Protein quality and quantity

Protein is an essential components of our bodies. Besides being used in many tissues, it also forms signal molecules, hormones, etc. This is why protein quality and quantity are important.

What is protein

A protein is a collection of peptides, which are collections of amino acids. These can be non-essential (i.e. we can make them in our bodies) or essential (i.e. we need to obtain them from food).

Protein is also one of the 3 macronutrients (protein, carbohydrates, fat), i.e. the components in food that contribute to energy intake. Each gram of protein contributes ~17kJ (4kcal). Foods that are high in protein include meat, fish, eggs and dairy products. Legumes, nuts and seeds and also relatively high in protein.

When we eat foods that contain protein, the digestive process breaks it down into amino acids. The amino acid composition of different food varies. Notably, animal proteins contain all essential amino acids, while most plant foods are deficient in one or more.

Roles in the body

The main roles of amino acids in the body are:

Form and provide strength and flexibility to tissues (e.g. muscles, bones, organs, tendons, ligaments, nails, hair, skin). This, in turn, allows for growth and movement.
Make enzymes and hormones that regulate metabolism
Make antibodies, essential for immune function
Transport oxygen, electrolytes, waste products, etc.
Act as signal molecules
Source of energy

Protein quality

Protein quality is not only determined by the presence of essential amino acids, but also by the digestibility and availability. Plants tend to contain less protein and are less digestible than animal proteins. Diets that rely heavily in protein of plant origin (>70%) contain an inadequate amount of lysine (1).

Factors affecting protein digestibility

The digestibility of proteins is affected by factors that are naturally occurring and introduced during processing. These include (2):

Natural:

Trypsin inhibitors in legumes, cereals, potatoes and tomatoes, which can be lowered by heat
Tannins in sorghum, millet, beans and peas, which are difficult to eliminate
Phytic acid in seeds, grains and nuts, which can be reduced in some foods with processing treatments

Processing:

Maillard reactions occurring between sugars and lysine in foods such as dairy, eggs and cereals
Protein-bound D-amino acids and lysinoalanine, which occurs when exposing food proteins to heat and/or alkaline treatments.

Protein Digestibility Corrected Amino Acid Score (PDCAAS)

This used to be the default method for determining the quality of proteins since 1989. However, it had several limitations due to the assumptions that it made (3, 4).

Digestible Indispensable Amino Acid Score (DIAAS)

This is the method recommended by the Food and Agriculture Organization (FAO) to determine protein quantity, according to the formula:

DIAAS % = 100 x [(mg of digestible dietary indispensable amino acid in 1 g of the
dietary protein) / (mg of the same dietary indispensable amino acid in 1g of the
reference protein)] (4).

In this calculation, digestibility refers to true digestibility at the ileum or end of the small intestine (3, 4). Additionally, digestibility is subject to variation between individuals (4).

Below are the scores of selected foods analysed by Mathai et al (3):

Milk protein concentrate (120)
Whey protein concentrate (107)
Skimmed milk powder (105)
Whey protein isolate (100)
Soya flour (89)
Soya protein concentrate (84)
Pea protein concentrate (62)
Wheat (45)

It is no wonder that foods that rank high according to their DIAAS, such as milk, whey, casein and soy, taken post-resistance exercise stimulate muscle protein synthesis. However, there is variability in the rate of amino acid uptake, potentially due to the proportion of other components (such as fat) in the food.

Protein quantity

High protein diets can help with weight/fat loss, lean mass preservation, blood pressure control, blood sugar control, lower waist circumference and triglycerides (5).

Individual protein requirements depend on age, gender, life stage (e.g. pregnancy, breastfeeding), weight, health conditions, physical activity, etc. However, a general rule is to aim for 20-30g of protein per main meal (breakfast, lunch, dinner) (5). In addition, regular physical exercise also helps maintain muscle mass, particularly in ageing populations (5, 6). Moreover, 20g of protein after exercise provides the greatest stimulus for muscle protein synthesis (6).

Using mathematical models, de Gavelle et al. proposed that the amount of protein is more important than the quality of the protein, when considering the amount of lysine in it. In this sense, they recommend eating legumes, nuts and seeds vs cereals as the main source of protein in a largely plant-based diet (1).