Amino acids are the building blocks that make up proteins. To get the amino acids needed for cell and tissue growth and maintenance, our body needs the protein found in food. Our needs for protein in our diets vary as we age. Adults should eat at least 0.83 g of protein per kilogram of body weight per day, according to the European Food Safety Authority (EFSA) (58 g/day for an adult weighing 70 kg). The quality and digestibility of proteins derived from plants and animals differ, although most people don’t normally care if their protein intake is sufficient overall. Our goal should be to eat a diverse range of protein sources that are good for the environment and our health.
What materials make up proteins?
Proteins are composed of numerous
Proteins are made up of many different amino acids linked together. There are twenty different of these amino acid building blocks commonly found in plants and animals. A typical protein is made up of 300 or more amino acids and the specific number and sequence of amino acids are unique to each protein. Rather like the alphabet, the amino acid ‘letters’ can be arranged in millions of different ways to create ‘words’ and an entire protein ‘language’. Depending on the number and sequence of amino acids, the resulting protein will fold into a specific shape. This shape is very important as it will determine the protein’s function (e.g. muscle or enzyme). Every species, including humans, has its own characteristic proteins.
Amino acids are classified as either essential or non-essential. As the name suggests, essential amino acids cannot be produced by the body and therefore must come from our diet. Whereas, non-essential amino acids can be produced by the body and therefore do not need to come from the diet.
What do proteins do for the body?
Our bodies are made up of thousands of different proteins, each with a specific function. They make up the structural components of our cells and tissues as well as many enzymes, hormones and the active proteins secreted from immune cells (figure 1).
These body proteins are continually being repaired and replaced throughout our lives. This process (known as ‘protein synthesis’) requires a continuous supply of amino acids. Although some amino acids can be recycled from the breakdown of old body proteins, this process is imperfect. This means we must eat dietary protein to keep up with our body’s amino acid demand.
As protein is essential for cell and tissue growth, adequate intake of protein is particularly important during periods of rapid growth or increased demand, such as childhood, adolescence, pregnancy, and breastfeeding.1
What foods are high in protein?
Protein can be found in both plant and animal-based foods. Figure 2 shows the protein content found in a typical serving of common animal and plant-based foods. For more information on how to estimate healthy portion sizes, see measuring portion sizes with your hands.
Is there a difference between animal and plant-based proteins?
As we can see in Figure 2, both animal and plant-based foods can be rich sources of protein. But do they have the same quality?
The quality of a protein can be defined in many ways; however, all definitions relate to the distribution and proportion of essential and non-essential amino acids they contain. In general, animal-based proteins are of higher quality as they contain higher proportions of essential amino acids compared to plant-based proteins.
There is a common misconception that plant-based proteins completely lack certain essential amino acids. In fact, most plant-based proteins will contain all 20 amino acids but tend to have a limited amount of certain essential amino acids, known as their limiting amino acid(s). This means, if a small number of plant foods are consumed as the only protein sources, they are unlikely to supply enough essential amino acids to meet our requirements. For people who consume little to no animal-based foods, such as vegans or vegetarians, it is important that they consume protein from sources with complementary limiting amino acids. For example, consuming rice (limited in lysine and thiamine but high in methionine) and beans (limited in methionine, but high in lysine and thiamine) will provide complementary amino acids that can help meet essential amino acid requirements.
Animal and plant-based proteins also differ in their bioavailability and digestibility. The digestible indispensable amino acid score (DIAAS) is the recommended method for determining dietary protein digestibility and is expressed in values below or sometimes even above 100.3 A DIAAS of over 100 indicates that the protein has very high digestibility and quality and is a good complement protein to those that have lower qualities. Animal-based proteins tend to have higher DIAAS scores compared to plant-based proteins (Table 2). As most people consume protein from a variety of sources the quality and digestibility of protein is not usually a concern.
How much protein should we eat every day?
EFSA has developed dietary reference values (DRVs) for protein. The DRVs for protein at different life stages are summarised in table 3. For an average adult, the recommendation is to consume at least 0.83 g of protein for every kilogram of body weight per day.1 In other words, a 70 kg adult should aim to eat at least 58 g of protein everyday day. This is the equivalent to the protein found in around 200 g of chicken breast or 240 grams of mixed nuts.
During periods of growth, such as childhood, pregnancy, and breastfeeding, protein requirements are relatively high.
What are the health benefits of protein?
Eating enough protein to meet our bodies’ requirements is important for many body functions. However, there is evidence to suggest that in certain situations increasing protein intake above required levels could provide additional health benefits.
Protein and Weight Control
Eating protein-rich foods has been shown to increase our feeling of fullness (also known as satiety) more than foods high in fat or carbohydrate. There is good evidence from short term studies that diets high in protein (i.e. 1.2 – 1.6 g/kg per day; 84 – 112 g per day for a 70 kg adult) can help reduce overall calorie intake and prompt weight loss. 5 However, the evidence for long-term weight maintenance is less clear.5 Like all diets, a high protein diet is only effective if it is stuck to, which can be difficult for some people and low adherence may partly explain the limited benefit observed for long term weight maintenance.5
Protein and sarcopenia
Sarcopenia is a disorder characterised by the progressive loss of muscle mass and physical function that is commonly associated with older adults. Sarcopenia is associated with increased frailty, risk of falls, functional decline and even early death.6 As protein is essential for the repair and maintenance of muscle mass, it is no surprise that low intake of protein is associated with an increased risk of developing sarcopenia.6 Similarly, increasing protein intake, as well as increasing physical activity can help maintain muscle mass and strength as we age, decreasing our risk of sarcopenia and skeletal disorders.
Protein and athletic performance
Protein has long been associated with athletic performance. Protein plays a key role in helping to repair and strengthen muscle tissue after exercise. Although protein is critical for building muscle, to maximise the benefits it should be considered in the context of the whole diet, which includes the right amount of carbohydrates, fats, vitamins and minerals. Optimum protein intake will depend on the type (e.g. endurance or resistance training), duration and intensity of exercise, with more not always being better. A protein intake of 1.4–2.0 g per kg body weight a day (e.g. 98 – 140 g per day for a 70 kg adult) is thought to be sufficient to meet the needs for most exercising individuals.7 Athletes should aim to achieve protein intakes through consuming a balanced diet, with protein supplements being used for individuals who need to keep protein high but limit total calorie intake.