Introduction
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. [1] 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. [3]
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. [11]
- Animal proteins are more digestible (>90%) compared to plant proteins(45-80%). Certain anti-nutrients, fibres, and other factors reduce the digestibility of plant proteins. [1]
- Mixing different plant-based proteins could compensate for the lower anabolic capacity of these protein sources. For example, cereal proteins (deficient in lysine) when consumed with legumes (deficient in sulfur amino acids) could improve the protein absorption rate. [6]
- However, once the antinutritional factors are removed, purified plant protein sources (soy protein isolate, pea protein concentrate, and wheat gluten) display a digestibility rate similar to that of animal-based protein sources. [9]
- Also, the human body can absorb a maximum of 20 – 25 grams of protein per meal and anything in excess of that is not absorbed efficiently. [2]
- In a 70 kg adult man, the body protein pool is around 10-12 kg of body weight, of which
43% resides in the skeletal muscle, 15% in both skin and blood tissues, 10% in liver and kidneys,
and the remaining protein is distributed across other organs. [3,4,5] - Also, four proteins (myosin, actin, collagen, and haemoglobin) represent almost 50% of the total protein content in the body; collagen may comprise 25% of the total. [4]
- The body takes longer to break it down than carbohydrates, due to their complex structure and, hence, they are a much slower and longer-lasting source of energy than carbohydrates. [2]
- Excess protein intake is stored as fat in the body. [2]
- Amino acids are irreversibly lost in the faeces (25-30 %), by metabolic oxidation (70-75 %)
and relatively small amounts of losses through urine, sweat, skin, nails and hair loss. [3,5]
Body Functions
Recommended Daily Intake
Deficiency Symptoms
Protein Food Sources
Excessive Intake/ Toxicity Side Effects
Optimal Muscle Mass Protein Intake
The primary variable responsible for regulating the maintenance or gain in skeletal muscle mass is muscle protein synthesis. [7]
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.
Nutrient Profiles For Food Groups
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