Structure and Function
Proteins are large molecules comprising amino acids (AAs) linked together by peptide bonds, and thus serve as the ‘building blocks’ of proteins (1). A protein generally contains differing amounts of a total of 20 individual AAs (2), consisting of 9 essential AAs (EAAs) and 11 non-essential AAs (NEAAs)(1). EAAs must be consumed via dietary means since they cannot be produced in the body (3).
Protein is essential for the growth and repair of tissues and maintenance of good health (1). Importantly, for muscle growth, besides providing the building blocks for muscle, ingestion of protein provides the trigger to stimulate muscle protein synthesis (MPS), the physiological process of adding new amino acids to muscle proteins (4). The biochemical activity of a protein is governed by its conformation, which is determined by the arrangement and properties of its amino acid constituents (1). Proteins have both necessary structural and functional responsibilities within every cell in the human body. Structural proteins serve to maintain cell shape and constitute structural elements in connective tissues like cartilage and bone in vertebrates. Other types of protein in the body include enzymes, which catalyse or “speed up” biochemical reactions in cells; and hormones, chemical signals which serve to regulate specific physiological processes such as growth and development, metabolism, and reproduction (5).
Reference Nutrient Intakes / Protein Requirements
Dietary reference values in place are based on estimation of daily protein needs, which varies throughout a lifetime (1). Protein recommendations are generally given relative to your body weight, rather than in absolute terms. For all adults, the Reference Nutrient Intake (RNI) is set at 0.75g of protein per kilogram of body weight per day (1).
-If an adult weighs 74 kg, they will need: 74 x 0.75 g/day = 55.5 g protein per day (1)
-Supposing they have a total energy intake of 2000 kcals per day, expressed as a percentage (%) of energy, their protein intake (55.5g x 4kcals = 222kcals) would equate to 11% of daily energy intake.
Notably, the guidelines set are based on the “sedentary” individual, therefore do not acknowledge the need for a greater protein intake recommended when accounting for exercise. For the majority of exercising individuals, for the maintenance or growth of muscle mass, an overall daily protein intake in the range of 1.4–2.0 g protein/kg body weight/day (g/kg/d) depending on the level of physical activity is deemed sufficient, which aligns with the Acceptable Macronutrient Distribution Range published by the Institute of Medicine for protein (3).
Disadvantages of an Inadequate Intake
Consuming an inadequate amount of protein in the diet thus failing to meet the RNI for protein may result in protein deficiency and have negative health implications (6). This problem is often worsened when coupled with an insufficient energy intake (6).
Some of the most common consequences include muscle atrophy and high risk of infection (6). Given that dietary protein is required to repair and build muscle mass, an insufficient protein intake will likely lead to a reduction in lean body mass, muscular strength and function. When energy intake is also low, the body will break down muscle protein for energy to support other bodily functions, thus causing muscle atrophy or “wasting” over time (7). An adequate protein intake is required for the immune system to function optimally, thus, it is suggested that a less than adequate intake may impair immune function. The outcome is an increased risk of infection as the immune response to combating infection is reduced (7).
Sources (High and Low quality)
Dietary proteins are often found in both animal-based and plant-based food sources, however levels of different AAs and AA combinations between foods varies (1). Protein quality (i.e. EAA content) is a key determining factor in the anabolic response to a protein (i.e. MPS) (8), and is determined by the level and relative proportions of its EAA content (1). Protein sources containing higher levels of the EAAs are considered to be higher quality sources of protein (3). Animal-based protein sources include meat, dairy products, eggs, poultry (e.g. chicken), seafood (e.g. oily fish), and other products, which provide greater quantities of the complete range of EAAs required by the body, than plant-based protein sources, referred to as “incomplete” proteins, including rice, wheat, corn, potato, vegetables, cereals, beans, peas, processed soy products, nuts, and seeds (9, 10).
In the UK diet, protein intake is derived from a variety of foods with differing proportions of AAs. Thus, for those following strictly plant-based diets (i.e. vegan and vegetarian), combining different plant-based sources with a comparatively less sufficient EAA content e.g. pulses and cereals, will largely compensate for the limited EAA content of one or the other source and result in a higher biological value protein combination (1).
To minimise saturated fat intake, It is important to source protein from lower fat foods, such as lean meats (e.g chicken) or reduced fat dairy products (e.g. skimmed milk) to minimize saturated fat intake (1).
British Nutrition Foundation (2018). Protein - British Nutrition Foundation - Page #2. [online] Available at: https://www.nutrition.org.uk/nutritionscience/nutrients-food-and-ingredients/protein.html?start=5.
Cosar, A. and Ozcan, O. (2014). “Reliable cut-off selection for total plasma homocysteine levels in prepubertal chilren”. Nutrition, 30(2), p.240.
Jäger, R., Kerksick, C., Campbell, B., Cribb, P., Wells, S., Skwiat, T., Purpura, M., Ziegenfuss, T., Ferrando, A., Arent, S., Smith-Ryan, A., Stout, J., Arciero, P., Ormsbee, M., Taylor, L., Wilborn, C., Kalman, D., Kreider, R., Willoughby, D., Hoffman, J., Krzykowski, J. and Antonio, J. (2017). “International Society of Sports Nutrition Position Stand: protein and exercise”. Journal of the International Society of Sports Nutrition, 14(1).
Trommelen, J. (2019). Perfecting Protein Intake in Athletes: How Much, What, and When?. [online] Stronger by Science. Available at: https://www.strongerbyscience.com/athlete-protein-intake/.
Khan Academy. (n.d.). Introduction to proteins and amino acids. [online] Available at: https://www.khanacademy.org/science/biology/macromolecules/proteins-and-amino-acids/a/introduction-to-proteins-and-amino-acids.
Wu, G. (2016). “Dietary protein intake and human health”. Food & Function, 7(3), pp.1251-1265.
Leal, D. (2019). The Effects of Protein Deficiency. [online] Verywell Fit. Available at: https://www.verywellfit.com/what-are-the-effects-of-protein-deficiency-4160404.
Gorissen, S., Horstman, A., Franssen, R., Crombag, J., Langer, H., Bierau, J., Respondek, F. and van Loon, L. (2016). Ingestion of Wheat Protein Increases In Vivo Muscle Protein Synthesis Rates in Healthy Older Men in a Randomized Trial. The Journal of Nutrition, 146(9), pp.1651-1659.
FAO (2013). Dietary Protein Evaluation in Human Nutrition: Report of an FAO Expert Consultation. [online] Available at: http://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf.
Li, X., Rezaei, R., Li, P. and Wu, G. (2010). “Composition of amino acids in feed ingredients for animal diets”. Amino Acids, 40(4), pp.1159-1168.