We can see why our science teacher feels his diet is preferable to eating meat. But how does he do it, and why is it possible to substitute other sources of protein for meat?
To understand the answer, we will have to take a look at exactly what protein is.
To a chemist, it’s easy to recognize protein. He or she would look under a microscope and would note molecules in the form of long chains. The molecules would all combine the following four elements: carbon, oxygen, hydrogen, and nitrogen. In addition, various particular proteins might also include zinc, sulfur, iron, or potassium. These molecule chains would be formed from shorter chains. These smaller chains are called amino acids. They are the basic building blocks of protein.
You and I cannot look under a microscope to check out the shape of molecules and the carbon, hydrogen, nitrogen, and oxygen content of foods; and amino acids have no particular flavor like sugar or salt. They come combined in food with other nutrients. We can’t tell just by tasting food whether or not it contains amino acids or which ones; we have to rely on what the food scientists tell us.
And what they say is that those building blocks, the amino acids, are the most important factors in determining the nutritive value of protein foods. Amino acids can be found in all protein-containing foods, in different combinations. A protein food’s value lies in the particular combination of amino acids it contains, not in the source from which it is derived. That is why it is possible to get all the protein we need from combining nonmeat sources containing different amino acids. Since proteins in foods are usually found in combination with other nutrients, chemists look at protein foods and divide them into three categories:
Simple proteins contain only amino acids.
Conjugated proteins include another substance such as fat or carbohydrate in addition to amino acids.
Derived proteins are intermediate forms created by the partial breakdown of other proteins.
Most of the foods we eat contain conjugated proteins. But nutritionists like to classify proteins according to which amino acids they contain since the amino acids are so important in the problem of getting adequate protein.
The protein we eat contains twenty-three different amino acids in many combinations. Our body needs all of them. But we only have to worry about eight, since the body can manufacture the other fifteen by itself.
The 8 Essential Amino Acids
The eight that concern us we call the essential amino acids because it is essential to our health that we get all of them in our food. In fact, we need each of them every day. When we talk about getting enough protein, we’re really talking about consuming these amino acids regularly in the proper proportion and in the proper quantity.
You don’t need to remember their names and special qualities. But in case you are interested, the eight essential amino acids are valine, lysine, leucine, threonine, isoleucine, tryptophan, phenylalanine, and methionine. (Histidine is a ninth that is essential for children because of the role it plays in growth and physical development.)
Your body can use them most effectively if your food contains them in these proportions:
one part tryptophan
two parts threonine and phenylalanine
three parts methionine, lysine, valine, and isoleucine,
three and a half parts leucine.
Egg whites provide these eight essential amino acids in just about these proportions. (That’s not surprising egg white is the protein food nature supplies to the chick embryo; the embryo uses the egg white as raw material for building itself into a ready-to-hatch chick.)
The proper quantity your body needs is .9 grams of protein per kilogram or 2.2 pounds of body weight. We’ll discuss this question further later.
Fortunately, you don’t have to learn the amount and proportions of each of these amino acids contained in various foods in order to eat properly. You only have to learn which ones combine best with each other. Luckily, this seems to coincide with which ones taste best together!
What could be more harmonious than a peanut butter sandwich on whole-wheat bread (better yet, with a glass of milk, assuming you’re not allergic to dairy)? Than the rice and beans, or corn tortillas and refried beans, of Mexican fare? Then the Italians’ favorite soup, pasta e Fagioli, combining beans and wheat pasta? With the major cuisines of the world as your guide, the right combinations are easy to learn. We’ll also discuss them in more detail later.
Nutritionists have divided all protein-containing foods into two groups for our convenience, according to the presence of the eight essential amino acids.
Complete proteins contain all the essential amino acids in approximately the correct proportions, and can therefore (in theory) satisfy all our protein requirements by themselves. This group includes meat, eggs, fish, and milk, and other dairy products, as well as soybeans used as tempeh and tofu.
Incomplete proteins have some of the essential amino acids, but not all of them, and not in the necessary proportions. Some incomplete sources are cereal grains, nuts, seeds, legumes, and many vegetables. Taken individually, none of these sources would be a fully life-sustaining and building protein source. Taken in combination, they feed millions of people.
You can see now, based on this distinction, why the complete protein foods especially meat, fish, eggs, and milk have such a good reputation as sources of protein. They’re the rugged individualists of the protein world they can get the job done all alone. But with a little cooperation, the vegetable sources can do it, too.
The Special Advantages of Vegetable Protein
In fact, perhaps they get the job done even better. Vegetable sources of protein are conjugated proteins: they contain other nutrients. But unlike animal sources, they are usually conjugated together with fiber and carbohydrate (or in the case of nuts and seeds, mostly unsaturated fats), rather than saturated fats. Their extra fiber means extra bulk you feel full after eating less. Often, you consume fewer calories, and thereby gain less weight. In countries where most protein comes from non-meat sources, such as legumes (beans and peas) and cereals (wheat, rice, corn, etc.), obesity is much rarer than in the United States. Americans consume, on average, over 3,000 calories a day far more than we need. Much of that comes from the “empty” calories in fat and sugar think of the meat-and-dessert-centered meals most of us still eat. No wonder so many of us are overweight.
The fact that incomplete proteins can be combined with each other to form complete proteins is what makes the science teacher’s diet such a nutritious one, one that can sustain his energy for teaching, gardening, cooking, socializing, regular athletic participation, and all his other activities.
All of the proteins we eat are made up of twenty-three amino acids. These, in turn, are chainlike molecules containing the elements carbon, hydrogen, oxygen, and nitrogen.
There are eight essential amino acids that our bodies require every day, in the right proportions, in order to keep every cell in our bodies functioning properly. These eight (and their proportional relationships) are;
(plus histidine for children)
Egg whites contain all these amino acids in just about these ratios. The complete protein foods meat, poultry, fish, eggs, dairy products, and soybeans in the form of tofu or bean curd contain all eight. Incomplete proteins have some of them, in less perfect proportions. But if you combine two or more complementary protein foods, you are completing their proteins.
their proteins. Although you may not know it, you already do this whenever you eat a peanut butter sandwich on whole-wheat bread; sprinkle spaghetti with cheese; serve whole wheat bread with Boston baked beans, or pour milk over your oatmeal.
Vegetable proteins are nutritionally superior in that they come packaged by nature together with vitamins, minerals, complex carbohydrates, and fiber.