Protein in a nutshell, is a nitrogen-containing macronutrient made up of smaller individual chemical chains known as amino acids.
Dietary protein has many vital roles and functions in the body including the growth, repair and maintenance of our cells, bone, skin and tissue, which is particularly important as we age.
Protein is delivered to us from an array of animal and plant-based forms from all food groups. Examples include egg, chicken, beef, nuts, fish, tofu, legumes (such as chickpeas or kidney beans) and some cereal-based products. Despite this, it is important to recognise that not all dietary proteins (or supplements) are created equal.
Protein quality often trumps protein quantity regarding its overall impact on our health, as the prevalence of protein deficiency among the general population in Australia is quite rare. However, if present, the main symptoms of protein deficiency are sarcopenia (muscle atrophy or wastage), oedema (excess fluid build-up such as swelling of the feet or ankles) and anaemia .
As we get older our protein requirement increases, yet consumption often declines. This can be due to a lack of appetite, loss of taste or smell, a decline in interest in food or cooking, poor dentition or reduced mobility . In such circumstances the quality of the protein and the food matrix in which it is consumed are very important considerations in the maintenance of an individual’s good health.
Protein quality can be simply defined by how well or how poorly a given protein can perform its metabolic function in the body. This is underpinned by the digestibility and quantitative supply of essential amino acids for providing proteins in the correct ratios for human consumption [3,4].
There are 20 ‘metabolically essential’ amino acids required for protein synthesis and for sustaining our body’s nitrogen equilibrium. However, only nine of these; histidine, leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine, need to be obtained from the diet, otherwise known as ‘indispensable’ or essential amino acids (EAAs). The other amino acids are conditionally essential (especially in times of illness or stress) or non-essential because they can be self-synthesised in the body.
However, in order to sustain protein quality integrity, all essential amino acids must be present in the diet. Complete protein sources contain all EAAs and are therefore considered ‘high quality’, whereas ‘low quality’ protein sources are incomplete proteins as they do not contain all EAAs.
Examples of high quality protein sources include:
Examples of low quality protein sources include:
Therefore, for individuals to appropriately meet both their minimum and/or additional dietary protein requirements, it is recommended to consume high quality proteins or a combination of at least two or more low/insufficient protein sources to adequately attain all EAAs through the formation of complementary proteins. The American Dietetic Association in 2003 concluded that complementary proteins do not all need to be consumed during the same meal and can be spread throughout the day to meet protein needs while also ensuring adequate nitrogen retention .
It is also worth considering the many benefits of consuming enough high-quality protein sources to meet our needs. Eating meals containing high quality protein can help increase satiety levels more than carbohydrate or fat sources; this can assist in reducing overall energy consumption and the desire to consume extra snacks. Adopting a moderately higher protein diet can also help to maintain and grow lean muscle mass and reduce body fat .
There are a range of different methods currently used to determine protein quality.
The most common is referred to as the ‘Biological Value’ (BV) protein scoring system. BV expresses the body’s nitrogen utilisation as a percentage (nitrogen retention for tissue formation is divided by the nitrogen absorption from food).
The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is an internationally standardised system which assesses protein quality via combining estimates of protein digestibility with an age-related amino acid profile pattern, also factoring in bodily protein excretion [6,7].
DIAAS, or the Digestible Indispensable Amino Acid Score, DIAAS uses a mathematic equation to calculate the “individual indispensable amino acids in the dietary protein relative to human reference amino acid requirement pattern” . Dietary proteins are ranked according their score, to indicate dietary protein quality. This is beneficial when looking for foods with higher scores to complement less nutritious proteins.
Acquiring a deeper understanding of the importance of both protein and protein quality can lead to better food choices to support our physiological health and general wellbeing.