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Getting enough protein in our diets is essential for adequate nutrition. What is less well known is that protein represents a group of nutrients, the amino acids, each of which needs to be consumed in sufficient amounts. Here, we look at how we digest protein, the importance of amino acids, and show that protein quality, not just quantity, is vital.

Protein, alongside carbohydrates and fat, is one of the dietary macronutrients found on the nutrition label of all commercially-produced food. The recommended daily intake (RDI) for protein on these labels varies between authorities, but is usually around 50 g. This allows food companies to easily calculate and display on packaging what percentage of your protein RDI is supplied by their product.

But what is meant by ‘protein’ on these labels? And where do these RDIs come from?

Protein and amino acids

Proteins are a group of molecules essential to all life, distinguishable from carbohydrates and fats by containing nitrogen. The use of proteins in our bodies is broad: they form our tendons and ligaments as collagen, break down our food as digestive enzymes, and protect from infection as antibodies, among many other roles.

Every protein is composed of a string of smaller molecules, amino acids, folded into a functional shape. The amino acids in the string and the folded shape of the protein are specific to the function of that protein.

When we discuss protein as a dietary macronutrient, we are really referring to the supply of amino acids in the foods we eat, rather than the protein per se. The protein content seen on food packaging should really be seen as the sum of the amounts of each amino acid in the food.

Protein digestion and use

Protein is present in the majority of foods we eat. The amount and type of protein varies depending on the food, but all are subjected to the same digestive processes once eaten.

Protein digestion begins in the stomach. The body produces the enzyme pepsin, which starts the breakdown of proteins with the help of the stomach’s acidic conditions. Digestion continues in the small intestine, with the enzymes trypsin and chymotrypsin continuing the breakdown of proteins to individual or very short strings of amino acids (dipeptides and tripeptides).

These small molecules, rather than the original proteins, are absorbed by the intestine and transported around the body in the bloodstream. Once absorbed, amino acids are used to construct the many proteins needed by the body.

Consuming adequate protein in the diet is essential. Our bodies do not store protein in the way we can store fat or carbohydrates. Instead, there is a constant cycling of protein construction, breakdown and excretion. This protein turnover cycle leads to around 250 grams of new protein being produced each day, either using recycled amino acids from body protein breakdown, or from the amino acids derived from newly digested dietary protein. If dietary protein is lacking, this can lead to an overall depletion of body protein over time.

The importance of each amino acid

The most common way of calculating protein RDI is by bodyweight. For example, a frequently heard recommendation is that you should eat 0.8 g of protein each day for each kg of bodyweight. Thus, a 75 kg man should consume 0.8 x 75 = 60 g of protein each day. However, there is a lack of consensus around the value of 0.8 g, with many arguing that intake should be at least 1 g, particularly for athletes and older adults.

This calculation around protein RDI hides the more specific amino acid requirements of the body. There are 20 common amino acids, 9 of which are essential. Essential means that the body cannot effectively make these amino acids itself, so must obtain them from the diet.

There are RDIs for each essential amino acid, based on the amount required for body protein production. However, these RDIs are not displayed on food products, as this would be difficult to calculate for each food and make understanding nutrition labels more difficult. Instead, the protein RDI approximates what is needed based on the amino acid content of an average diet. This approximation was designed for a population that consumes a diverse diet over time. It is less fitting for day-to-day protein consumption of the individual, particularly those who consume only a limited range of protein sources. As an individual, it’s important you obtain enough of each essential amino acid each day.

What happens if we don’t get enough of a certain amino acid?

The result of deficiency in amino acids is best explained through an analogy.

Imagine you are assembling toy cars. The process involves painting the body of the car green, and then putting on the wheels. You have a box of car bodies, a pot of green paint, and a box of wheels.

As you are assembling these cars, you come to a point where you still have car bodies and wheels, but you have run out of green paint. However, with a little more effort, you can make more green paint by mixing some blue and yellow paint you have. With this newly made green paint, the assembly process can continue.

However, if you come to a point where you have car bodies and paint, but have run out of wheels, you cannot continue to assemble the cars. No matter how much of the other two components you have, the wheels are essential, so car assembly must stop until you have more wheels.

