Scientifically speaking, proteins are large, complex molecules that make up 20% of our body weight in the form of muscle, bone, cartilage, skin, as well as other tissues and body fluids.
During digestion, protein is broken down into at least 100 individual chemical building blocks known as amino acids that form a little pool within our liver and are used to build muscle, skin, hair, nails, eyes, hormones, enzymes, antibodies, and nerve chemicals.
Some research has found that inclusion of small amounts of protein during prolonged activity can help enhance performance by sparing muscle glycogen as well as aiding fluid uptake.
Protein also can help mute hunger that arises during longer efforts.
Be careful about overdoing protein, however, as large amounts slow gastric emptying and can precipitate a “backlog” of nutrients of gut and consequent stomach distress and muscle fatigue/cramping. Proteins are essential nutrients for the human body even if you have a sedentary lifestyle.
According to US & Canadian Dietary Reference Intake guidelines:
These Recommended Dietary Allowances (RDAs) were calculated based on 0.8 grams protein per kilogram body weight and average body weights of 57 kg (126 pounds) and 70 kg (154 pounds), respectively.
However, this recommendation is based on structural requirements but disregards use of protein for energy metabolism. This requirement is for a normal sedentary person. In the United States, average protein consumption is higher than the RDA. According to results of the National Health and Nutrition Examination Survey (NHANES 2013-2014), average protein consumption for women ages 20 and older was 69.8 grams and for men 98.3 grams/day.
Active people need more protein
Several studies have concluded that active people and athletes may require elevated protein intake (compared to 0.8 g/kg) due to increase in muscle mass and sweat losses, as well as need for body repair and energy source.
Suggested amounts vary from 1.2-1.4 g/kg for those doing endurance exercise to as much as 1.6-1.8 g/kg for strength exercise, while a proposed maximum daily protein intake would be approximately 25% of energy requirements i.e. approximately 2 to 2.5 g/kg.
In addition, some have suggested that athletes using restricted-calorie diets for weight loss should further increase their protein consumption, possibly to 1.8–2.0 g/kg, in order to avoid loss of lean muscle mass. They are one of the building blocks of body tissue and can also serve as a fuel source.
As a fuel, proteins provide as much energy density as carbohydrates: 4 kcal (17 kJ) per gram; in contrast, lipids provide 9 kcal (37 kJ) per gram.
The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition. Proteins are polymer chains made of amino acids linked together by peptide bonds.
There are nine essential amino acids which humans must obtain from their diet Protein is a nutrient needed by the human body for growth and maintenance.
Aside from water, proteins are the most abundant kind of molecules in the body.
Protein can be found in all cells of the body and is the major structural component of all cells in the body, especially muscle. This also includes body organs, hair and skin.
Proteins are also used in membranes, such as glycoproteins. When broken down into amino acids, they are used as precursors to nucleic acid, co-enzymes, hormones, immune response, cellular repair, and other molecules essential for life. Additionally, protein is needed to form blood cells.
Testing in foods
Kjeldahl method and the Dumas method. These tests determine the total nitrogen in a sample.
The only major component of most food which contains nitrogen is protein (fat, carbohydrate and dietary fiber do not contain nitrogen). If the amount of nitrogen is multiplied by a factor depending on the kinds of protein expected in the food the total protein can be determined. This value is known as the "crude protein" content.
On food labels the protein is given by the nitrogen multiplied by 6.25, because the average nitrogen content of proteins is about 16%. The Kjeldahl test is typically used because it is the method the AOAC International has adopted and is therefore used by many food standards agencies around the world, though the Dumas method is also approved by some standards organizations.