Protein: we need quality, not just quantity


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.

FoodAmino acid bioavailability
(% of total consumption that is absorbed)
Roasted beef94 – 99  
Fish81 – 94
Cooked kidney beans64 – 100
Oats70 – 88
Potatoes47 – 66
Rice75 – 99
Skim milk78 – 97
Cooked soyabeans71 – 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.

FoodDIAASLimiting amino acid(s)
Beef (roasted)99Leucine and Valine
Pea protein concentrate82Methionine and Cysteine
Rice (cooked)60Lysine
Skim milk powder105Methionine and Cysteine
Soya protein isolate84Methionine and Cysteine
Wheat45Lysine

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.


Glossary

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

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

Higher atmospheric CO2 changes the nutritional quality of vegetables

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Vegetables grown at higher carbon dioxide (CO2) concentrations may grow better but may not have the same nutritional benefits. 

Increasing atmospheric CO2 concentrations have prompted research into the effect of this phenomenon on plant growth. In general, elevated COis good for plant growth, increasing yield and environmental stress tolerance. However, a review of the research in this field has found that elevated CO2 also reduces the magnesium, iron and zinc content of vegetables. This reduction was as much as 31% for iron in leafy greens. 

In specific vegetables, the review found that sugar content of lettuce, tomatoes and potatoes increased at higher atmospheric CO2 concentrations, while protein content decreased. Other factors, such as antioxidant content, were strongly affected, but this effect was different between different vegetable cultivars. 

Higher yields with lower protein content have also been found for staple crops and grains grown at elevated CO2 concentrations. These changes occurred alongside reduced iron and zinc content. 

In a future with increased atmospheric CO2 concentrations, our crops and vegetables may grow larger and sweeter, but the amounts of other essential nutrients that we get from them may decrease. This could lead to higher caloric intakes required to obtain the same amount of nutrients from these foods. While the concentrations reviewed in this publication were high compared to those expected in the near future, we should be prepared for some degree of impact on our crops. Targeting crop varieties which make the best of the changing conditions is being explored.

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The role of sugar in a sustainable food system

It is commonly heard that high consumption of sugar is linked to obesity, type 2 diabetes and other life-threatening diseases. However, a less discussed problem comes from the fact that sugar displaces more nutrient-rich foods in the food system and in individual diets.

The DELTA Model shows that based on all food currently produced, there is sufficient energy and macro-nutrients to meet the nutrient requirements of the global population – assuming that all food were distributed evenly. However, shortages lie in key micro-nutrients and trace elements such as iron, calcium and zinc. This creates a challenge to meet nutrient requirements without a problematic excess of calories, and within planetary resource constraints.   

Because sugar supplies us with energy and carbohydrates, but little else, it provides minimal nutritional value. The most important aspect of a sustainable food system is that it must meet the nutrient requirements of the population. It can therefore be claimed that sugar is unsustainable in the fact it does very little to meet this requirement. According to the FAO, approximately 2.4 billion tonnes of raw sugar cane and beet is produced per year. This is nearly a quarter of the total food biomass from the world’s farms and oceans, and utilises 31 million hectares of land. This land could instead be used to produce more nutrient-rich foods like pulses, which are high in vitamins and minerals such as zinc, iron and folate.  

On a more individual level, poor choice or lack of choice can lead to an excess of calorie-dense and nutrient-poor foods, such as those high in sugar. If this displaces nutrient-rich foods, ‘hidden hunger’ or micro-nutrient deficiencies can arise, despite the individual eating sufficient calories. It would be unrealistic to believe you could remove sugar from the food system altogether – some sugar and sugar derived production will be needed. However, sugar and other energy-rich and nutrient-poor foods should be prioritised as a target for reduction.


Glossary

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Plant-based beverages are not suitable as milk replacements for young children

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The North American Society for Paediatric Gastroenterology, Hepatology, and Nutrition (NASPGHAN) has released a nutritional position paper highlighting the deleterious effects of using plant-based alternatives to milk on infant development and health.

There are many of beverage choices available to Western consumers. These include plant-based products that are positioned as alternatives to milk. Influenced by trends such as ‘plant-based’ or ‘animal free’ and with perceived health benefits associated with the name ingredient, sales of these products have seen rapid growth.

The consumption of plant-based beverages is a matter of consumer choice and preference, appropriate in a diet that contains a balance of nutrients. However, problems arise when they are used as a replacement for dairy in cases where milk is the primary source of nutrition: for infants and young children. 

By association and with the use of dairy terms, plant-based beverages leverage the nutritional credentials of milk. This leads consumers to believe they are getting (or providing for their children) a nutritional equivalent to milk, but in a healthier way, or with a lower carbon footprint, due to the perceived halo of plant-based products. However, this is often not the case. Many plant-based alternatives fall well short of dairy nutrient content, without considering differences in the bioavailability of nutrients between plant and animal sources.  

The paper puts emphasis on protein, which is particularly important in the growth of young children.  Due to a combination of low protein content and poor protein quality, one serve of almond or rice beverage may provide only 2% or 8% of the dietary protein of an equal sized serve of cows’ milk, respectively. The paper also recommends bioavailability studies for products that have been fortified to match other nutritional characteristics of milk (e.g. calcium). 

The paper makes clear the need for consumer education to ensure that children are given the right foods for their nutrient needs. 

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Glossary

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Microbiomes and Sustainable Nutrition


Did you know that around 10% of your daily energy intake is supplied by intestinal microbes? Or that many plants and animals that we rely on for food are dependent on microbes for their survival? Although the connections between the microscopic world and the global scale of sustainable nutrition are not obvious, microbes play a significant role in the way our food is produced, processed and digested.

