Protein is an essential macronutrient that constitutes approximately 18% of total body weight in an average person. Contrary to carbohydrates and fats, proteins contain nitrogen atoms. Proteins should provide about 15 to 20% of total daily calories of an average person. One gram of protein generates 4 calories.
Amino Acids:
Amino acids are the building blocks of proteins. They bind together through peptide bonds to build different proteins. There are two types of amino acids, essential and nonessential. Essential amino acids cannot not be synthesized by the body and they should be provided from external sources. Eight amino acids in adults and nine amino acids in infants are considered essential. Nonessential amino acids can be synthesized by the body and they do not need necessarily to be provided from outside (see the table below).
Amino Acids:
Essential
Histidine*
Lysine
Leucine**
Isoleucine**
Valine**
Methionine
phenylalanine
Threonine
Tryptophan
*This amino acid is essential in infants, not in adults.
**The three amino acids leucine, isoleucine and valine are called BCAAs (branched chain amino acids).
Nonessential
Alanine
Arginine
Aspartic acid
Citrulline
Cysteine
Cystine
Glutamine
Glutamic acid
Glycine
Hydroxyproline
Ornithine
Proline
Serine
Tyrosine
Types of Proteins:
If a protein contains all nine essential amino acids, it is called “Complete Protein”. These types of proteins support tissue growth and repair, as they are higher – quality proteins that maintain nitrogen balance. All animal-based proteins are complete proteins.
The proteins that lack one or more essential amino acids are called “Incomplete Proteins”, and they do not support growth. All plant-based proteins are considered incomplete proteins, except “Soy” and “Spirulina”, which are complete proteins
Why Do We Need Proteins?
As essential macronutrients, proteins have a variety of functions in the body. They include:
- Energy . Every one gram of protein provides 4 calories. During our daily activities and exercise, the primary source of energy is carbohydrates followed by fats. When the body runs out of its reservoirs of carbohydrates and fats, proteins come into play.
- Tissue growth and maintenance . Protein serves as a constituent of all cells and tissues in the body. Though proteins make up about 18% of the body weight, the contents of protein in different cells varies. The cells require proteins for their daily constant turnover. Moreover, rapid growth in infancy and childhood need proteins. When the body is in “catabolic states”, such as burns, after surgeries, advanced cancers, and traumatic injuries, proteins play an important role in tissue growth and healing.
- Hormones and enzymes are proteins, as they are made of amino acids.
- Immunity . The body produces antibodies in response to foreign bodies. Antibodies are a major part of the immune system and they are proteins.
- Acid and base balance . Proteins act as blood buffers, eliminating excess hydrogens and maintaining pH within normal limits.
- Fluid balance . Proteins, such as albumin and globulin, have a key role in maintaining fluid balance by exerting oncotic pressure, which helps keep water inside the vessels and prevent edema.
- Transportation . The carriers that transport nutrients in the blood are made of proteins. For instance, hemoglobin (which carries oxygen), lipoproteins (which carry fats), and vitamin carriers.
How Much Protein Do We Need A Day?
Contrary to carbohydrates and fats, human body does not have “reservoirs” of proteins. This is why it is highly important to take enough protein daily. The daily requirement of protein depends on person`s body weight. The recommended daily allowance (RDA) for protein in adults is 0.8 grams per one kilogram of body weight.
Children and adolescents need some extra amounts of proteins. Also during pregnancy and all months of breast feeding, the RDA increases by 25 grams per day.
At the Canadian Academy of Sports Nutrition, we use the terms the “right amount of protein” for the first time. The “right amount of protein” (RAP) is the daily amount of protein required by the body to maintain nitrogen balance. Nitrogen balance occurs when nitrogen intake equals to nitrogen loss. Nitrogen is usually lost in urine, feces, and sweat.
