.

There are certain things we take as universal truths when it comes to dieting and health: kale is good; ice cream is bad. Until now. When Drs. Segal and Elinav published their groundbreaking research on personalized nutrition, it created a media frenzy. They had proved that individuals react differently to the same foods-a food that might be healthy for one person is unhealthy for another. In one stroke, they made all universal diet programs obsolete. The Personalized Diet helps readers understand the fascinating science behind their work, gives them the tools to create an individualized diet and lifestyle plan (based on their reactions to favorite foods) and puts them on the path to losing weight, feeling good, and preventing disease by eating in the way that’s right for them.



Welcome to the Future of Dieting

Imagine that there was no single food that was bad for everyone or good for everyone—not chocolate, not kale, not cookies, not a big salad, not a banana, not coffee. Imagine something you love to eat—something you think is a terrible dietary choice (but that constantly tempts you, like a juicy, fat steak or a bowl of mint-chip ice cream)—is actually okay to eat and won’t have a negative impact on your health. What if a food you hate—something you force down because you think it is good for you and will help you lose weight or avoid health problems, something like rice cakes or steamed fish—is exactly the wrong thing for you? What if we told you that carb-loading with pasta before endurance sports might be bad for you and slow you down, that diet soda might be directly contributing to your weight gain, or that sushi might be making your blood sugar spike in a way that could increase your risk of diabetes?

Imagine no longer having to suffer through painful diets that restrict too many foods. Imagine never having to go through another cleanse, another “induction phase,” another fast, another starvation diet. Imagine eating carbs again, eating fat again, or eating meat again, if that is what you’ve been longing for. And imagine not having to pay attention to the never-ending stream of confusing and contradictory dietary information telling you the foods to eat, or not eat, in order to lose weight or fight chronic disease. Imagine that science has finally begun to scratch the surface of the complex question about the optimal diet and that you no longer have to wonder what is right for you to eat, because you finally understand that there is no single correct diet philosophy that will work for all people. What if each person requires a different diet tailored to his or her own body composition? And that science is only beginning to discover a methodology so an individual can determine exactly what his or her diet should be? What if you finally understood how and why optimal nutrition must (and can) be personalized?

What if you could use that information for the benefit of your own health and weight-loss efforts, right now?

We are Dr. Eran Segal and Dr. Eran Elinav, researchers and colleagues at the Weizmann Institute of Science, an internationally renowned, multidisciplinary research institution dedicated to advancing science for humanity’s benefit. We have been collaborating on an ambitious and far-ranging research effort called the Personalized Nutrition Project that we believe has the potential to shift the very foundations of nutrition science.

In The Personalized Diet, we will explain how we arrived at our conclusions; give you the genuine, hard science behind the surprising claims we are now able to make; and show you how you can get a jump on those changes now, in your own life and for the sake of your own health, by applying our personalized nutrition approach to the way you eat and the lifestyle choices you make. The insights we gained in our studies, based on new, large-scale data that we collected, may be life-transforming, as it may have you looking at your dietary choices in a completely new way. It’s very likely that many of the foods you love, that you think you shouldn’t eat, aren’t harmful for you at all. It’s possible that many of the foods you thought were healthy aren’t good for you—not the general “you,” but you personally. How can you know for sure? This is the future of dieting. What we discovered in our groundbreaking and internationally publicized research has the potential to change your health, your weight, your energy level, and your sleep quality—indeed, your life.

Most people want to lose weight, get healthier, feel better, and generally get control of their appetites and lower their risk for chronic disease. That’s why scientists and research institutions have spent countless hours and billions of dollars researching and publishing studies to answer one simple question: What is the best diet for humans?

Maybe you think you know. Maybe you are already in the low-carb camp, the vegan camp, or the Mediterranean diet camp, or you have worked with a dietitian and that person has told you what to eat. In any case, perhaps you are sure science knows. After all, the question sounds straightforward and direct enough. With all the scientific advancements that have been made over the centuries, surely we know the answer to this seemingly small question by now.

The reality is that although there are many convincing books, articles, and websites written by people claiming to know the truth, many of them citing tens and sometimes hundreds of scientific studies to prove their theories, there isn’t one definitive answer. Some of those who support one diet over another are doctors, or dietitians, or nutritionists, or exercise trainers, and some of them are people who have successfully lost a lot of weight and want to share how they did it. Each claims to know what really works, the absolute truth. It’s no wonder that so many people flock to this kind of information, even constantly changing their opinions and strategies based on the latest thing they’ve read. When one diet or philosophy doesn’t work, they jump to another, and then another, and then another, thinking they are being discerning because they are listening to experts.

