What is protein and how does the body use it?
Protein is a macronutrient that provides four calories per gram. Along with carbohydrates and fats, protein is one of our primary sources of energy. The body also uses protein to perform the following functions:
- Tissue development, including muscle, bone, and skin
- Enzyme functions, including metabolism, digestion, and blood clotting
- Hormone development
- pH balancing
Proteins are long chains of amino acids. There are 20 types of amino acids used to build proteins, and dietitians categorize them as either essential or non-essential. Essential amino acids are required in the diet, because the body cannot make them. In contrast, non-essential amino acids can be made by the body and are not required in the diet.
Essential amino acids
Non-essential amino acids
How much protein do I need?
Adequate Daily Grams Per Pound of Lean Body Weight
|Category||Adequate Intake (g/lb) 1, 2, 3|
|Endurance & Strength-training Athletes||0.5 - 0.8|
|Vegetarian Endurance & Strength-training Athletes||0.55 - 0.88|
Which foods are good sources of protein?
Good sources of protein include meat, soy, nuts, beans, dairy, and eggs.
Foods that contain all 20 amino acids are called complete proteins, while those missing amino acids are referred to as incomplete proteins. When incomplete proteins are combined to provide all 20 amino acids, they are called complementary proteins.
Complete Protein Examples
- Animal-based foods (including meat, dairy, and eggs)
- Soy (including tofu and tempeh)
Complementary Protein Examples
- Bread and peanut butter
- Rice and beans
- Ezekiel bread (wheat, barley, beans, lentils, millet, and spelt)
Vegan diets are often low in the essential amino acids methionine and lysine. Vegans can meet their methionine needs by eating brazil nuts, hemp seeds, oats, and sesame seeds; and they can reach their lysine goals by eating tempeh, seitan, lentils, and black beans.
What is the upper limit for protein?
The Food & Nutrition Board (FNB) of The National Academies of Sciences, Engineering, and Medicine has not set an upper limit for protein. They found "no evidence that amino acids derived from usual or even high intakes of protein from food present any risk." However, they urge caution due to limited research on the subject.5
Recent studies suggest that healthy adults can tolerate daily protein intake as high as 1.6 grams per pound of lean body weight.6
What is protein deficiency?
Protein deficiency is not a big concern in developed nations. Most people, including vegetarians, consume more than enough protein per day. However, the elderly, the extreme poor, and those on highly-restrictive diets may be at risk for not meeting their amino acid needs.
In countries with high rates of poverty, protein deficiency is more common. In extreme cases, a condition known as kwashiorkor can result. Symptoms of this disorder include a protruding abdomen, fluid retention, thin hair, weight loss, slowed growth, and discolored hair & skin. If kwashiorkor is left untreated, it can cause stunted growth in children, mental impairments, and even death. Kwashiorkor is easily prevented and, if caught early, can be reversed. Many organizations, including UNICEF, are working to eradicate this cruel condition.
- Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Institute of Medicine of the National Academies.
- Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance (March 2009). Journal of the American Dietetic Association, 109(3), 522.
- Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance (March 2009). Journal of the American Dietetic Association, 109(3), 510.
- Nutrition and Athletic Performance. American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine.
- Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Institute of Medicine of the National Academies, 737.
- Dietary protein intake and human health. Food and Function