Protein

Protein= chain of amino acids, contains nitrogen, used to build muscle, enzymes are proteins, sources include:meat, fish, dairy, eggs, beans, and grains. 1g=4calories

Protein has a wide variety of functions inside the human body:

1. DNA transcription, translation, destruction
2. Hemoglobin for oxygen transport
3. Enzymatic activity
4. Protein synthesis
5. Muscle activity (actin, tropomyosin, myosin)
6. Energy
7. Hormones
8. Antibodies
9. Cell Signaling

It is possible to not get enough protein which results in Kwashiorkor, a nasty disease causing muscle loss, fatigue, diarrhea, decreased immunity, and many other symptoms, even death.  For athletes, especially weightlifters, a higher protein intake provides greater training results by giving your body's cells enough material for growth.

Protein has a high thermic effect (20-35%) compared to carbohydrates (5-15%) and fats (0-5%). (1)

1 Bodybuilding.com

2 Harvard Protein