The construction of the toy cars from components is analogous to the construction of a protein in the body from individual amino acids. In the assembly of a protein, several different amino acids are required. Like the green paint, if the body runs out of a non-essential amino acid, then it can produce more from other amino acids, although less efficiently. However, if the body runs out of an essential amino acid (those that must be derived from the diet), protein synthesis is halted – much like running out of wheels in the toy car assembly.

If you do not obtain sufficient essential amino acids from your diet, synthesis of necessary proteins can be halted. The wheels in the toy car analogy are the ‘first limiting’ component in car assembly. In humans, it is often the amino acid lysine that is the first limiting amino acid to protein synthesis. This is because lysine is required in a large number of proteins and is not always readily available from the diet. A person can be protein deficient by being deficient in just one essential amino acid, regardless of the amount of the other amino acids they consume. And since the body is unable to store protein, an excess of unused amino acids consumed will be wasted by the body when it cannot immediately use them. Getting enough of each essential amino acid is required for optimal health.

How do I ensure I get enough of each amino acid?

Different foods contain different distributions of amino acids. For example, chickpeas are higher in lysine than oats, but the reverse is true for the amino acid cysteine. Plant foods are more often limited in certain essential amino acids than animal foods, due to the similar proteins required by animals and our own bodies. If plant-sourced foods are your main source of protein, it is important to understand their amino acid profile. Plant foods with complementary amino acid profiles can be consumed together to make up for their individual deficiencies.

Another important consideration is amino acid bioavailability; the percentage of the total amino acid that is available to the body from different food protein sources. The efficiency of the protein digestion process varies depending on the structure of the protein consumed and the food matrix proteins are contained in. Extensive research has been performed on the bioavailability of each amino acid in human foods. The table below gives a summary of bioavailability values for some selected foods.

Food	Amino acid bioavailability (% of total consumption that is absorbed)
Roasted beef	94 – 99
Fish	81 – 94
Cooked kidney beans	64 – 100
Oats	70 – 88
Potatoes	47 – 66
Rice	75 – 99
Skim milk	78 – 97
Cooked soyabeans	71 – 90

Bioavailability of amino acids can vary widely between foods. Therefore, it is useful to have a score for each food reflective of the overall amino acid availability, commonly referred to as protein quality. The DIAAS score (Digestible Indispensable Amino Acid Score) is recommended by the UN Food and Agriculture Organisation for this purpose. The digestibility of each essential amino acid in a food is calculated and compared to a reference protein, and the DIAAS is the lowest of these calculated values. The score is thus reflective of the digestibility of the most limiting essential amino acids in the food.

A DIAAS score of 100 or more indicates excellent protein quality, with high digestibility of all the essential amino acids. Scores between 75 and 100 are considered good sources of protein, but consuming complementary proteins would improve their profile. Scores below 75 are of lower quality. Some example foods with their DIAAS are given below.

Food	DIAAS	Limiting amino acid(s)
Beef (roasted)	99	Leucine and Valine
Pea protein concentrate	82	Methionine and Cysteine
Rice (cooked)	60	Lysine
Skim milk powder	105	Methionine and Cysteine
Soya protein isolate	84	Methionine and Cysteine
Wheat	45	Lysine

Generally, animal-sourced foods have higher DIAAS scores than plant-sourced foods. This means that the profile of amino acids is better suited to human digestion and to fulfilling our needs for protein synthesis.

At a global scale, producing enough of each amino acid is critical to the ability of the food system to meet nutrient needs. When considering possible future scenarios, the DELTA Model predicts the supply and bioavailability of essential amino acids, as well as total protein.

Take home message

The single macronutrient protein consists of a group of essential nutrients: the amino acids. These molecules are what is needed in our diet to construct the diverse body proteins, essential to bodily function, health and life.

Getting enough protein in your diet is not just about reaching the protein RDI. Instead, you need to reach the RDI for each essential amino acid. This is most easily achieved by eating high-quality protein, or combinations of protein sources with complementary amino acid contents.

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Glossary

References for DIAAS values: 1, 2, 3, 4

Green car photo by MW on Pixabay. All other photos courtesy of the Riddet Institute.