The problem is that these books, articles, and websites all seem to champion completely different and often directly contradictory information. Even well-constructed research on any one nutritional principle or strategy can almost always easily be refuted by different research on a different nutritional principle or strategy. There are numerous studies that support or oppose every single dietary intervention available.

So, what is the real answer to the question about the best diet? Maybe science would have uncovered an irrefutable answer by now, were it not for one increasingly unavoidable reality that science is only beginning to uncover: There is no answer to the question of the perfect diet because it is the wrong question.

But before we get to the right question—the truly important question, and the question that actually has an answer that may transform your life—we would like to introduce ourselves.


Before I ever conceived of the notion of personalized nutrition, I was a scientist and a marathon runner married to a clinical dietitian. Because of my wife’s profession, I was fairly certain I already knew how to eat healthfully, and I thought I made good decisions about my meals. But a few years ago, I became interested in ways I could improve my athletic performance, and in my free time, I took to researching sports physiology. This led me to start thinking about how diet might improve my performance. I wondered if adjusting what I ate could give me more energy to sustain my long runs or make me faster. If I could find good evidence for any dietary changes that might increase my speed and endurance, I was willing to try them.

Being a scientist, I am not that interested in popular literature about diet and fitness fads, so instead I turned to books with a more scientific slant, with solid research backing up their claims. I wanted to know what real, hard science had to say about the question of diet for athletic performance—specifically, my own. I respect science, and therefore I trusted science to tell me the truth. I approached this new personal project with energy and expectation, hoping to find something interesting and useful for my life.

However, the more I researched the question of how diet might help or hinder athletic performance, the more I realized that the dietary advice that was widely available for athletes (and everyone else) was often contradictory. Some of it even sounded suspiciously inaccurate. As I investigated further, I discovered, to my surprise, that the science upon which this advice was supposedly based was sometimes not up to standard, involved very small studies with only a handful of subjects, had been misinterpreted by writers and journalists, or was outdated. What at first looked like good solid science in many cases turned out to be, when more carefully examined, not very scientific at all. Most shocking to me was the discovery that the dietary advice I had always practiced (almost religiously, because I was confident that it was based in science) had no real scientific underpinnings. How could this be? How could I have missed this? How could professional curriculums about nutrition, government guidelines for diets, and nutritional advice from exercise science be based on what seemed increasingly to me to look like nothing? I had taken for granted that mainstream dietary advice was true—that is, based in proven scientific principles. The more I read, the more I realized it was not.

Many of the contradictions, misinterpretations, and especially what I perceived to be missing science had to do with dietary carbohydrates. These are the sugars, starches, and fiber in food that the body breaks down, to varying degrees, into glucose to feed the cells. Athletes think about carbs a lot. Many of us “carb-load” the night before a big athletic event like a marathon and don’t worry much about eating carbs because we have been taught that carbs equal energy. Dieters often focus on carbs as well, either emphasizing them as a replacement for fat (such as with many vegetarian or low-fat diets) or eliminating them because of the belief that they are responsible for weight gain and health issues (such as with the many iterations of low-carb diets). The more I investigated, the more I saw that there was plenty of evidence both for and against carbohydrates, as well as many different approaches to carbohydrates, including some that considered them all the same and others that considered some “good” and some “bad.” What was a scientist to make of all this seemingly well-researched and scientifically supported but conflicting information?

But I was still primarily interested, for personal reasons, in the exercise aspect of carbohydrates, so I decided to focus on that. For example, I read a study (this was long ago and I can’t recall the source) in which people ate dates, which contain fast-digesting (or “simple”) carbs, 30 to 60 minutes before running or doing intense exercise. The effect of eating these dates at first seemed inconclusive—some people eating the dates were energized and had better workouts, but others felt exhausted to the point that, a few minutes into their runs, they had no energy and had to stop. I remember stopping to think about this. Why would people respond so differently to the same food when doing the same activity at approximately the same intensity? I wondered whether this might be related to differences in how people’s blood sugar levels responded to dates, because blood sugar crashes are associated with low energy. If eating dates gave one person a moderate rise in blood sugar, then that could indeed provide energy during strenuous activity. But if another person had a huge spike in blood sugar and then an imminent blood sugar drop, this could result in exhaustion. I thought about this in my own life. Sometimes I felt energized from carbohydrates, and other times I felt the opposite. Maybe you have noticed something similar in your own experience—do certain carbohydrate-rich foods give you energy while others seem to sap your strength? The more I thought about it, the more I realized that some of the foods that seemed to give me the most energy were not always carbohydrate-heavy. Sometimes they were foods higher in protein and/or fat. Interesting.

