Protein, a Sports Nutrition Perspective

What is Protein?

Protein is a macronutrient essential for all living organisms. The word protein is derived from the Greek word proteios, which means 'the most fundamental and important', or first. Proteins are fundamental, integral and essential food components - they provide amino acids and energy, which are essential for maintenance, repair and growth of all tissues in the human body. All of the body's enzymes, and some hormones, are composed of protein. About 15% of human body weight is protein, with the major proportion of body proteins contained in skeletal muscle.

Basic Protein Chemistry

Proteins are complex organic chains of amino acids, linked together by peptide bonds (see Figure 1).

Amino acids, the backbone of proteins, always contain carbon, oxygen, nitrogen and occasionally phosphorus, sulphur, zinc, copper and iron. The nitrogen content distinguishes protein from fat and carbohydrate and is responsible for the unique physiologic function of protein¹.

Energy production, muscle recovery, growth and fat loss as well as number of neurological functions are critically and intimately linked with amino acids.

An amino acid is a small molecule with an amine group (NH2) at one end and a carboxylic acid (COOH) group at the other end (see Figure 2). The peptide bonds formed between these groups create chains, or polymers, of amino acids - a polymer of 50 or more amino acids is considered to be a protein.

There are 23 amino acids known to be necessary for normal functioning of the human body². Amino acids can be classified in two simple groups³ (Table 1).

The first group are essential amino acids (EAA), which means human body cannot produce them and they must be supplied from dietary sources.

The second group are non-essential amino acids (NEAA), which means the human body can synthesise them internally. Within the non-essential amino acids there are those described as conditionally essential amino acids (CEAA) such as L-Glutamine. Under some circumstances, these amino acids become essential and need to be obtained from dietary sources.

What are BCAAs?

Leucine, isoleucine and valine are the branched chain amino acids (BCAAs). Together they form one third of muscle tissue and are able to be utilised during intensive exercise as a fuel source, and therefore need to be replaced. BCAAs taken before and during exercise may increase mental and physical performance⁴, although additional supplementation with BCAA's may be unnecessary as the characteristic feature of a high quality protein is a high level of BCAAs.

Table 1: Essential and Non Essential Amino Acids

Protein Metabolism and Turnover

Dietary proteins are hydrolysed to small peptides and their constituent amino acids in the stomach and intestine. Proteins are absorbed into the blood stream in the form of amino acids (about 33%) and small peptides (about 67%) (Figure 3). Amino acids are transported to various locations in the body where they are resynthesised into the proteins required for muscle and connective tissue development, enzyme and hormone formation etc.

The body does not rely on dietary protein to provide all of its requirements; rather the majority of the body's amino acid supply comes from breakdown (catabolism) of its own cells.

The daily protein turnover (Figure 3)⁵ of a moderately active adult is 1- 2% of total body protein: 125-250g of body protein is broken down and re-synthesised each day. Protein turnover is rapid (3-4 days) in some tissues, such as the liver and intestines and less rapid (several months) for muscular proteins, bones and connective tissue1.

Nearly 25% of the amino acids released during protein breakdown are completely degraded; the remainder are re-utilised to synthesise new protein.

The average adult needs to consume 30-60g of protein per day to replace those amino acids that are normally catabolised⁶.

Do Athletes Need More Dietary Protein?

The protein requirement of athletes is higher than that of moderately active or sedentary people. This increased need is likely to be due to increased amino acid oxidation and perhaps an increase in the rate of muscle protein synthesis to repair any muscle damage induced by prolonged exercise⁷. The protein utilisation during exercise depends on the type, frequency, intensity and duration of exercise, the age, sex, level of training and the environmental conditions under which an exercise is performed⁸.

Different athletes have different nutritional needs. Athletes engaged in endurance exercise require 1.2 to 1.4 g of protein per kg body weight daily as an auxiliary fuel source.

As intense muscular activity increases protein breakdown and protein use as an energy source, protein requirements are higher for body builders and other athletes engaged in strength exercise. The recommended intake increases to 1.6 to 2.0g of protein per kg body weight daily (Table 2).

Table 2: Daily Protein Requirement of Athletes

Although the daily protein requirement for athletes exceeds the current RDA of 0.8g per kg body weight, there is no indication that it will cause any adverse side effects. However, excessive prolonged loading of protein (more than 2g per kg body weight/day) may cause increased fat storage and increased calcium loss.

An Athlete's Protein Supplementation Requirements

There are several different dietary sources of protein available to an athlete. A diet based on traditional sources of protein will provide an adequate intake, but may not be as effective as one that includes a protein supplement for the following reasons:

1. Common dietary proteins may be deficient in some of the essential amino acids
2. Common dietary protein is not a concentrated source of protein. It contains other constituents such as saturated fat, cholesterol, purines, etc., which many athletes try to avoid
3. Intact dietary proteins are less bioavailable than protein supplements even though both contain the same amount of amino acids

Protein supplementation is a very convenient and effective way to assist in meeting protein requirements.

