6.4: Protein Recommendations

Learning Objectives

• Identify the RDA and AMDR for protein.
• Describe nitrogen balance.
• Identify foods high in protein.
• Compare and contrast incomplete and complete proteins.
• Describe various vegetarian diets.

How Much Protein Does a Person Need in Their Diet?

The Recommended Dietary Allowance (RDA) for protein is 0.8 grams of protein per kilogram of body weight. The Acceptable Macronutrient Distribution Range (AMDR) for protein is 10-35% of total calories. Although the RDA is the same for all non-pregnant and non-lactating adults, there is a growing body of evidence that this number is inadequate to meet the needs of some athletes.¹ 

Using the information above, how much protein does a 150 pound sedentary adult need? Steps to answer the question include:

1. convert the person's weight in pounds to kilograms by dividing their weight in pounds by 2.2 (150 pounds / 2.2 = 68.2 kg)
2. multiply the person's weigh in kilograms by the appropriate RDA (68.2 kg x 0.8 grams/kg = 55 grams protein per day)

Most Americans meet or exceed the RDA for protein.

Protein Input = Protein Used by the Body + Protein Excreted

The appropriate amount of protein in a person’s diet is that which maintains a balance between what is taken in and what is used. The RDAs for protein were determined by assessing nitrogen balance. Nitrogen is one of the basic elements contained in all amino acids. When proteins are broken down and amino acids are catabolized, nitrogen is released. Remember that when the liver breaks down amino acids, it produces ammonia, which is rapidly converted to nontoxic, nitrogen-containing urea, which is then transported to the kidneys for excretion. Most nitrogen is lost as urea in the urine, but urea is also excreted in the feces. Proteins are also lost in sweat and as hair and nails grow. The RDA, therefore, is the amount of protein a person should consume in their diet to balance the amount of protein used up and lost from the body. For healthy adults, this amount of protein was determined to be 0.8 grams of protein per kilogram of body weight. You can calculate your exact recommended protein intake per day based on your weight by using the following equation:

(Weight in lbs. ÷ 2.2 kg/lb) × 0.8 g/kg

The National Academy of Medicine used data from multiple studies that determined nitrogen balance in people of different age groups to calculate the RDA for protein. A person is said to be in nitrogen balance when the nitrogen input equals the amount of nitrogen used and excreted (Table \(\PageIndex{1}\)). A person is in negative nitrogen balance when the amount of excreted nitrogen is greater than that consumed, meaning that the body is breaking down more protein to meet its demands. This state of imbalance can occur in people who have certain diseases, such as cancer or muscular dystrophy. A person healing from a severe wound may also be in negative nitrogen balance because protein is being used up to repair tissues. Someone who has a low-protein diet may also be in negative nitrogen balance as they are taking in less protein than what they actually need. Positive nitrogen balance occurs when a person excretes less nitrogen than what is taken in by the diet, such as during child growth or pregnancy. At these times the body requires more protein to build new tissues, so more of what gets consumed gets used up and less nitrogen is excreted.

Table \(\PageIndex{1}\): Nitrogen Balance

Term	Nitrogen Balance Equation	Associated Situations
Nitrogen Balance	Nitrogen intake = nitrogen used	healthy individuals eating a varied diet
Negative Nitrogen Balance	Nitrogen intake < nitrogen used	fasting, cancer, wound healing
Positive Nitrogen Balance	Nitrogen intake > nitrogen used	pregnancy, periods of high growth

Dietary Sources of Protein

The protein food group consists of foods made from meat, seafood, poultry, eggs, soy, beans, peas, nuts and seeds. Fruits and vegetables are not high in protein, but help to spare protein by providing carbohydrate. The overall suggestion is to eat a variety of protein-rich foods to benefit health. Different protein-containing foods provide different nutrients. For example, animal proteins contain Vitamin B₁₂ and plant proteins contain a higher amount of fiber. In addition, recommendations suggest choosing leaner/less fatty cuts of meat.

Protein Quality

While protein is contained in a wide variety of foods, it differs in quality. High-quality protein contains all the essential amino acids in the proportions needed by the human body. The amino acid profile of different foods is therefore one component of protein quality. Foods that contain all 9 essential amino acids are called complete protein sources, or high-quality protein sources. Foods that are complete protein sources include animal foods such as milk, cheese, eggs, fish, poultry, and meat, and a few plant foods, such as soy and quinoa.

Foods that do not contain all of the essential amino acids in sufficient amounts to support growth and health are called incomplete protein sources. Most plant-based foods are deficient in at least one essential amino acid and therefore are incomplete protein sources. For example, grains are usually deficient in the amino acid lysine, and most legumes do not contain enough methionine or tryptophan. Because grains and legumes are not deficient in the same amino acids, they can complement each other in a diet. The process of combining two or more incomplete protein sources to make a complete protein is known as mutual supplementation. Some examples of mutual supplementation are provided in Table \(\PageIndex{2}\). Complementary protein sources do not have to be consumed at the same time—as long as they are consumed within the same day, you will meet your protein needs.

Table \(\PageIndex{2}\): Mutual Supplementation Combinations

Foods	Lacking Amino Acids	Complementary Food	Complementary Menu
Legumes	Methionine, tryptophan	Grains, nuts, and seeds	Hummus and whole-wheat pita
Grains	Lysine, isoleucine, threonine	Legumes	Cornbread and kidney bean chili
Nuts and seeds	Lysine, isoleucine	Legumes	Stir-fried tofu with cashews

Key Takeaways

• The RDA set for protein for adults is 0.8 grams per kilogram of body weight and represents the amount of protein in the diet required to balance the protein that is used up by the body and that is excreted.
• Protein-containing foods include meat, seafood, poultry, eggs, soy, beans, peas, nuts, and seeds.
• Most animal-based proteins are complete protein sources and most plant-based proteins are incomplete protein sources.

References

1. Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise. 2016;48(3):543-568. doi: 10.1249/MSS.0000000000000852.