What's the diet craze this week? You name it, there's a book selling it—and people buying it, hoping for a "magic bullet" to help them shed excess pounds. But how can everybody be right? More to the point, is anybody right? What are we supposed to eat? How can we lose weight, keep it off—and not feel hungry all the time? What's the best diet for our health and well-being?

For more than twenty years, as an avid researcher of health, nutrition, and fitness, I have been working to answer these questions. I started this quest because I wanted to get past all the hype, confusion, and political posturing swirling around dietary opinion. I was looking for facts; the simple, unadulterated truth. The answer, I found, was hidden back in time—way back, with ancient humans who survived by hunting wild animals and fish and gathering wild fruits and vegetables. These people were known as "hunter-gatherers," and my research team and I recently published our analysis of what many of them (more than 200 separate societies) ate in The American Journal of Clinical Nutrition. We were astonished at the diversity of their diet. We were also amazed at what they did not eat—which we'll get to in a minute and which may surprise you.

Health Secrets of Our Ancestors

What do Paleolithic people have to do with us? Actually, quite a lot: DNA evidence shows that genetically, humans have hardly changed at all (to be specific, the human genome has changed less than 0.02 percent) in 40,000 years. This means that the genetic makeup ofPaleolithic people is virtually identical to our own. Literally, we are Stone Agers living in the Space Age; our dietary needs are the same as theirs. Our genes are well adapted to a world in which all the food eaten daily had to be hunted, fished, or gathered from the natural environment—a world that no longer exists. Nature determined what our bodies needed thousands of years before civilization developed, before people started farming and raising domesticated livestock.

In other words, built into our genes is a blueprint for optimal nutrition—a plan that spells out the foods that make us healthy, lean, and fit. Whether you believe the architect of that blueprint is God, or God acting through evolution by natural selection, or by evolution alone, the end result is still the same: We need to give our bodies the foods we were originally designed to eat.

Your car is designed to run on gasoline. When you put diesel fuel into its tank, the results are disastrous for the engine. The same principle is true for us: We are designed to run best on the wild plant and animal foods that all humans gathered and hunted just 500 generations ago. The staples of today's diet—cereals, dairy products, refined sugars, fatty meats, and salted, processed foods—are like diesel fuel to our body's metabolic machinery. These foods clog our engines, make us fat, and cause disease and ill health.

Sadly, with all of our progress, we have strayed from the path designed for us by nature. For instance:

• Paleolithic people ate no dairy food. Imagine how difficult it would be to milk a wild animal, even if you could somehow manage to catch one.
• Paleolithic people hardly ever ate cereal grains. This sounds shocking to us today, but for most ancient people, grains were considered starvation food at best.
• Paleolithic people didn't salt their food.
• The only refined sugar Paleolithic people ate was honey, when they were lucky enough to find it.
• Wild, lean animal foods dominated Paleolithic diets, so their protein intake was quite high by modern standards, while their carbohydrate consumption was much lower.
• Virtually all of the carbohydrates Paleolithic people ate came from nonstarchy, wild fruits and vegetables. Consequently, their carbohydrate intake was much lower and their fiber intake much higher than those obtained by eating the typical modern diet.
• The major fats in the Paleolithic diets were healthful, monounsaturated, polyunsaturated, and omega 3 fats—not the saturated fats that dominate modern diets.

With this book, we are returning to the diet we were genetically programmed to eat. The Paleo Diet is more than a blast from the past. It's the key to speedy weight loss, effective weight control, and, above all, lifelong health. The Paleo Diet enlists the body's own mechanisms, evolved over millions of years, to put the brakes on weight gain and the development of the chronic diseases of civilization. It is the closest approximation we can make, given the current scientific knowledge, to humanity's original, universal diet—the easy-to-follow, cravings-checking, satisfying program that nature itself has devised.

The Problems with Most Low-Carb Diets

The Paleo Diet is a low-carbohydrate diet—but that's where any resemblance to the current glut of low-carbohydrate fad diets ends. Remember, the Paleo Diet is the only diet based on millions of years of nutritional facts—the one ideally suited to our biological needs and makeup and the one that most closely resembles hunter-gatherer diets. How does the Paleo Diet compare with the low-carb fad diets and the average U.S. diet?

Modern low-carbohydrate weight-loss diets are really high-fat diets that contain moderate levels of protein. They don't have the high levels of protein that our ancestors ate—the levels found in the Paleo Diet. Actually, compared with what our ancestors ate, the carbohydrate content of these modern weight-loss diets is far too low. Even worse, almost all of these low-carbohydrate diets permit unlimited consumption of fatty, salty meats (such as bacon, sausage, fatty ribs, and lamb chops) and dairy products (cheeses, cream, and butter) while restricting the consumption of fruits and vegetables. Cancer-fighting fruits and vegetables! This dietary pattern is drastically different from that of our ancestors.