Types of Protein Supplements

Commercially available protein supplements can be classified in three basic groups:

• Intact Proteins: These are polypeptides containing purified forms of native proteins such as whey, casein and soy protein concentrates
• Protein Hydrolysates: These are hydrolysed (pre-digested) proteins with portion of the protein existing as small chains of amino acids such as dipeptides and tripeptides
• Free Form Amino Acids: these are singular amino acids with levo-rotatory structure (L form).

Different Sources of Proteins

Commercially available proteins are obtained from a range of animal and plant sources.  Proteins from plant sources i.e. soy, wheat, are generally balanced but may lack one or more essential amino acids.   Proteins obtained from animal sources are more complete with regard to their essential amino acid profile. They are widely used as food ingredients eg. gelatine, egg white, milk (casein and whey), fish (surimi) etc. Among animal proteins, isolated milk proteins are the most common for use in manufacture of supplementary protein products due to their excellent functional and nutritional properties.

The Nutritional Value of Proteins

In a general sense, protein 'quality' refers to how well or poorly the body is able to use the protein. A number of indices are used to measure and compare the quality and nutritional properties of different proteins i.e. chemical, biological and microbiological methods¹⁰.  The most commonly used are Biological Value (BV), Protein Efficiency Ratio (PER), Protein Digestibility Corrected Amino Acid Score (PDCAAS) etc. (Table 3).

Table 3: Comparative Nutritional Values of Different Proteins

Different Types of Milk Protein

Bovine milk, by far the most common source of milk in the westernised human diet, contains 3 - 4% protein, of which 80% are casein proteins and 20% are whey proteins. To separate these two distinctly different proteins, milk is acidified to a pH of 4.6 and the casein protein (no longer soluble) is precipitated. The remaining protein, which is soluble under these acid conditions, is referred as whey. A wide variety of products can be manufactured from both casein and whey protein by altering the processing conditions downstream during manufacture¹¹.

Casein:

Casein is the principal protein of milk, commercially produced from skim milk by either:

• acid precipitation at pH 4.6-4.8 (acid casein)
• conversion of the milk sugar lactose to lactic acid, lowering pH and precipitating proteins (lactic casein)
• enzyme coagulation process at neutral pH (rennet casein)

Casein proteins, as opposed to caseinates, exist in a colloidal form known as a 'micelle', and exhibit different properties from caseinates.  Caseinates are made from lactic casein or acid casein. Casein is rinsed and treated with various alkali salts, and then spray dried to produce sodium, calcium, potassium and magnesium caseinates¹².

Whey:

Whey in its native state consists of 93% water and only 0.6% whey protein. Whey protein molecules are soluble, globular proteins. Whey must be concentrated to extract the whey protein portion. The whey protein can then be purified into a number of different whey protein types based on composition.

Purification of whey:

To temporarily stop bacterial growth whey is processed as soon as possible after collection or cooled to ~ 5°C. Whey contains residual casein protein, which is removed by separating devices otherwise it will have an adverse effect on fat separation. After separation of casein and fat, whey is either chilled for short term storage or pasteurised, for longer storage. Whey can either concentrated and dried to produce whey powder or processed further for protein recovery.

Fractionation of total solids by ultrafiltration: Whey is first ultrafiltered. The whey protein, called the 'retentate', is retained on the filters due to the large size of the protein molecules. Water, lactose, minerals and small molecular weight nitrogenous materials flow through the membrane. Whey protein concentrates (WPC) are powders made by drying the retentates filtered from the ultrafiltration process. The WPC from this process contains approx 40% protein and high amounts of lactose and minerals.

Diafiltration of Retentates:

To obtain an 80-85% whey protein concentrate, the liquid whey is concentrated 20-30 times by ultrafiltration then diafiltration. Diafiltration is a membrane filtration process that increases the protein concentration by adding water continuously to the retentate stream to wash out the lactose and minerals. Diafiltration enhances the whey protein purity.

Whey Protein Isolate (WPI) Manufacture

Further processing of high protein WPC through two additional processing operations: microfiltration (to remove fat) and lactose hydrolysis (to remove lactose) produces WPI. WPI is low in fat (<1%) and lactose (< 1%). After spray drying, the product is called microfiltered whey protein isolate.

WPI is also produced by cationic ion exchange treatment prior to the ultrafiltration. Whey is acidified and the whey proteins are positively charged. The whey is then reacted with negatively charged resin beads. The whey proteins get attached to the resin beads while fat lactose and minerals remain in solution. The protein loaded resin beads are separated and treated with an alkaline solution to detach the protein from the resin, followed by elution, ultrafiltration, diafiltration and drying. This type of whey protein is called Ion Exchange WPI¹¹.

Both microfiltered and ion exchange whey protein isolate disperse slowly in water. To enhance dispersibility, WPI is treated with lecithin. This readily dispersible WPI is called Instantised Whey Protein Isolate (IWPI).

