Protein: What is it?
Proteins are complex molecules that make up tissues of the body such as muscle mass, bones, blood and hormones. Proteins are constantly being broken down, repaired, replaced or maintained. This is most easily seen in improved muscle tone and muscular development with exercise. These proteins in are body are composed of amino acids – the nitrogen-containing building blocks that form proteins. There are 2 classes of amino acids:
1. Essential Amino Acids
Meaning essential that these amino acids come from the diet because the body cannot make them. They include:
- Histidine
- Isoleucine
- Leucine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Tryptophan
- Valine
2. Nonessential Amino Acids
Meaning not essential that they come from the diet because they can be manufactured by the body in sufficient quantities. They include:
- Alanine
- Arginine
- Asparagine
- Aspartic Acid
- Cysteine
- Glutamic Acid
- Glutamine
- Glycine
- Proline
- Serine
- Tyrosine
The body can synthesize nonessential amino acids through the process of transamination – in which the amine group from one amino acid is removed and transferred onto another acid and side chain.
Where does it come from?
Common sources of dietary protein include beef, pork, poultry, seafood, eggs, beans/legumes, tofu, nuts and dairy products. The quality of protein is assessed through multiple methods such as the protein efficiency ratio, the protein digestibly, corrected amino acid score and the net protein utilization. Factors affecting protein quality include the amount of essential amino acids and the digestibility. Animal and dairy protein sources are highly digestible. Complementary proteins occur when at least 2 foods are combined and together contain all 9 essential amino acids required to make a complete protein. Examples include rice and legumes – rice is low in lysine but high in methionine – conversely, legumes are low in methionine but high in lysine. Other examples include peanut butter and bread, rice and lentils, etc.
Why is it important?
As previously mentioned, protein is vital to cell growth, maintenance and repair. Because the turnover of cells in our bodies is so constant, dietary intake of protein is needed to accommodate.
Less commonly known facts:
Enzymes – Enzymes are proteins and enzymes are catalysts of chemical reactions in the body.
Hormones – Some hormones, such as insulin, are composed of amino acids.
Immune System – The immune system also relies on proteins. Antibodies are made of proteins and they help fight against foreign bacteria, viruses, allergens and toxins.
Fluid Balance – Proteins also play a role in fluid balance and blood pressure regulation. If protein intake is insufficient, low levels of proteins in the blood will not be able to draw fluid from the tissues and across the blood vessels. Therefore, a condition known as edema can occur as seen with a swollen appearance from fluid collecting inside the tissues.
Acid-Base Balance – Proteins also act as buffers, maintaining acid-base balance in the body and blood. The negative side chains in protein molecules neutralize the positive charged hydrogen ions found in acids.
Energy Source – Although carbohydrates and fat remain the primary energy sources of the human body, proteins can also be used for energy. Through a process known as deamination, in which the nitrogen is removed from the amino acid and excreted through the urine by the kidneys. Then, the remnant carbon, hydrogen and oxygen are converted to carbohydrates or directly metabolized for energy.
How much do you need?
The Recommended Dietary Allowance (RDA) for sedentary adults is 0.8 grams of protein per kilogram body weight (1kg = 2.2lbs). For example: 154lb female = 132/2.2 = 70kg 70kg female = 56g of protein per day Dietary protein needs change with type, level, and intensity of activity. Also, certain health conditions and injuries (i.e. pregnancy, lactating, broken bones, burns, etc.) will change the protein requirements. The Acceptable Macronutrient Distribution Range (AMDR) for protein for sedentary adults (>19 years) is 10-35% of daily calories coming from protein.
Recommendations more centered on exercise and fitness have also been provided by leading organizations. The position of the International Society of Sport Nutrition that exercising individuals ingest protein ranging from 1.4 to 2.0 g/kg/day. Individuals engaging in endurance exercise should ingest levels at the lower end of this range, individuals engaging in intermittent activities should ingest levels in the middle of this range, and those engaging in strength/power exercise should ingest levels at the upper end of this range.
References
- National Academy of Sciences. Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for carbohydrate, fiber, fat, fatty acids and protein. 2005; cited 2010 September 6.
- International Society of Sports Nutrition Position Stand: Protein & Exercise. Journal of the International Society of Sports Nutrition20074:8