Protein is a major functional and structural component of every cell in the body.
It is made up of a long chains of amino acids and there are 20 of them, of which 9 are considered to be nutritionally indispensable (essential) in humans because the body is not able to synthesise them. [1,3]
Amino acids are foundational blocks of proteins and proteins are the building blocks of muscle mass which helps metabolism.  The most important aspect of protein is its amino acid’s composition. [1,2]
Thus, the quality of a dietary protein source is determined by its digestibility and its amino acids composition. [1,3,4]
The food sources that contain all the essential amino acids are referred to as complete proteins. Most plant-based sources are categorised as incomplete proteins, except for soy and quinoa. Hence, vegans and vegetarians are recommended to combine different dietary protein sources throughout the day to meet their daily protein intake requirements. 
Also, Protein Digestibility Corrected Amino Acid Score (PDCAAS) and Digestible Indispensable Amino Acid Score (DIAAS) are two composite metrics used to rate the dietary protein source based on its essential amino acid composition, true faecal digestibility and its ileal digestibility. [4,5]
According to FAO, UN, plant-based foods are the leading source of protein and comprises 57% of daily protein intake, followed by meat (18%), dairy (10%), fish and shellfish (6%), and other animal products (9%) globally. 
The primary variable responsible for regulating the maintenance or gain in skeletal muscle mass is muscle protein synthesis. 
It is the naturally occurring process in which protein is produced to repair muscle damage caused by exercise. [1,3,7]
Of all the 20 amino acids needed for preserving or gaining muscle mass, leucine is particularly one of the most important ones. The other 2 being methionine and lysine. [6,7]
Recent research suggests consuming plant-based foods that could provide at least 3g of leucine per meal(equivalent to 23g of whey) to maximise postprandial muscle protein synthesis. [1,7]
The below charts highlights the dietary plant food sources of leucine along with the recommended intake.