Hydrolysed Whey Protein (HWP) Manufacture

WPC can be hydrolysed by acid, alkali or enzymes to form peptides and/or amino acids. HWP prepared by enzymatic hydrolysis is preferred over that produced by chemical hydrolysis due to improved nutritional properties.

Enzymatic hydrolysis can produce hydrolysates with well-defined peptide profiles. To manufacture whey protein hydrolysates using enzymes, WPC is hydrated in water and solubilised by pH adjustment. An enzyme (a protease) is added at the appropriate temperature and allowed to react in a specified time. This enzyme is then deactivated through heat treatment and the resulting milk protein hydrolysates are pasteurised. Further processing may include filtration, clarification, evaporation, flavour reduction and spray drying.

Whey Protein Fractions

The principle fractions of whey proteins are b-lactoglobulin, a-lactalbumin, immunoglobulins and bovine serum albumin¹⁹:

• Lactoglobulin represents about 50% of the total whey protein content in bovine milk.
• Lactalbumin is a complex protein molecule that is transported from the blood to the milk to provide health benefits to the new-born.
• Immunoglobulins²⁰ bind to foreign organisms which trigger an immune response, producing large amounts of specific antibodies to help fight disease.  Immunoglobulins are sensitive to pH and heat, and easily lose their antibody carrying properties as they denature.
• Bovine serum albumin is known as the glutathione precursor. Glutathione is an antioxidant produced by the body that is important in antimetabolic cycles to assist in the prevention of malignant growths. It normally constitutes about 2-4% of the whey proteins.

Minor Whey Proteins

Whey protein contains several minor protein fractions and although present in very small amounts, they are the most biologically active of all whey protein fractions²¹. 

Glycomacropeptide²¹, is released by enzyme action (chymosin or rennin) on kappa-casein during the manufacture of cheese and rennet casein. There is more GMP in sweet whey than in acid casein. GMP contains a compound called sialic acid, which acts as an anchor for viruses reducing the chance of infection. GMP has also been shown to influence digestion by directly controlling the release of gastric acid, pancreatic enzymes (insulin) and regulating protein digestion in the stomach.

Lactoferrin: Bovine lactoferrins are considered to be important in human nutrition. They are iron binding proteins and have antibacterial properties^{22, 23}.  They also act as immunomodulators and have been implicated in assisting tissue regrowth²¹.

Lactoperoxidase: in combination with natural hydrogen peroxide and/or thiocyanate, forms a strong anti-microbial and anti-bacterial agent²¹.

Lysozyme: is an enzyme that cleaves the carbohydrate polymers of bacterial cell walls. Growth factors (IGF)²⁰ assist growth of new tissues in the body. They are very sensitive and easily destroyed during processing.

The Nutritive Value of Whey Protein

All proteins contain amino acids, which can be utilised by the body. During digestion, proteins are hydrolysed to small peptides and their constituent amino acids.  To be efficiently utilised by the body, dietary proteins must supply adequate amounts of essential amino acids in appropriate proportions and in a digestible form.  The quality of a protein is mainly based on the BCAA and essential amino acid content. A wide variety of tests are available to measure protein quality. Whey proteins show very high values in both analytical and biological nutritional tests²⁵.

Why are whey proteins are more popular than casein proteins in sports nutrition products?

Whey proteins are technically superior to caseinates with respect to the following physicochemical and functional characteristics:

1. Whey proteins are water soluble over a range of pH's whereas caseinates have variable solubility profiles at different pH levels.
2. Whey proteins have excellent foam stability when compared to caseinates.
3. The overall viscosity profile for caseinates is pH dependent whereas a change in pH does not alter the viscosity of whey proteins.
4. Caseinates may taste chalky due to their high mineral content.

Whey proteins are more popular than caseinates¹⁷ due to their excellent nutritional properties:

1. Whey proteins are absorbed faster than caseinates in the gastrointestinal tract. In humans, plasma amino acids levels are high, rapid and transient after intake of whey proteins, vs lower, slower and prolonged levels after intake of caseinates. This has lead to the classification of whey as a fast protein and casein as a slow protein^13,14,15. This variation in absorption may be due to precipitation of caseins in the acidic pH of the stomach, which results in a delay in stomach emptying¹⁶.
2. Whey proteins contain higher proportions of both branched chain and essential amino acids.
3. Whey proteins, especially whey protein isolate, contain a high percentage of immunoglobulins to support the body's immune system.
4. Whey protein isolate contains quadrapeptides, which have been shown to have opioid (pain-killing) effects. This may help to decrease the sensation of muscle soreness following intense weight training.
5. Whey proteins have the ability to enhance endogenous glutathione (naturally occurring antioxidant) production.
6. Whey protein isolates contains less sodium, lactose and fat compared to caseinates.
7. The bioavailability of whey is superior to that of caseinates (Table 3).

References

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