Proteins in Foods

Proteins in Foods

• Nearly all foods contain protein, but no naturally occurring food is 100% protein.
• Protein comprises about 20 to 30% of the weight of a piece of beef, 25% of the weight of drained tuna fish, and 12% of an egg's weight.
• Animal foods generally provide higher amounts of protein than similar quantities of plant foods.

High-Quality vs. Low-Quality Proteins

• A high-quality protein contains all essential amino acids in amounts that support protein deposition in muscles and other tissues, as well as a young child's growth.
• High-quality proteins are well digested and absorbed by the body.
• Meat, fish, poultry, eggs, and milk and other dairy products contain high-quality proteins.
• Egg protein generally rates very high for protein quality because it is easy to digest and has a pattern of essential amino acids that closely resembles that needed by humans.
• A low-quality protein lacks one or more of the essential amino acids or contains inadequate amounts of these nutrients.

Limiting Amino Acids

• The essential amino acids that are in relatively low amounts are referred to as limiting amino acids because they reduce the protein's ability to support growth, repair, and maintenance of tissues.
• In most instances, lysine, tryptophan, threonine, and the sulfur-containing amino acids methionine and cysteine are the limiting amino acids in foods.

Plant Foods as Protein Sources

• Certain parts of plants contain more protein than other parts.
• Seeds, tree nuts, and legumes supply more protein per serving than servings of fruit or the edible leaves, roots, flowers, and stems of vegetables.
• A 3-ounce serving of almonds, dry-roasted peanuts, or sunflower seed kernels supplies about 20 g of protein.
• Mature peas, lentils, and most kinds of beans contain more protein and complex carbohydrate than fat.

Quinoa and Soy Protein

• Quinoa is an exception to the rule that most plant foods are not sources of high-quality proteins.
• The quality and amount of protein in quinoa seeds are superior to those of many cereal grains.
• Soy protein is comparable to that of most animal proteins.
• Processed soybeans are used to make a variety of nutritious foods, including soy milk, infant formula, and meat substitutes.

Understand the difference between high-quality and low-quality proteins, identify rich food sources, and learn about limiting amino acids in plant foods.