I decided it was time for an experiment, with myself as test subject. The first thing I tried was changing what I ate prior to my long runs (approximately twenty miles). I wanted to see what would happen if, instead of carb-loading, I ate protein and fat. The reason I tried this particular experiment was because I had heard more and more “low-carb athletes” claiming they could burn fat instead of carbs for energy and that it was even more efficient. It sounded strange, but I was curious enough to try it. I wanted to know how it might affect my physical hunger and my motivation, as well as my performance. I was a little hesitant to do this because I had always carb-loaded before exercise, eating three or four large bowls of pasta the night before a run and having a few dates or energy bars the next morning about 30 to 60 minutes before a run. I almost always felt extremely hungry about 15 to 30 minutes after my run, but I figured that was just because I had burned up all those useful carbs, and I was ready for more. After a run, I would always eat even more carb-rich foods, thinking I was responding to my body’s needs. I had always believed that this was necessary to give me enough energy to run that distance, but what if I (and all those other athletes and coaches and fitness professionals I knew) was wrong?

So, one evening, instead of carb-loading, I ate a big salad with lots of fat sources like tahini, avocado, and nuts. In the morning, I set out for my twenty-mile run without eating anything (against the advice of many professional running coaches).

I was surprised at what a positive effect this diet had on both my energy level and my performance! During my run, I had just as much energy, if not more, than I had with the carb-loading. Moreover, my ravenous postrun hunger completely disappeared. After my run, I couldn’t believe that I wasn’t hungry. I surmised that my body must have switched to burning fat rather than carbohydrates, and this must have been responsible for these significant changes in my energy level and hunger.

I then considered what I knew about how the human body works. When we eat carbs, we store some of that energy in our livers in the form of glycogen, for use during strenuous activity. However, we can store only 2,500 to 3,000 kilocalories’ (what we typically call calories) worth of glycogen. Over the course of a twenty-mile run, it is easy to burn 2,500 calories or more, so if your source of fuel is glycogen, you can see how those stores could be depleted quickly. This could certainly trigger fatigue and postrun hunger.

Even lean people have about 60,000 Kcals (calories) of fat available for energy. This is a much bigger storehouse of energy, so it makes sense that burning fat rather than carbs is more efficient for long-term exertion. If we deplete 2,500 Kcals of fat, we consume only a small percentage of the available fat energy stores, and the need for replenishment will feel (and be) less urgent.

It all made sense to me. Switching my body from burning glycogen to burning fat on a run might finally be the answer I had been seeking. As an endurance athlete, I felt like I had hit on a eureka moment. I continued eating low carb in my daily life and noticed that I had more energy, even when I wasn’t exercising. This was an unexpected benefit. I also lost some of the excess weight I was carrying, and best of all, my athletic performance steadily improved until I met my goal of running a marathon in under 3 hours: In 2013, I finished the Paris Marathon in 2:58! Then, in 2017, I broke that 3-hour mark again running a marathon in Vienna.

As I continued with my life and athletic pursuits, I couldn’t help noticing that there were some successful athletes I encountered—as well as friends and colleagues—who were not eating the way I was eating. Despite my low-carb evangelizing, some of them swore by their carb-rich diets and seemed to do just fine… even fantastically, including some vegans performing at a very high athletic level after carb-loading. Maybe my eureka moment wasn’t universal. Maybe it was personal. Maybe not everyone would react to this kind of dietary adjustment the way I did. Perhaps I had found the optimal Eran Segal Diet, but maybe I still had not discovered the optimal universal diet. Based on my observations so far, I couldn’t be sure.

I began to think more seriously about dietary carbohydrates. Were they, as I had previously believed, the primary and most desirable source of energy for the athlete—the across-the-board best source of fuel for the body and brain—or was a diet based on carbohydrates (even the complex type I had always thought were so valuable, such as oatmeal, pasta, and whole-grain breads) inhibiting my athletic performance, energy level, muscle growth, and brain function?

I was still in the mind-set that a diet based on complex carbohydrates as a primary energy source was good for the human body, neutral, or bad. But I kept coming back to all that contradictory research. Carbohydrates could not possibly be both good and bad.

Or could they?

