Proteins

Nutritional Science

PROTEINS

Definition

 

A protein is a linear molecule built from about 20 amino acids joined by peptide bonds. Proteins contain nitrogen unlike fats and carbohydrates.

 

Function

Protein has many roles in the body: as a structural material, enzymes for biochemical reactions, hormone production, antibodies, regulators of gene transcription, and acid-base balance.

An adequate supply of protein is essential for normal growth and health.1 Nine of the 21 amino acids in protein are considered essential because they cannot be synthesised in the human body and therefore must be obtained from the diet. Non-essential amino acids are synthesised from available carbon and nitrogen precursors in the body and some amino acids are “conditionally essential”, because they become essential under certain conditions such as stress or critical illness (Table 1).

Table 1. Essential, non-essential and conditionally essential amino acids

Essential Conditionally-essential Non-essential
Histidine Arginine Aspartic acid
Isoleucine Cysteine Glutamic acid
Leucine Glutamine Alanine
Lysine Glycine Asparagine
Methionine Proline Serine
Phenylalanine Taurine  
Threonine Tyrosine  
Tryptophan    
Valine    

Table adopted from Mueller CM, The ASPEN Adult Nutrition Support Core Curriculum, 2nd edition (2012)2

Glutamine is the most important amino acid for kidney ammonia and bicarbonate production and therefore for maintaining acid-base balance.3

Arginine is a known stimulus for release of growth hormone.4

Leucine plays an essential role in the signalling pathway for muscle protein synthesis (MPS)6 and modulates the phosphorylation level in the MPS pathway.7

Glutathione is a major antioxidant in the body while cysteine is the limiting amino acid for the in vivo production of glutathione.8

Protein needs to be digestible to be of use as nutrition. The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is the preferred method for measuring protein value in human nutrition.9 It is calculated by comparing the indispensable amino acid content of the protein to an individual’s requirements. The PDCAAS has been adopted by the FAO/WHO as the preferred method for the measurement of protein value in human nutrition.9-11 The highest possible PDCAAS is 1.0, which indicates that the amino acid profile of the protein meets or exceeds the requirements for essential amino acids.

Types of protein

Whey protein is a high quality protein with a PDCAAS of 1. About half of its amino acids are essential. Whey protein is high in leucine, isoleucine, and valine, the three branched chain amino acids.12 It also contains glutathione and cysteine. Whey protein has a higher cysteine content than other major proteins such as casein and soy. Whey protein is rapidly digested because it does not coagulate like casein when exposed to gastric acid and is not associated with curd production. Gastric emptying time is therefore faster with whey than with casein, which may lead to improved tube feeding tolerance.13

Casein and soy protein isolate contain all of the essential amino acids in the amounts required to support growth, maintenance and repair and are considered high quality with a PDCAAS of 1).14 These proteins also have unique amino acid profiles and therefore exert their own health benefits: casein is high in glutamine and soy is high in glutamine and arginine.15 It has been suggested that glutamine plays a role in regulating muscle protein balance and muscle glutamine concentrations are correlated with skeletal muscle protein synthesis.16

Soy protein in a diet low in saturated fat and cholesterol may reduce the risk of coronary heart disease (CHD) by lowering blood cholesterol levels.17 The mechanism is not firmly established but may be related to the amino acid composition, modulation of LDL receptor levels and bile acid or cholesterol absorption or excretion, and presence of soy isoflavones.18

Protein deficiency or overload

When low protein intake is coupled with a severely calorie-restricted diet, diseases related to protein energy malnutrition (PEM) can occur. Protein cannot be stored in the body and therefore when energy is limited, dietary protein cannot be utilized for anabolic functions. PEM is a common occurrence in geriatric patients and is associated with poor health outcomes such as reduced bone mass and cognitive function, a disproportionate loss of muscle compared to fat mass, impaired muscle and immune function, anaemia, poor wound healing and recovery after surgery, and mortality.19-21

Catabolism is the breakdown of muscle and other tissuesand takes place to provide an adequate supply of amino acids within the bloodstream to support the normal functions of protein. During catabolism, amino acids are mobilized from the periphery and transported to the liver for gluconeogenesis. During this process, lean tissue loss is inevitable and the patient’s nitrogen balance is disrupted.22

Nitrogen balance is nitrogen input minus nitrogen output. A positive nitrogen balance value is often found during periods of growth or tissue repair, whereas a negative value can be associated with burns, fevers, or illness.A nitrogen:energy ratio of 1:160 and a nitrogen:non-protein energy ratio of 1:133 is an appropriate intake for restoration of lean body mass and optimising negative nitrogen balance in depleted and stressed patients.23

If sufficient carbohydrate and fat are provided in the diet, protein will be used for building or repairing muscle and other tissues, rather than for producing energy. This is called protein sparing and is important to prevent loss of lean body mass.

Role of proteins in illness

The adequate level of protein for the average healthy population is 0.8 g/kg per day. In 2018, ESPEN stated that the majority of elderly patients require at least 1g of protein/kg/day.24

Protein requirement in healthy adults is 10-35% of total daily calories.25 Protein needs increase during disease due to the increased requirements for support of immune function and tissue synthesis. Older adults (>65 years) need more dietary protein compared to younger adults to promote recovery from illness as they experience a decline in anabolic response to protein intake, undergo age-related changes in metabolism, immunity, hormone levels and frailty.26

In this population, determining the appropriate protein levels is important because inadequate intake contributes to increased risk for sarcopenia, osteoporosis, and impaired immune response.26 Research also shows that older adults often have inadequate protein intake due to anorexia, appetite loss, GI disturbance, reduced ability to use available protein (eg insulin resistance, protein anabolic resistance, high splanchnic extraction, immobility), or a greater need for protein (eg inflammatory disease, increased oxidative modification of protein).26

Protein Guidelines

Current recommendations by expert members from the Dietary Protein Needs With Aging group (PROT-AGE) suggest an average daily intake in the range of 1.0 to 1.2g protein per kg of body weight per day26 based on evidence that shows support for lean body mass maintenance in older adults with higher protein intakes compared to the RDA.30-32

2018 ESPEN Guidelines recommend 1.3g/kg/protein /day for both the acute and chronic phases of critical illness.

2016 ASPEN Guidelines recommend in critical illness:

Target per kg/day based on BMI

BMI Protein (g) Protein Weight Reference
<30 1.2-2 ABW
30-40 2 IBW
40-50 2.5 IBM
>50 2.5 IBW