We all understand that growing children need protein, but we often underestimate how much protein we needs as adults. We need a constant supply of protein to maintain and repair tissues, and all the proteins in our bodies need to be replaced at least every year. Many of us don't eat enough protein, which puts us at risk of muscle weakness and poor metabolic health, but can also affect other aspects of health, including digestive health and mental wellbeing. Here we describe why protein matters and how to ensure you eat an optimal amount every day to keep healthy and strong!
Protein and amino acids
Proteins are large, complex molecules that are needed for the structure and function of all cells in the human body. Protein is one of the fundamental macronutrients, along with carbohydrates and fats. Proteins are made up of smaller units called amino acids, which are linked together in specific sequences.
Our bodies are roughly made up of 60% water, with half of the remaining 40% made of protein. All the proteins in our bodies need to be replaced at least every year, so we need a constant supply of good quality protein from food in order to replace and renew all of the proteins in our bodies on a regular basis.
The protein we eat provides the essential amino acids we need to build or replace proteins in the body. There are twenty amino acids that help form the thousands of different proteins in the human body. Nine amino acids are considered essential as the body cannot make them, so they have to be eaten in food.
Protein and health
It is impossible to enjoy good health without an adequate intake of good quality protein. We are unable to store protein, and humans can only survive for seventy days, at most, without protein. Here are some of the major ways that protein is essential for health.
Muscles, bones, tendons, ligaments and other structures - Protein is needed for the maintenance, repair, and growth of muscles and bones and provides structural support to cells, tissues, and organs. Collagen is a protein that is a major component of connective tissues, such as skin, tendons, and bones. Proteins, such as actin and myosin, are essential for muscle contraction
Digestion and gut health - Although this may sound surprising, eating enough good quality protein is also essential for digestive health. Proteins are needed to make digestive enzymes which are crucial for proper digestion. Amino acids from protein contribute to the maintenance of gut integrity as they support the repair and regeneration of the mucosal lining of the gastrointestinal tract, which is essential for a healthy digestive system.
Immune health - Our immune systems relies on proteins for proper functioning, as amino acids are used to make Immunoglobulins, antibodies, and other immune-related proteins that are crucial for defending the body against pathogens.
Hormones – protein is needed to make hormones like insulin, which regulates blood glucose and thyroid hormones, which help regulate your metabolic rate and can affect the secretion of growth hormone and bone health.
Neurotransmitters – Proteins are needed to make neurotransmitters such as dopamine, serotonin, and adrenaline, which are so important for the brain and for regulating neurological development, mood and sleep.
Transportation - Some proteins act as carriers, transporting molecules such as oxygen (haemoglobin in red blood cells), nutrients, and ions across cell membranes.
Maintaining fluid balance and pH regulation - Proteins help regulate fluid balance in the body by controlling the movement of water between the blood and surrounding tissue. They also help maintain the body's pH balance within a narrow and optimal range.
Overall, proteins are fundamental to the structure and function of cells and are involved in virtually every aspect of the body's biological processes. So, you can see how if you’re not getting enough protein, it can affect your health in many different ways.
Protein and weight loss
Eating enough protein is also really helpful in weight loss. Clinical trials show that high protein diets are more satiating than traditional calorie-controlled diets. By eating plenty of protein, you are less likely to over-eat, as feelings of hunger will be suppressed. Higher protein increases levels of peptide YY, a gut hormone that drives feelings of fullness after eating, and reduces ghrelin, which drives hunger. The thermic effect of protein will also lead you to burn more of the calories you consume. By eating more protein, you also increase your metabolic rate.
Protein turnover in the body
The human body needs to make around 300g protein each day to repair or replace proteins in the body. Every protein in the body is replaced on a regular basis and then rebuilt. The frequency of the turnover depends on the tissue. For example, some liver proteins break down and are replaced every hour, whilst muscle protein turns over approximately every 30 days and collagen turns over approximately every 100 days.
Some of the proteins which are broken down are re-used and together with protein consumed in meals, are used to make new proteins in the body.
The body prioritises the liver, heart, brain kidney and gastro-intestinal tract, and if not enough protein is consumed, skeletal muscle growth and repair will be compromised.
Protein turnover requires anabolic and catabolic processes:
How we use protein to build and maintain muscles
Muscle tissue is dynamic and in a constant state of turnover and remodelling with muscle proteins being both synthesised and broken down simultaneously throughout the day.
