Too much protein can be harmful too. The an- imal foods rich in high-quality protein are often rich in saturated fats, and high in calories. Meats, milk, and eggs are also devoid of fiber. Thus the same foods that can “cure” the hungry in impov- erished countries—foods such as hot dogs, fried chicken, pizza, cheeseburgers, and ice cream— are excessive in the American diet.
In contrast, much of the malnutrition in devel- oping countries stems from a diet that contains only plant foods—foods that contain fiber, and are generally low in fat and calories. The typical American diet can be improved by more of these foods.
The traditional diets of many countries contain moderate amounts of protein. In these diets, animal sources of protein are not the center of the meal as they tend to be in the American diet. Rather, they are often just one of several ingredients in the main course. Examples include Chinese stir-fried broccoli with bits of beef, an Indian dish of lentils and vegetables mixed with small cubes of cheese, and the Middle Eastern dish of couscous—a flavorful mound of crushed grain topped with vegetables mixed with bits of meat.
We can do the same with our American menus by, for example, eating smaller portions of meat and more generous portions of the plant foods that we often consider the “side dishes.” A prudent diet is one that contains a variety of foods, that includes only a moderate amount of animal proteins.
Summary
Proteins are made within cells. The blueprints to make proteins are encoded in the structure of DNA, the genetic material that lies in the center— the nucleus—of cells. Human DNA contains about 20,000 blueprints to make proteins. Each
blueprint—the directions to make a particular protein—is called a gene.
DNA is a long double-stranded chain of chemical units, each of which contains a substance called a base. There are four kinds of bases found in DNA, and it’s the sequence of these bases along the chain of DNA that encodes the directions to make protein.
The bases are “read” in groups of three. This provides 64 possible combinations of the four bas- es—more than enough to code for the 20 kinds of amino acids needed to make protein.
A particular sequence of three bases (triplet) is the genetic code for a particular amino acid. With this genetic code in hand, one can determine the sequence of amino acids in a protein by “read- ing”—three at a time—the sequence of bases in the protein’s blueprint.
One kind of base will join only with one other kind of base. In this way, a complementary copy of a precise sequence of these bases is made.
The first step in making a protein is to obtain such a copy of the particular sequence of bases in DNA that encodes the directions to make that protein. This copy is made in the nucleus of the cell and is known as messenger RNA.
The messenger RNA moves out of the nucleus into the cytoplasm, and attaches itself to a ribo- some, the site of protein synthesis. As the trip- lets of bases are “read” on the messenger RNA, the corresponding amino acids are delivered via transfer RNA, and the amino acids are connected. When the entire messenger RNA is read, the new- ly-formed protein is released from the ribosome.
Our dietary protein requirement provides the amino acids necessary for protein synthesis. For adults, the dietary protein requirement is gener- ally that amount needed to maintain the body’s proteins. Relative to body weight, children and pregnant women need more protein because they are adding—rather than just maintaining—body proteins. Nursing mothers also need additional protein in order to make milk protein.
Athletes do need a little more protein, but typically the protein in their diets alone far exceeds their need. When protein intake is excessive, the excess isn’t stored. Rather, the amino acids are
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converted to fat or carbohydrate-like compounds, and are then broken down to produce energy or are stored as body fat. Because protein-rich foods are generally expensive, such uses of protein are economically wasteful.
The RDA for protein, like the RDA for other nutrients, is generous. The average protein intake in the United States is well above the RDA.
Many of the vegetarians in the U.S. include dairy products and/or egg in their diet (lacto and/ or ovo vegetarians), so can easily get adequate amounts of vitamin B12, calcium, and high-qual-
ity protein. Strict vegetarians (vegans) have to be more vigilant, especially in getting vitamin B12,
because B12 isn’t found naturally in plant foods.
In many developing countries, severe protein deficiency is common. The protein deficiency is usually accompanied by deficiencies of other nutrients and inadequate calories. Inadequate calories makes the protein deficiency worse, be-
cause the body’s first priority is fulfilling energy needs—the body will break down amino acids to meet the need for energy rather than using the amino acids to make protein. This manifests itself in large numbers of children with the ills of pro- tein-energy malnutrition.
Breastfeeding can be a matter of life or death for the infants—breast milk can be their only de- pendable source of high quality protein and other essential nutrients.
Moderation is a key to good nutrition. Protein intake is no exception. Animal sources of protein are generally high in protein, but have no fiber, and are generally high in fat. Since the typical American diet is high in protein and fat and low in fiber, our diet could be improved by eating smaller portions of meats and high-fat dairy products, and eating larger portions of whole grains, vegetables, and fruit.
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