When nitrogen intake exceeds nitrogen loss, it is called “positive nitrogen balance”. It is often seen in growing children and during pregnancy, breastfeeding, recovery from diseases, and resistance training. A positive nitrogen balance is the optimal state for building muscles.
When nitrogen loss exceeds nitrogen intake, it is called “negative nitrogen balance”. A negative nitrogen balance may occur during burns, fever, starvation, infections, dieting, fasting, muscle wasting diseases, and diabetes.
The right amount of protein (RAP) is not necessarily equal to the recommended daily allowance (RDA) of protein. The RDA is based on body weight, and the metabolic status of the person is not taken into consideration. The RAP relies on both body weight and metabolic status of the person.
Protein Requirements:
| Age or Conditions |
Required Protein (grams/Kg of Body Weight) |
|---|---|
|
Infants |
|
|
0 – 6 months |
2.2 |
|
6 – 12 months |
1.6 |
|
Children |
|
|
1 - 3 years old |
1.2 |
|
4 – 6 years old |
1.2 |
|
7 – 12 years old |
1.0 |
|
Adolescents |
|
|
(13 – 18 years olds) |
0.9 |
|
Adults |
|
|
Non – athletic, healthy |
0.8 |
|
Kidney failure without dialysis |
0.6 |
|
Kidney failure with dialysis |
1.2 |
|
Weight management |
1.5 |
|
Metabolic stress (eg, burns, infections, surgery, trauma, critical illness) |
1.5 |
|
Recreational exerciser |
1.2 |
|
Athletes |
|
|
Endurance sports |
1.4 |
|
Ball sports |
1.6 |
|
Strength sports |
2.0 |
Protein Sources:
Protein is found in a wide range of foods, even in fruits and vegetables. For foods high in protein, see the table below.
Protein Sources:
| Foods | Serving Size |
Protein (in grams) |
|---|---|---|
|
Animal – based |
||
|
Dairy products |
||
|
Cheese, cottage |
1 cup |
28 |
|
Cheese, hard |
1 oz* |
8 |
|
Milk |
1 cup |
8 |
|
Yogurt, regular |
1 cup |
10 |
|
Yogurt, Greek |
1 cup |
16 |
|
Eggs |
||
|
Egg white |
1 medium |
3 |
|
Whole egg |
1 medium |
6 |
|
Meats |
||
|
Beef |
1 oz |
8 |
|
Chicken |
1 oz |
8 |
|
Fish |
1 oz |
8 |
|
Luncheon |
1 oz |
5 |
|
Lamb |
1 oz |
8 |
|
Pork |
1 oz |
8 |
|
Turkey |
1 oz |
8 |
|
Plant – based |
||
|
Beans (legumes) |
1 cup |
16 |
|
Nuts |
1 cup |
35 |
|
Spirulina |
1 cup |
60 |
|
Tempeh |
1 cup |
40 |
|
Tofu, firm |
3 oz |
10 |
*1 oz (ounce) equals to 28 grams.
Protein Quality:
Protein quality refers to the ability of a specific dietary protein to support body growth and maintenance. The three most common methods for measuring protein quality are:
- Biologic Value (BV).
- Protein Efficiency Ratio (PER).
- Protein Digestibility-Corrected Amino Acid Score (PDCAAS).
Biologic Value (BV):
This is considered the most valid method and it measures the amount of nitrogen retained in comparison to the amount of nitrogen absorbed. The biologic value of a whole egg is considered as a value of 100, and other proteins are compared with that of a whole egg.
Proteins with higher BVs are considered “high – quality”, and they come from animal sources.
Protein Quality Based on Biologic Value:
| Food items | Biologic Value (BV) | |
|---|---|---|
|
Whey protein |
||
|
Isolate |
158 |
|
|
Concentrate |
104 |
|
|
Hydrolyzed |
100 |
|
|
Whole egg |
100 |
|
|
Milk |
91 |
|
|
Meats |
79 _ 83 |
|
|
Casein |
77 |
|
|
Soy |
74 |
|