That’s when I thought: Why do some people seem to thrive on diets that are high in carbohydrates, while others quickly gain excess weight or suffer from low energy? Why were some of those people who ate the dates energized and some were exhausted? I knew people who, for example, were vegetarian and ate only fruit, vegetables, and plant foods such as legumes and whole-grain rice. They lived primarily on foods rich in carbohydrates, with relatively low levels of protein and fat. Some of them seemed to thrive, some claimed to have reversed their heart disease, and some had significant muscle and strength. Others didn’t seem very healthy and were always tired and pale.

On the other hand, I also knew some “low-carb” people who didn’t eat any grain-based foods or legumes and consumed hardly any fruit. They lived on green vegetables, meat, nuts, and seeds, and added fat, such as olive oil, coconut oil, and even lard. Many of them were exceptionally vigorous athletes with excellent endurance, and many were quite lean. Others gained excess body fat and suffered from dangerously high cholesterol.

How could this be? Either some of these people were lying about what they were eating—the cheating vegan secretly eating meat on the sly, or the cheating Paleo aficionado sneaking cookies and toast under dark of night—or some of these people were simply not responding positively, personally, to the dietary philosophy they had adopted. I didn’t think the people I knew were lying about what they ate. Many of them were smart people with dietary knowledge, and they were likely to be choosing good, high-quality, high-nutrient sources of carbs, protein, and/or fat.

What else could be going on?

Perhaps, as I was beginning to suspect, it wasn’t just about the food. Perhaps, it was also about the person eating the food. This led me to a completely new line of thinking, as I wondered:

What are the effects of different foods on different people?

Now this was an interesting and far more complex question than what I had first considered as I sought out the best foods for my exercise performance. As I began to apply myself to this new question, I considered how many factors could influence how any one person reacted to food. For example:

• As a scientist, my research focused on studying the human genome—the genetic map of the human—so I already knew that genetic differences can affect the way some people respond to food. For instance, some people are missing the DNA pieces that produce particular enzymes to digest certain foods, like milk. Maybe there were many more genetically based conditions relating to food digestion that we didn’t yet understand. Is that what I was observing in the people who did or did not thrive on various diets?

• I had also been reading about the newly emerging field of science that studies the microbiome, the collection of thousands of different bacteria we all have inside our gastrointestinal system. I knew that new sequencing technologies have opened avenues of exploration into the influence of these microbes on digestion and metabolism (the way the body extracts energy from food). I wondered if different collections of gut microbes could also influence how someone might react to various types of diets, or even individual foods. That seemed a fascinating and promising area of further study as well.

• What about lifestyle? Could the level of physical activity influence the body’s reaction to food? What about sleep patterns, stress levels, mental engagement? Could preexisting disease processes, age, weight and height, or the diet someone ate as a child have an impact?

If the individual, rather than the food, was the wild card, then perhaps the question of how any one person will react to any one food was too complex to answer. So how was I ever going to figure out what to eat to be a better marathoner? The more I kept coming back to my original, personal reasons for investigating these questions, the more I could feel the scientist in me getting intrigued and engaged.

But the more I read, the more I realized that there was not enough data on the subject. I knew that a data-driven approach, without prejudice or bias, was the only way to answer my questions. If I really wanted to find out more, and nobody had the answer yet, I might just have to do it myself. I would need to find something that would measure an individual response to food that would include and encompass personal genetics, individual microbiomes, and clinical parameters like blood tests, weight, and age, and lifestyle factors like physical activity, sleep, and stress. It was a lot to consider. Would such an experiment even be possible?

Because I have a background in computer science, it made sense to approach this problem by using machine learning and algorithms—basically, in these fields, we take large amounts of data and try to get computers to identify patterns and rules in the data. The interesting thing about this is that when given large amounts of data, these algorithms can identify patterns that are impossible for people to find because we can’t take in and process that much information. A computer’s ability to see patterns and derive rules far better than what we can see is why computers are now better than people at games like chess and the Chinese game Go.

I’d never seen this data-driven approach applied to nutrition research, but I thought, Why not? Nutrition is a complex issue with many variables. What better way to sort it all out than with big data and a computer algorithm? I thought this might be just the way to plug the right data into the right places to learn for sure what foods would and would not increase athletic performance, as well as improve health and support weight control, for any given person. I had no idea what information such an approach might yield, but I was already eager to find out when I met Dr. Eran Elinav.