And although low-carbohydrate diets may be successful in promoting weight loss, many dieters are achieving short-term weight loss at the expense of long-term health and well-being. Here's what the sellers of these diet plans don't want you to know: When low-carbohydrate diets cause weight loss in the short term, it's because they deplete the body's reserves of muscle and liver glycogen (carbohydrate), and the weight you're losing rapidly is mostly water weight.

When low-carbohydrate diets cause weight loss in the long run (weeks or months), it's because more calories are being burned than consumed, plain and simple. Low-carbohydrate diets tend to normalize insulin metabolism in many people, particularly in those who are seriously overweight. This normalization prevents swings in blood sugar that, in turn, may cause some people to eat less and lose weight. It is the cutback in total calories that lowers total cholesterol and low-density lipoprotein (LDL) cholesterol (the bad cholesterol) levels. Also, reductions in dietary carbohydrates (whether calories are cut or not) almost always cause a decline in blood triglycerides and an increase in blood high-density lipoprotein (HDL) cholesterol (the good cholesterol).

So, if low-carbohydrate diets cause someone to consume fewer calories, they may help produce weight loss and improvements in blood chemistry, at least over the short haul. However, dieters beware: When low-carbohydrate, high-fat diets are followed without a decrease in the daily consumption of calories, they are, according to the American Dietetic Association, "a nightmare." Let's see why.

Low Carb Doesn't Mean Low Cholesterol

Despite what anybody tells you—despite the outrageous claims of the low-carbohydrate, high-fat diet doctors—if you eat a lot of the saturated fats found in cheeses, butter, and bacon and don't cut your overall calorie intake, your cholesterol will go up. The medical community has known this for more than fifty years. It's been demonstrated in hundreds of clinical trials, including metabolic ward studies, in which people are locked into a hospital wing and only allowed to eat foods that have been carefully weighed and analyzed. Many of the low-carbohydrate diet doctors claim that these clinical trials are invalid because none of them reduced the carbohydrate content sufficiently. These doctors should know better; low carbohydrates don't guarantee low cholesterol.

Dr. Stephen Phinney and colleagues from the Massachusetts Institute of Technology conducted a normal caloric intake metabolic ward trial involving nine healthy, lean men. These men consumed nothing but meat, fish, eggs, cheese, and cream for thirty-five days. They had a low carbohydrate intake—less than 20 grams a day—but it didn't matter. Their blood cholesterol levels still went up, from 159 to 208 on average in just thirty-five days. This study and others prove beyond a doubt that diets high in saturated fats—no matter how low the carbohydrate content—will raise blood cholesterol levels when caloric intake levels are normal. What does this mean for the people on these diets? Possibly, serious health risks. Eventually, even the most obese subjects stop losing weight on low-carbohydrate diets. Eventually, they must return to a normal caloric intake (otherwise, they would die of starvation)—and when they do, watch out. Their high-saturated-fat diets will raise their blood cholesterol levels and increase their risk of heart disease.

So, at best, low-carbohydrate, high-fat diets are a temporary fix. At worse, they can cause big trouble in the long run.

Healthy Fats, Not Lethal Fats

One major difference between the Paleo Diet and the low-carbohydrate, high-fat diets we just talked about is the fats. In most modern low-carbohydrate weight-loss diets, no distinction is made between good fats and bad fats. All fats are generally lumped together; the goal is simply to reduce carbohydrates and not worry about fats.

But you should worry about fats. Not all fats are created equal, and the impact of fat on blood cholesterol—and the odds of developing heart disease—is enormous. The problem is, fats are confusing for many people trying to make good dietary decisions. For one thing, many of them sound alike. How are saturated fats different from monounsaturated—or even polyunsaturated—fats? How are omega 6 fats different from the omega 3 variety?

• Monounsaturated fats are good. They're found in olive oil, nuts, and avocados, are known to lower blood cholesterol, and help prevent artery clogging or atherosclerosis.
• Saturated fats are mostly bad. They're found in meats and whole dairy products; most of them are known to raise cholesterol.
• Polyunsaturated fats are a mixed bag—some are more beneficial than others. For example, omega 3 polyunsaturated fats (the kind found in fish oils) are healthy fats, which can improve blood chemistry and reduce your risk of many chronic diseases. But omega 6 polyunsaturated fats (found in vegetable oils, many baked goods, and snack foods) are not good when you eat too much of them at the expense of omega 3 fats.