Muscle protein synthesis (MPS) - is a metabolic process in which amino acids are incorporated into muscle protein, supporting the maintenance or building of muscle mass
Muscle protein breakdown (MPB) – is the opposing process to MPS and involves the breakdown of muscle protein.
What happens as we age?
As we get older, our response to eating protein or undertaking resistance exercise becomes blunted, and this is known as Anabolic Resistance. This may lead to a chronic imbalance between MPB and MPS and a loss of muscle. Over time the progressive and generalised loss of skeletal muscle mass and strength, called sarcopenia, leads to frailty, problems with mobility, a greater risk of fractures and falls and a reduction in metabolic rate.
Factors that contribute to anabolic resistance are shown below:
However, whilst anabolic resistance is associated with ageing, the process of resistance actually starts in our 40’s. And it worsens over time!
It is also important to ensure you eat enough food overall as without sufficient energy intake anabolic resistance will worsen.
To understand what we can do about the effect of anabolic resistance, we need to look at how the body makes new muscle protein.
How we make new muscle protein
As we get older it becomes more difficult for us to make new muscle protein and certain conditions need to be met to stimulate MPS. New muscle protein is only created under certain conditions:
Sufficient protein must be consumed at a single meal.
The protein in the meal must include enough of the amino acid Leucine.
The leucine threshold
Research shows that a particular amino acid, Leucine, is needed in order to stimulate the synthesis of protein (MPS), and a meal has to contain enough leucine or otherwise MPS will not be triggered. It is now known that leucine plays a key role, which is to trigger mTOR, (the mechanistic Target of Rapamycin) which is a key regulator of fundamental cell processes such as MPS and autophagy.
If the amount of leucine contained in a single meal is insufficient, MPS will not be triggered. The dose of protein either contains enough leucine and it will trigger MPS, or it does not, and then MPS will not be triggered.
When we’re in our twenties or thirties we need 1.7g of leucine to stimulate mTOR and initiate MPS, however as we get older the threshold for triggering MPS is at least 2.5g of leucine at a single meal.
How much protein do we need to eat every day?
Whilst government guidelines recommend we consume 0.75g per kg of bodyweight, this is really a minimum requirement. Today, many nutrition experts consider these levels too low and a higher intake of protein is considered optimal for most adults, and particularly for older adults. Research now suggests that an optimal level of protein is 1.2 - 1.6g per kg of ideal bodyweight.
Recommended intake of protein
A daily intake of 1.2 – 1.6 g of good quality protein per kg of ideal bodyweight is recommended for adults, with older adults aiming for at least 1.6g/kg of bodyweight per day.
(You can use your actual weight to estimate your protein requirements, however if you are either underweight or overweight it is recommended that you use the weight recommended for your height).
A woman of average weight (9.5 stone) will need approximately 75 - 96g protein per day, with older women aiming for 96g/kg per day.
A man of average weight (11stone) will need approximately 80 - 112g of protein per day, with older men aiming for 112g/day.
Distribution of protein and the importance of breakfast!
It is important that daily protein intake is spread over the day, rather than mainly eaten in the evening, as quite often is the case. To maintain healthy muscles and bones, adults are recommended to include at least 30g of protein at two or more meals each day. This will ensure that the leucine threshold is met and MPS stimulated at a minimum of two times every day.
Research suggests that 90g of protein eaten in three equal servings at breakfast, lunch and dinner will more successfully stimulate MPS than a skewed distribution of 90g protein over three meals (10g, 20g and 60g) as shown in the diagram below.
Protein distribution at meals. A) Ingestion of 90 grams of protein, distributed evenly at 3 meals. B) Ingestion of 90 grams of proteins unevenly distributed throughout the day. Stimulating muscle protein synthesis with three equal portions of protein is more likely to provide a greater 24 hour protein anabolic response than the unequal protein distribution (Layman, 2009, adapted from Paddon-Jones & Rassmussen Curr Opin Clin Nutr Metab Care 2009, 12: 86–90.)
Breakfast or the first meal of the day, whenever taken, is an important meal for dietary protein. This is because the body is in a catabolic state after an overnight fast. A meal with at least 30 g of good quality protein is needed to switch the body from a catabolic to an anabolic state, and trigger muscle protein synthesis which will enable the body to replenish proteins during the early part of the day.
Recommendations:
Distribute protein intake across the day
Aim for 30-40g of good quality protein at breakfast.