I came into the world of personalized nutrition from a completely different angle than my colleague Dr. Segal. As far back as I can remember, I have been intrigued by the complexity of machines. As a child, I once opened my grandfather’s transistor radio and took it apart without asking permission. I did the same with my parents’ record player, only to discover a multitude of wonderfully colorful and strangely shaped metallic components, interwoven with wires. I was amazed and delighted by the complexity created by human beings just like me. Of course, after dismantling many appliances, I was left with a handful of neglected parts following my attempts at reconstruction.

But no machine compared, in my estimation, to the mysterious human body. Even as a child, I thought of the body as the ultimate complex machine, containing seemingly endless hidden parts, concealed from my eyes yet easily perceptible—the beat of my heart; the wheezing sounds my lungs would produce when I had a cold; even the feelings, dreams, and sensations emerging from my brain and nervous system. The body was a machine I couldn’t take apart, of course (at least not until I got to medical school), but it occupied much of my thought and imagination. When I found my grandparents’ old encyclopedia of the human body, I was elated. I spent hours flipping through the pages, gazing at the many differently shaped and colored organs, tubes, and structures perfectly fitted together. Bodies were even more complex than I realized. I wondered if I would ever truly understand them.

It was no surprise to me or to anyone around me that biology became my passion and the focus of my studies. Following a four-year military service on a submarine (another fascinating machine), I joined the Hebrew University of Jerusalem School of Medicine and finally found a place where I could attain answers to the many years of questions about the functions and intricate secrets of the human body. I embraced my studies, voraciously consuming the thousands of anatomical details I was finally able to see directly in dissection classes, the endless cellular structures I discovered through the light microscope in my histology classes, and the multitude of strange-sounding medical terms revealed to me in my pathology classes. The human machine was gradually being revealed in front of my eyes.

Yet, I found that the more I learned, the less clear the big picture became. The more I zoomed in on the intricacies of the human body, the more the rules of its function became pixilated and blurry to me. The more answers I received, the more questions I had. I felt like I must be missing something. When you take apart a record player, at some point you understand it completely. Why was the human body still so elusive?

My favorite courses were in microbiology. My professors of microbiology and infectious disease revealed a world full of hidden enemies. You can’t see a virus or bacteria, but they can conquer a human, sometimes in a matter of days. A living world of tiny, strangely shaped and named invisible creatures—ordered into families and groups, including bacteria, viruses, fungi, and archaea (microbes with no cell nucleus)—was coming into focus before my eyes. This was next-level anatomy! And it was an exciting world—hostile, deadly, and obscure. My teachers were like the cavalry riding in to fight in this invisible war against our ultimate adversaries, teaching us medical students how to wield sophisticated antibiotic weaponry against our enemies, even as these foes developed resistance and emerged more potent and deadly than ever.

Next, I entered a phase of clinical practice, putting all those hours of studying, memorizing, and practicing into practical use. During these grueling years as an intern and resident in internal medicine, and as a gastroenterology fellow, I had a revelation: Even more complex than the secrets of the human body are the principles of its inner battle against dysfunction.

During this time, I was exposed to human suffering at its utmost severity. Especially troubling was a set of diseases collectively termed the metabolic syndrome. This included severe obesity, adult-onset diabetes, hyperlipidemia, fatty liver, and the many complications that come from all these conditions. I dealt with diabetes-associated limb amputations and blindness, kidney failure and the associated need for daily hemodialysis, heart attacks, heart failure, stroke, and sudden death. The vast majority of patients admitted to the internal medicine department where I worked suffered from this common syndrome, and the illnesses associated with it often caused severe debilitation and sometimes death. The need to deliver lifesaving cardiopulmonary resuscitation became almost a daily routine for me. This degree of suffering would have been unimaginable to me, had I not witnessed it. What was happening to people? Yet, I was surprised and disturbed that the treatments we offered these many patients, who were clearly in agony, focused on treating their many complications rather than doing anything to impact the course of their primary disease. My colleagues and I became increasingly frustrated by our inability to do anything about the vast epidemic itself and its horrible consequences. We were mopping up the mess after the fact rather than preventing the disasters before they could happen.


On Sale
Dec 26, 2017
Page Count
384 pages

Eran Segal, PhD

About the Author

Eran Segal, PhD, is a Professor at the Weizmann Institute of Science and the recipient of numerous honors and awards. He travels and lectures extensively worldwide and has published over 120 scientific articles in leading peer-reviewed journals.

Dr. Eran Elinav, MD, PhD, is a professor at the Weizmann Institute of Science and a board-certified physician. He has published over 120 papers in leading scientific and medical journals, and he is a popular international lecturer and the recipient of many prestigious awards.

Learn more about this author