People in the Paleolithic age ate a lot of monounsaturated fats; they had saturated and polyunsaturated fats in moderation—but when they did have polyunsaturated fats, they had a proper balance of the omega 3 and omega 6 fats. They consumed far fewer omega 6 polyunsaturated fats than we do today.

How important are fats in the diet? Here's a modern example: People who live in Mediterranean countries, who consume lots of olive oil, are much less likely to die of heart disease than Americans or northern Europeans, who don't consume as much olive oil. Instead, our Western diet is burdened by high levels of saturated and omega 6 fats and sadly lacking in heart-healthy, artery-protecting omega 3 fats.

Our studies of hunter-gatherers suggest that they had very low blood cholesterol and relatively little heart disease. Our research team believes that dietary fats were a major reason for their freedom from heart disease.

Disease-Fighting Fruits and Vegetables

A major problem with low-carbohydrate weight-loss diets is what they do to health-promoting fruits and vegetables—they nearly eliminate them. Because of a technicality—a blanket restriction of all types of carbohydrates, even beneficial ones, to between 30 and 100 grams per day—fruits and veggies are largely off-limits. This is a mistake. Fruits and vegetables—with their antioxidants, phytochemicals, and fiber—are some of our most powerful allies in the war against heart disease, cancer, and osteoporosis. Yet just one papaya (59 grams of carbohydrate) would blow the daily limit for two of the most popular low-carbohydrate diets. Eating an orange, an apple, and a cup of broccoli and carrots (73 grams of carbohydrate)—just a drop in the bucket to hunter-gatherers, whose diets were rich in fruits and vegetables—would wreck all but the most liberal low-carbohydrate diets.

Humanity's original carbohydrate sources—the foods we survived on for millions of years—didn't come from starchy grains and potatoes, which have high glycemic indices that can rapidly cause blood sugar to spike. Instead, they came from wild fruits and vegetables with low glycemic indices that produced minimal, gradual rises in blood sugar. These are the carbohydrates that you'll be eating on the Paleo Diet. These nonstarchy carbohydrates normalize your blood glucose and insulin levels, promote weight loss, and make you feel energized all day long.

The Osteoporosis Connection

One of the greatest—and least recognized—benefits of fruits and vegetables is their ability to slow or prevent the loss of bone density, called "osteoporosis," that so often comes with aging. Recently, Dr. Katherine Tucker and colleagues at Tufts University examined the bone mineral status of a large group of elderly men and women. These scientists found that the people who ate the most fruits and vegetables had the greatest bone mineral densities and the strongest bones.

But what about calcium? Surely eating a lot of cheese can help prevent osteoporosis? The answer is a bit more complicated. One of the great ironies of the low-carbohydrate, high-fat diets is that even though they allow unlimited consumption of high-calcium cheeses, they almost certainly will be found to promote bone loss and osteoporosis in the long run. How can this be? Because getting a lot of dietary calcium from cheese, by itself, isn't enough to offset the lack of fruits and vegetables.

Nutrition scientists use the term "calcium balance" to describe this process. It's the difference between how much calcium you take in and how much you excrete. Most of us have gotten the message about consuming calcium. But the other part of the equation—how much calcium you excrete—is just as important. It is quite possible for you to be in calcium balance on a low calcium intake if your calcium excretion is also low. On the other hand, it's easy for you to fall out of calcium balance—even if you load up on cheese at every meal—if you lose more calcium than you take in.

The main factor that determines calcium loss is yet another kind of balance—the acid-base balance. If your diet has high levels of acid, you'll lose more calcium in your urine; if you eat more alkaline foods, you'll retain more calcium. A study in the New England Journal of Medicine by my colleague Dr. Anthony Sebastian and his research group at the University of California at San Francisco showed that simply taking potassium bicarbonate (an alkaline base) neutralized the body's internal acid production, reduced urinary calcium losses, and increased the rate of bone formation. In a follow-up report in the New England Journal of Medicine, Dr. Lawrence Appel at Johns Hopkins University reported that diets rich in fruits and vegetables (these are alkaline foods) significantly reduced urinary calcium loss in 459 men and women.

See Appendix A for a list of common foods and their acid-base values.

Cereals, dairy products, legumes, meat, fish, and eggs produce net acid loads in the body. By far the worst offenders on this list are the hard cheeses, which are rich sources of calcium. Again, unless you get enough fruits and vegetables, eating these acid-rich foods will actually promote bone loss and osteoporosis.

Virtually all fruits and vegetables produce alkaline loads in the body. When you adopt the Paleo Diet, you won't have to worry about excessive dietary acid causing bone loss—because you'll be getting 35 percent or more of your daily calories as healthful alkaline fruits and vegetables that will neutralize the dietary acid you get when you eat meat and seafood.