Protein quality
Whilst we often refer to protein, it is really the amino acids in the protein-rich foods that are important, and not all proteins are equal when it comes to the range and quantity of amino acids! Whilst animal sources of protein contain all nine essential amino acids in good quantities, plant sources of protein are generally incomplete as they do not contain all nine amino acids in sufficient quantities. Anti-nutrients in plant foods, such as oxalates and tannins may also reduce amino acid availability.
Protein quality is measured by assessing the digestibility and bioavailability of the amino acids in the protein. Animal-based proteins are highly bio-available and contain all nine essential amino acids in good quantities, including leucine, which is required for MPS. Plants generally have a lower digestibility score than animal foods. For example, studies show that whey supplements increase MPS more than soy protein supplements.
Getting enough protein as a vegetarian or a vegan
It is certainly possible to meet daily protein requirements asa vegetarian, particularly if dairy foods and eggs are included in the diet. Higher levels of protein are needed on a plant-only diet and foods are best eaten in combination so that the all nine essential amino acids are eaten at each meal, as is recommended.
Soy, which is used to make foods such as tofu, has historically been considered the best plant-based source of protein as it contains all nine essential amino acids, although low in methionine. Another good choice is Quorn or mycoprotein, which is a fungal protein that has a high protein density and a similar amino acid profile to dairy foods.
Combining plant sources of protein ensures a meal contains all nine essential amino acids – for example, pairing legumes with grains or legumes with nuts or seeds. More advice on this is available from the Vegetarian Society.
Total Protein and leucine content of foods
Foods which are derived from animal proteins, such as meat, fish and dairy roughly contain 20 - 30g protein per 100g of food, so these foods are around 25-30% protein. So, for example, if you eat 100g of chicken that will provide 32.1g of protein as shown in the infographic below. Plant sources usually contain 10-20g protein per 100g of food, with soy beans having one of the highest proportions of protein amongst plant based sources.
The specific amino acid Leucine, which is needed to trigger MPS, will also vary by food type – aim for a minimum of 2.5g of leucine per meal.
Summary of Recommendations
Protein is vital for good health, and we need a constant supply of good quality protein every day. Our protein requirements increase as we get older as anabolic resistance from our forties onwards limits our ability to trigger muscle protein synthesis.
For optimal health the following is recommended:
References
Layman DK. Dietary Guidelines should reflect new understandings about adult protein needs. Nutr Metab (Lond). 2009 Mar 13;6:12. doi: 10.1186/1743-7075-6-12. PMID: 19284668; PMCID: PMC2666737. https://pubmed.ncbi.nlm.nih.gov/19284668/
Government Dietary Recommendations Government recommendations for energy and nutrients for males and females aged 1 – 18 years and 19+ years.
Rogeri, P.S, et al, Strategies to prevent Sarcopenia in the aging process: Role of protein intake and exercise. Nutrients 2022, 14, 52. https://doi.org/10.3390/nu14010052
Kim,I.Y. et al .Quantity of Dietary Protein Intake, but Not Pattern of Intake, Affects Net Protein Balance Primarily through Differences in Protein Synthesis in Older Adults. Am. J. Physiol. Endocrinol. Metab. 2015, 308, E21–E28
Paddon-Jones D, Rasmussen BB. Dietary protein recommendations and the prevention of sarcopenia. Curr Opin Clin Nutr Metab Care. 2009 Jan;12(1):86-90. doi: 10.1097/MCO.0b013e32831cef8b. PMID: 19057193; PMCID: PMC2760315. https://pubmed.ncbi.nlm.nih.gov/19057193/
Putra, C eet al ( 2021) Protein Source and Muscle Health in Older Adults: A Literature Review. Nutrients 2021, 13(3), 743; https://doi.org/10.3390/nu13030743
Coelho MOC, Monteyne AJ, Dunlop MV, Harris HC, Morrison DJ, Stephens FB, Wall BT. Mycoprotein as a possible alternative source of dietary protein to support muscle and metabolic health. Nutr Rev. 2020 Jun 1;78(6):486-497. doi: 10.1093/nutrit/nuz077. PMID: 31841152.
Drummen M, Tischmann L, Gatta-Cherifi B, Adam T, Westerterp-Plantenga M. Dietary Protein and Energy Balance in Relation to Obesity and Co-morbidities. Front Endocrinol (Lausanne). 2018 Aug 6;9:443. doi: 10.3389/fendo.2018.00443. PMID: 30127768; PMCID: PMC6087750.
Lyon, Gabrielle, Forever Strong, A new science -based strategy for aging well, Piatkus, London 2023