The term microbiome refers to a collection of microbes in a certain location. For example, the human gut microbiome consists of the microbial population living in our intestinal tract, which is receiving increasing attention as we recognise its importance in human health.

Microbiomes exist in diverse locations, many of which form part of the global food system. The role of these microbiomes in delivering sustainable nutrition for the global population is increasingly clear.

Cereal crops are a staple food source for the global population, providing predominantly energy and protein. These crops rely on soil nutrients, such as nitrogen, to grow and produce the protein we then consume. Often these nutrients are applied to cropland as fertiliser, produced either industrially or from animal sources. Management of fertiliser application is essential to avoid environmental damage caused by excess nutrients in soils and waterways.

Nitrogen can also be captured directly from the air by soil and root microbiomes, and microbes associated with roots can increase the availability of micronutrients to the plant. These microbes also increase the resistance of crops to soil pathogens. Moreover, soil microbes play a role in reducing soil erosion by producing products that bind the soil together. Current soil microbiome research is tackling the problem of reduced crop yields due to microbiome depletion and working to understand how the beneficial impacts of soil microbes can be harnessed. Learn more

In addition to plant-sourced food products, microbiomes are essential in the production of animal-sourced foods. An example of this is the rumen microbiome. Much of the forage consumed by ruminants cannot be digested by the animal’s own digestive enzymes; instead, the action of rumen microbes converts resistant plant matter, such as cellulose, to nutrients that can be absorbed by the animal’s digestive tract. These microbial products form the majority of energy intake for many domesticated ruminants. The action of the rumen microbiome is thus an important step in converting inedible plant material into animal-sourced food products in our own diet.

Rumen microbiome research currently has a strong focus on minimising the production of methane, a greenhouse gas and by-product of digesting plant material, by the rumen microbiome. This research is unpacking what causes certain microbiomes to produce less methane than others, and what the impact of different animal feeds is on methane production. Learn more

Continuing along the food supply chain, microbes are responsible for the production of common fermented foods. Fermented foods include cheeses, yoghurts, kimchi, sourdough and fermented meats, and are produced via the introduction of microbial populations to the raw food material. Apart from changing the taste, texture and appearance of these foods, the fermentation process enables perishable foods to be stored for longer periods, which can reduce food waste. The nutritional value of fermented foods is also enhanced in many cases. For example, the fermentation of cabbage to sauerkraut results in vitamin B12 synthesis, a nutrient not available in unfermented cabbage. There is also the probiotic capacity of fermented foods: their consumption can introduce beneficial bacteria to the human gut microbiome. Learn more

Microbiomes continue to play a role in the food system even after food is eaten. Although there are microbiomes in different sections of the human digestive system, the gut microbiome is intensively studied for its impacts on human nutrition and health. The make-up of our microbiome is in part determined by our diet, which forms the major food source for intestinal microbes. Just as our own ten trillion human cells require the nutrients we eat to carry out their function, so too do our equally numerous microbial cells. Current research is demonstrating increasing links between gut microbiome composition and various outcomes for human nutrition and health. This includes links to energy and nutrient yield from the diet, roles in intestinal disease and even impacts on brain function and mood. It is now recognised that we cannot have a full appreciation of human nutritional health without consideration of the gut microbiome. Learn more

A sustainable food system is one that ensures food security and nutrition for all, without compromising the future of the economic, social and environmental bases that the system depends on. Microbiomes are a critical element of a sustainable food system. Soil microbiomes enable and enhance crop growth, while playing a protective role in minimising the environmental damage of farming. Animal microbiomes are essential for the conversion of inedible plant material to animal-sourced foods, essential for food security in many developing parts of the world. Fermented foods are an integral constituent of the diet in many cultures and provide a means of preserving perishable foods, as well as adding nutritional and financial value. Finally, the human microbiome in part determines the nutrition we obtain from the foods we eat.

Microbiomes are present throughout the food system, and touch on all aspects of sustainability. As such, designing sustainable food systems for the future must involve consideration of the microbial element.


Glossary

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From a good idea to reaching millions: learning from CGIAR’s work on biofortification

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Consultative Group on International Agricultural Research (CGIAR) have been developing and implementing biofortified crops to address micro-nutrient deficiencies.

Deficiencies in micro-nutrients poses serious and widespread threats to health and economic development. This is known as ‘hidden hunger’. The conventional response has been supplementation or food fortification. However, these solutions involve high and recurrent costs, can be hard to organize in poor rural areas, and cannot always solve the problems.  CGIAR scientists proposed that the same health impacts could be achieved by breeding vitamins and minerals into the staple crops that people eat every day, such as sweet potato, wheat and rice. This is known as ‘biofortification’. CGIAR have been working on this for almost 25 years and invested $900m into development and implementation. More than 290 new varieties of 12 biofortified crops have been released or are in testing. This has benefited 10 million farming households globally to date.

The DELTA model can be used to scenario test various food systems with the view of adequate sustainable nutrition for the global population. This repeatedly demonstrates that on a global scale, animal-sourced foods are needed to meet nutrient requirements. However, this is based on the fact that current conventional crops do not have the same content of bioavailable micro-nutrients and trace elements that animal-sourced foods do. There may be potential for biofortified plants to better contribute to global nourishment and reduce requirements for animal foods. However, what is still unclear is whether those micro-nutrients in biofortified plant-based foods would have the enhanced bioavailability that characterises animal-sourced foods. In addition, biofortified plant-based foods may not have the ability to enhance the uptake of micro-nutrients from plant-derived sources, in the same way animal foods do as part of a meal. For example, haem iron from meat helps with the uptake of non-haem iron from plant sources. The ability of biofortified plants to do the same needs to be determined before concluding that biofortified crops can replace the role of animal foods in the global food system.

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Glossary

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