Proteins are the third universal food group. The foods richest in proteins come from the animal kingdom. Their most abundant source is meat. Among animal meat, beef is especially high in protein. The leaner cuts are extremely low in fat, but just as rich in protein.

Mutton and lamb are visibly more marbled, and this fat reduces their protein content. Finally, some cuts of pork, which are even fattier, are not rich enough in protein to belong to the elite group of protein foods.

Organ meats like liver, kidneys, tongue, sweetbreads, chicken hearts, and tripe are very rich in protein and low in fat and carbohydrates. However, liver contains a small dose of sugar.

Fish, particularly lean white-fleshed fish like sole, skate, cod, sea bass, or tilapia are a gold mine of proteins with a very high nutritional value.

Shellfish and other types of seafood are lean and carbohydratefree, and rich in protein.

Eggs are an interesting source of protein. The yolk contains fats and enough cholesterol that should you be predisposed to high cholesterol, you should avoid excessive consumption of the yolk.

On the other hand, egg white is the purest and most complete known form of protein, which gives it the status of benchmark protein, as it is used to classify all other proteins.

Plant proteins are found in most cereals and legumes, but these are far too rich in carbohydrates to be included in a diet whose effectiveness depends on the purity of proteins. Furthermore, apart from soy, these plant proteins are desperately short of certain vital amino acids, so they cannot be used exclusively over a prolonged period of time.

So how can we be vegetarian? If it means not eating anything from an animal that has been raised and slaughtered for us to eat, but consuming eggs and dairy products, this is sufficient for people who are not trying to lose weight. If vegetarian means eating only vegetables, that diet becomes very hard to follow, as there is no other choice but to use incomplete vegetable proteins that have to be very cleverly teamed up with cereals and legumes to ensure that all amino acids are consumed, because without all amino acids, it is impossible for the body to produce vital proteins.

It is important to realize that humans emerged from their animal condition by becoming carnivorous. Our apelike ancestors, like today’s great anthropoid apes, were essentially vegetarians, even if, occasionally, certain apes hunted other animals for food. Indeed, it was by becoming group hunters and meat eaters that humans were able to acquire uniquely human faculties. The human body possesses a whole system for digestion and elimination that still allows us today to eat unlimited quantities of meat and fish.

We are designed to eat meat, fish, and poultry, as far as both our metabolism and psychology are concerned. Yes, it is possible to live without hunting and without eating meat, but by doing so we give up a part of what our nature expects, and we lessen the emotional effect our body is programmed to produce when we give it what it expects.

Of all the food categories, the digestion of proteins is the most time consuming. It takes over three hours to break down and assimilate proteins. The reason for this is simple: protein molecules are long chains with well-soldered links, and to break down their resistance requires the combination of good chewing and the simultaneous attack of various gastric, pancreatic, and biliary juices.

This long process of calorie extraction taxes the system; it has been calculated that to obtain 100 calories from a protein food, the system must use 30 calories. We can say that the specific dynamic action of proteins is 30 percent, while it is only 12 percent for fats and just 7 percent for carbohydrates.

What we should remember from this is that when someone wanting to lose weight consumes meat, fish, or nonfat yogurt, the person has to work hard to simply digest and assimilate the food, and the calories they use doing this reduces the energy absorbed from the meal. This really works in the favor of anyone wanting to lose weight. We will explore this process at greater length when we explain how the pure protein diet works. What is more, this slow rate of digestion and assimilation delays the process of emptying the stomach and increases our sense of "feeling full” and our sense of satisfaction.

Of the three universal food groups, only proteins are indispensable for our existence. Carbohydrates are the least necessary nutrient, because our bodies can produce glucose—that is, sugar—from meat or fat. When we are deprived of food or are dieting, we draw upon our fat reserves, transforming them into the glucose that is vital for our muscles and brain. The same goes for fats: an overweight person is expert in both making and storing them.

On the other hand, we do not have the metabolic means to synthesize proteins. Just being alive and ensuring that our muscular system is maintained, that our red blood cells are renewed, that wounds heal, that hair grows, and even that memory functions—all these vital operations require proteins, a minimum of 1 gram per day for every 2 pounds of body weight.

Whenever there is not enough protein, the body is forced to draw upon its reserves, mainly the muscles, but it also uses skin or even bones. Recently, some diets have led people to believe that our bodies can be detoxified by eating just fruit and vegetables for a few days.

When you realize it has been scientifically proved that after eight hours without good-quality proteins, the body has to draw upon its own muscle reserves to ensure its vital functions, you can understand just how inappropriate such ideas are.

Anyone wanting to lose weight should therefore realize that however restrictive the diet, it should never supply the body with less than 1 gram of protein per day for every 2 pounds of body weight, and, most important, protein intake should be evenly distributed over the day’s three meals. A meager breakfast, lunch consisting of a pastry and a bar of chocolate, then pizza for dinner with fruit for dessert are all meals that lack protein and will make your skin dull and impair your body’s general strength.

A gram of protein provides only 4 calories, the same as sugar but half of what fat provides.

Only 50 percent of all meat, fish, and other food proteins are assimilated; the rest is waste or useless tissue. This means that 4 ounces of turkey or steak provide only 200 calories. When you take into account that your body has to contribute 30 percent of this caloric value—that is, 60 calories—just to assimilate it, only 140 calories are left from this tasty and filling food, the equivalent of 1 tablespoon of the dressing you deem so harmless when you add it to your salad.

A high-protein diet has two drawbacks:

Protein is expensive. The cost of protein is relatively high— meat, fish, and seafood can easily make a dent in a modest budget. Eggs, poultry, and offal, like chicken livers, are more affordable but are still expensive. Fortunately, nonfat dairy products make it possible to get excellent quality protein at a price that allows us to offset the high cost of protein meals.

Protein produces waste products that must be eliminated. When protein is digested, waste products, such as uric acid, remain in the system and have to be eliminated. In theory, eating higher amounts of protein foods would increase the amount of waste products and cause discomfort to certain people. In fact, the human organs, and particularly the kidneys, have mechanisms for elimination and are perfectly suited to this task. But for the kidneys to work efficiently, it is absolutely necessary that they have an increased quantity of water. The kidneys will filter and remove uric acid from our blood, provided that we increase our normal consumption of water.