Toxic Salt

Most low-carbohydrate, high-fat diets don't address the dangers of salt; some even encourage its use. And yet there is a ton of medical evidence linking salt to high blood pressure, stroke, osteoporosis, kidney stones, asthma, and even certain forms of cancer. Salt is also implicated as a factor in insomnia, air and motion sickness, Ménière's syndrome (an agonizing ear ringing), and the pre-eclampsia of pregnancy.

Salt is made up of sodium and chloride. Although most people think that the sodium portion of salt is entirely responsible for most of its unhealthful effects, chloride is just as guilty, if not more so. The average American eats about 10 grams of salt a day (this turns out to be about 4 grams of sodium and 6 grams of chloride). Chloride, like cereals, dairy products, legumes, and meats, yields a net acid load to the kidneys after it is digested. Because of its high chloride content, salt is one of the worst offenders in making your diet more acid.

Paleolithic people hardly ever used salt and never ate anything like today's salty cheeses, processed meats, and canned fish advocated by most of the low-carbohydrate weight-loss diets. Do your body a favor and throw out your salt shaker along with all the highly salted, processed, packaged, and canned foods in your pantry.

Lean Meat Helps You Lose Weight

It's taken half a century, but scientists have finally realized that when they stigmatized red meat, they threw out the proverbial baby with the bath water. Meat is a mixture of fat and protein. Lean meat—such as that found in wild game and seafood—is about 80 percent protein and about 20 percent fat. But fatty meats like lamb chops can pack a whopping 75 percent of their calories as fat and only 25 percent as protein. What should have been obvious—that it was the high level of saturated fat, not the protein, that caused health problems—was essentially ignored. Meat protein had unfairly become a villain.

Here again, there's a major lesson to be learned from looking at the distant past: For more than 2 million years, our ancestors ate a diet rich in lean protein. It gave them energy and, combined with fruits and vegetables, helped them stay healthy.

Protein Increases Your Metabolism and Slows Your Appetite

When scientists actually studied how lean protein influences health, well-being, and body weight regulation—and this has occurred only in the last decade—they found that our ancestors were right all along. It turns out that lean protein is perhaps our most powerful ally in the battle of the bulge. It has twice the "thermic effect" of either fats or carbohydrates, which means it revs up your metabolism. In other words: Protein's thermic effect increases our metabolism and causes us to burn more calories than if we ate an equal caloric serving of either fat or carbohydrate. Also, more than fats, more than carbohydrates, protein has the highest "satiating value"—that is, it does the best job of making us feel full.

The principles I have laid out in the Paleo Diet—all based on decades of scientific research and proven over millions of years by our ancestors—will make your metabolism soar, your appetite shrink, and extra pounds begin to melt away as you include more and more lean protein in your meals.

Lean Protein and Heart Disease

But this diet gives you much more than a slimmer figure. Unlike other low-carbohydrate diets, it's good for your heart. High-protein diets have been shown by Dr. Bernard Wolfe at the University of Western Ontario in Canada to be more effective than low-fat, high-carbohydrate diets in lowering total and bad LDL cholesterol and triglycerides while simultaneously increasing the good HDL cholesterol. My colleague Neil Mann at the Royal Melbourne Institute of Technology in Melbourne, Australia, has recently demonstrated that people who eat a lot of lean meat have lower blood levels of homocysteine (a toxic substance in the blood that damages the arteries and predisposes them to atherosclerosis) than vegan vegetarians. The net result is that high-protein diets produce beneficial changes in your blood chemistry that, in turn, reduce your overall risk of heart disease.

High-protein diets have been shown to improve insulin metabolism, help lower blood pressure, and reduce the risk of stroke. They have even prolonged survival in women with breast cancer.

Some people have been told that high-protein diets damage the kidneys. They don't. Scientists at the Royal Veterinary and Agricultural University in Copenhagen effectively put this myth to rest. Dr. Arne Astrup and colleagues put sixty-five overweight people on a high-protein diet for six months and found that their kidneys easily adapted to increased protein levels. Furthermore, kidney function remained perfect at the end of the experiment.

Isn't it time you got protein on your side? Eating lean meat and fish at every meal, just as your Paleolithic ancestors did, could be the healthiest decision you ever made.

Compared to the faddish low-carbohydrate weight-loss diets, the Paleo Diet includes all the nutritional elements needed to encourage weight loss while promoting health and well-being. The Paleo Diet is designed to imitate the healthful diets of our pre-agricultural ancestors. It contains the proper balance of plant and animal foods—and the correct ratios of protein, fat, and carbohydrate required for weight loss and excellent health.

So, don't be fooled by the low-carbohydrate fad diets. The Paleo Diet gives you the same weight-loss benefits, but it's also a delicious, healthy diet you can maintain for a lifetime.