PROTEINS are the body-building nutrients. They also serve as
enzymes, antibodies, hormones, transport molecules. 

COMPLETE proteins contain essential amino acids that can not be
synthesized by our body. They tend to be of animal origin. 
Animal protein sources often contain high amounts of saturated fat.
These proteins supply the most absorbable form of iron and 
almost all of our vitamin B12 intake. 
 
Plant foods such as dry peas and beans, grains, nuts and seeds also 
contain proteins but these are 'INCOMPLETE' because not all of 
the essential components are present.  Plant proteins contain much 
dietary fiber and magnesium. 

Overconsuption of proteins has no benefits and may pose health risks.

Soy proteins is complete despite its plant origin. Numerous studies
have shown that consumption of soy protein rather than animal protein
significantly decreases blood cholesterol, LDL cholesterol, and 
triglycerides.