Protein: Quality over Quantity. What does it mean. How do we measure it and what factors influence it?
Quality is not an act, it is a habit.
Not all protein is created equal. The quality of protein is arguably more important that the quantity. In our previous article, Protein - the nutrition facts, we discussed the total amount of protein needed in the diet to maintain our weight or stay in equilibrium. We are constantly making and repairing proteins just about everywhere in our bodies and this creates a turnover of protein that has to be replaced.
Think of hair that is growing and being lost each day, skin that flakes off all the time, nails growing, protein in our urine and bleeding from cuts. These are small protein losses that all have to be replaced. Our bodies are also constantly fixing cells, which requires protein. Proteins are made up of amino acid molecules linked together. We need to consume the protein, or more correctly, the amino acids that make up all proteins, to keep these processes fuelled.
Any googling around protein requirements will throw up terms such as the Protein Quality, Essential Amino Acid score, Protein Efficiency Ratio and many more. Confused? So were we. It’s taken quite a lot of head scratching to get to grips with this topic so here goes. This is how we think it all fits together. If you spot any flaws in our interpretation please let us know.
Protein nutrition can get very technical very quickly and the meaning gets lost behind the terminology. In a nutshell, to maintain our bodies we have to eat a certain amount of protein each day. It’s probably more useful to think about a need to consume amino acids each day, rather than protein in total. We need different amounts of each essential amino acid each day which add up to our total daily protein need. Different foods have a different mix of each of the amino acids. To meet our daily requirements we need to consume a mixture of protein that provides the correct amounts of each of the essential amino acids.
Remember protein is not just meat. In terms of protein it doesn't matter where we get the amino acids from, they all count.
100 g of meat does not equal 100 g of protein.
Salmon has a high protein content. However in 100 g of salmon roughly 20 g is protein, the rest is made of fats, minerals and water. The majority of the 100 g being water. If you want to know more about how they work out the protein content of food then read here.
We know that of the 22 amino acids needed to make human proteins, our bodies can produce 13 of these without having to ingest the individual amino acid. We therefore have to eat enough of the other 9 essential amino acids in our diet to satisfy our bodies needs for all that repairing and replacing.
All food contains different amounts of the essential amino acids, so we need a way to compare one food with another.
The World Health Organisation (WHO) and others do this by working out the amount of the essential amino acids in each food and comparing this to our daily requirement. The daily requirement has been worked out with various experiments over many years, and where differences exist the WHO have used the higher requirement as the daily recommended figure. If you want to read a lot more about this and how it’s done click here for the WHO report. The recommended daily amounts of each of essential amino acid are shown below in the table.
TABLE OF ESSENTIAL AMINO ACID REQUIREMENTS.
Protein Quality scores
The amount of amino acids in a food is only meaningful if we know how much of the amino acids our bodies can digest and absorb from that food when we eat it. This is known as the digestibility. When we take this into account we can compare foods with each other in terms of protein content and describe the quality of the protein.
There are several ways to describe the quality of a protein in any food. The WHO and other agencies around the world use this information for research and planning for emergencies, so they can provide the most efficient advice when referring to the needs of a population. The information is also used in research of population diets and health. On an individual level it's useful to get an idea of what groups of foods provide us with what amount of our daily amino acid needs.
Protein Digestibility Corrected Amino Acid Score
The most common protein quality measures are the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and the Digestible Indispensable Amino Acid Score (DIAAS).
The PDCAAS is worked out by calculating the amount of each of the essential amino acids contained in a gram of the food. This is compared to a ‘reference’, which is the amino acid needs of an infant human as they are considered to have the highest protein needs. Once you factor in the ‘digestibility’ of the food you can produce a score from 0 - 1, where 1 means that 1g of the food has enough essential amino acids to make or maintain 1g of protein in the body. Scores greater than one are rounded down to 1.
Digestible Indespensible Amino Acid Score
Digestible Indispensable Amino Acid Score is suggested as overcoming some of the criticisms of PDCAAS. It is calculated in a similar manner. The major difference is that they calculate the digestible part based on how much of the protein is absorbed in your small intestine, rather than the whole gut, as this is a more accurate measure of how much of the protein you can actually digest. It also provides scores greater than 1 allowing protein sources to be compared more accurately.
The take home message from these scoring systems is that they are designed to be used on a population level and act as a good rule of thumb when making important decisions such as food and nutrition policies in government. They are not perfect and there are always assumptions that have to be made when designing them, which introduces some degree of error. Given the intended use, assumptions are usually made so that they overestimate rather than underestimate protein needs, but that's entirely appropriate when applying it to a population.
It all adds up
Eating protein sources that have a quality score of less than 1 is by no means a bad thing and that's because the individual protein scores add up. So, if you eat some rice which has a PDCAAS score of 0.5, and combine this with peas which have a score of 0.7, you have a complete protein source that is the equivalent of eating something with a score of 1 to start with. Calculating the overall score for your own diet would be time consuming and probably pointless as the amino acid that one food is missing is likely to be provided by another food in your diet. Eating a good range of foods ensures you are covering all bases.
Whats the big picture?
To keep this all in context we need to remember that protein is only one of many nutritional needs for the body. While it's true that you could get all your protein needs by consuming just one source of protein that contains all the essential amino acids, it probably wouldn't be that fun. A 70 Kg adult needs about 58g of protein daily. Take milk for example. To meet that need you would have to drink about 1.7 litres of milk. That's also about 10 good sized eggs if you didn't fancy the milk option.
Countries eating a ‘western’ diet have an average protein intake of 90 - 100 g per day so it's unlikely that the vast majority of people need to worry about their total protein intake. Time would be better spent ensuring a good mix of different protein sources to cover all the essential amino acids.
Some real world numbers
The table below shows the amino acid content in 100 g of some common foods. The PDCAAS score for each of the foods is shown in brackets. Its easy to see how a combination of foods can easily provide your daily essential amino acid needs without a 'superfood' in sight.
|Amino Acid (s)||mg in 100g Potato (0.95)||% of daily need||mg in 100g Rice (0.5)||% of daily need||mg in 100g Peas (0.7)||% of daily need||Mg in 100g Chicken (0.9)||% of daily need|
|M Methionine + C Cysteine||38||3.6%||267||25.4%||114||10.8%||518||49.3%|
|F Phenylalanine + Y Tryosine||135||7.7%||681||38.9%||314||17.9%||1211||69.2%|
How does this apply to real life?
Protein needs are only one factor in our bodies nutritional requirements, so we shouldn't get too focused on just this one thing. Weight management concerns calories, not protein requirements per se. All food has a calorie content. Weight loss is brought about by consuming less calories than you burn. You still have a protein requirement even if losing weight, gaining weight or maintaining your weight. So reducing calories while maintaining the adequate amount of essential amino acid intake involves choosing food with high nutritional value but lower calories. In the case of protein, this would be high protein quality as discussed and lower calories. In terms of overall health, exercising more and eating a wide range of food that provides a good mix of amino acids is likely to be most sustainable. Its also likely to provide all the other nutritional needs apart from protein.