Protein quality

The quality of protein heavily depends on its composition of the EAAs. A complete protein contains all EAAs in sufficient amounts to support lean tissue maintenance.  An incomplete protein doesn’t contain all EAAs in sufficient amounts. All animal-based proteins are complete proteins, except for gelatin, plant-based proteins on the other hand are incomplete proteins. This is why it is vital for vegans to consume a good combination of foods to get adequate amounts of EAAs in the diet, for example, grains are low in lysine and threonine, but legumes are low in methionine, so combining these will ensure a sufficient quantity of EAAs, bare in mind that these do not need to be consumed in one meal and they can be instead consumed throughout the day. 
 

The most popular index for assessing protein quality is likely the protein digestibility-corrected amino acid score (PDCAAS), the highest score a protein can get is 1, a good ratio of EAAs to non-essential amino acids is about 1:1 for comparison. All soy, whey and casein have a PDCASS score of 1, which may lead you to believe that each protein is just as good as one another for muscle growth, however, it has been proven that whey protein is a superior protein source to casein and particularly soy protein when it comes to stimulating muscle protein synthesis, this may be due to the fast-digesting nature of whey, which means more leucine enters the circulation at one time which will activate protein synthesis to a greater degree. However, a blend of fast and slow-digesting proteins, such as mixing whey with casein, may prolong aminoacidemia (amino acids being elevated in the blood) which means fewer amino acids are used for energy and more can be put towards muscle protein synthesis, this will prolong protein synthesis. Casein is considered a slow-digesting protein as it clumps together in a gel when it enters the acidic environment of the stomach, so it takes longer to break down. Fast-digesting proteins particularly stimulate muscle protein synthesis for 3 hours after their consumption, whilst slow-digesting proteins particularly stimulate muscle protein synthesis for 6-8 hours after their consumption. It is important to note that we are not talking about muscle growth but muscle protein synthesis, even though it does seem obvious that these 2 go hand in hand they are not the same thing. Also it appears to be more important to get good high-quality protein than a particular source of protein.
 

Apart from the PDCASS there are also other methods of determining protein quality, which each have there own criteria, they usually take into account the proteins EAA composition, the digestibility of the protein and the bioavailability of the protein (the ability of it to be absorbed and used in the body). The protein efficacy ratio (PER) method takes a very unique approach, as it measures the weight gain in rats which are fed the protein in question. The biological value (BV) method divides the nitrogen retained in the body by the amount of nitrogen absorbed from the protein. Because each method has there own criteria combining some of the best methods will be the most effective way of determining protein quality, it is important to pay particular attention to the PDCASS and BV methods as these take into account the protein’s digestibility. The digestible indispensable amino acid score (DIAAS), is based on the proteins digestibility in the ileum, which makes it more reliable than other measures, however, this method is yet to be fully researched so it is not yet a reliable method.

Protein synthesis

Writing about the processes that lead to the stimulation of protein synthesis goes beyond the scope of this article, but we can discuss the process of protein synthesis.
 

We first need to get an understanding of DNA. We have 23 pairs of chromosomes located in the nucleus of cells, 1 of these pairs is known as the sex chromosomes and the other 22 are known as autosomes, these are responsible for creating the proteins essential to allow the body to function. Chromosomes are made up of DNA which has 2 strands which curl around each other in a formation known as a double helix. Each strand has nucleotides which connect to the other nucleotides on the other strand via hydrogen bonds. There are 4 types of nucleotides to concern ourselves with for now, these are A, T, G and C, and these are always bonded to another particular nucleotide. An A nucleotide is always bonded with a T nucleotide via 2 hydrogen bonds and vice versa, and a G nucleotide is always boned to a C nucleotide via 3 hydrogen bonds and vice versa. The first step of protein synthesis is known as transcription, this is when the enzyme RNA polymerase comes along and breaks the hydrogen bonds between 10-20 pairs of nucleotides and separates the 2 strands, one of these strands is called the coding strand, as it codes for amino acids and the other is known as the template strand, RNA polymerase reads the template strands and produces the apposing nucleotide, but this time instead of producing a T nucleotide to oppose an A nucleotide, it will instead produce a U nucleotide when it reads an A nucleotide. When RNA polymerase is finished reading the strand it would have created mRNA, which moves away from the DNA and the DNA closes back up with help from the enzyme DNA ligase. Parts of the mRNA can also be removed, these parts are known as introns, and the rest is recombined, this part is known as an exon. The next step of protein synthesis is known as translation. The mRNA leaves the nucleus and enters the cytoplasm, then enters a ribosome. tRNA moves over to the ribosome, carrying with it an amino acid, the tRNA has 3 nucleotides which match up with the corresponding nucleotides of the mRNA. The 3 nucleotides of the mRNA is known as the codon, whilst its complementary nucleotides on the tRNA is known as the anticodon. For every 3 nucleotides of the mRNA, there is 1 amino acid which it matches. It doesn’t matter which order the nucleotides are in for coding for particular amino acids, for example, the U, G, C nucleotides will code for the same amino acid as the C, U, G nucleotides, also different nucleotides can code for the same amino acid. The tRNA will read 3 nucleotides of the mRNA, then it will move on and read the next 3, and so on, until the whole mRNA strand has been read, and for every 3 nucleotides read of the mRNA the tRNA will make its amino acid combine with the next one, until it eventually creates a chain of amino acids, known as a protein. The different  characteristics of each amino acid make it fold in different ways, sometimes when the protein folds it leaves pockets, which molecules can bind to, or it can work as the active site of an enzyme, helping chemical reactions occur quicker. Proteins can leave or stay in the cell, where they are essential for building almost every component of every cell in the body and are also essential for almost every chemical reaction. In muscle, if protein synthesis exceeds the rate of muscle breakdown it will lead to an increase in muscle size.
 

Mutations can occur in the DNA through things such as UV light, certain chemicals, reactive oxygen species or it can even happen spontaneously, this changes the nucleotides of the DNA, so the wrong protein is often coded for which folds differently and than essential bodily functions can’t occur.

In muscles there are many pathways which lead to an increase in protein synthesis these are the PI3K/AKT pathway, the mTOR pathway, the MAPKs pathway and the calcium-dependent pathway, all of which are stimulated during resistance training.

Protein digestion

A protease is the name given to a group of enzymes responsible for breaking down proteins into small enough pieces so they can be digested. Proteases are first located in the stomach where they break down protein into large polypeptides, which then enter the duodenum, in the small intestine, where they are further digested into smaller polypeptides, on the surface of the enterocytes are more proteases to further break down the polypeptides, once the polypeptides are small enough or have been broken down into amino acids they can enter the enterocytes via channel proteins. Polypeptides enter through certain channel proteins along with hydrogen, which is then transported back out into the lumen in exchange for sodium through certain other channel proteins. The amino acids enter the enterocytes through a channel protein along with sodium. Because sodium is entering the enterocytes it draws water into the cell, sodium then enters the bloodstream in exchange for potassium via the sodium-potassium pump. In the enterocytes more proteases can finish the job by breaking the polypeptides down into amino acids, so they are small enough to diffuse from here into the bloodstream. In the bloodstream, some amino acids travel to the liver where they are synthesised into new proteins, mostly plasma proteins, and other amino acids travel to certain tissues for the synthesis of tissue-specific proteins. Proteins are constantly being renewed and the old proteins get recycled.

Proteins utilisation for energy

When too much protein is consumed in one meal or during energy shortage amino acids can be converted to keto acids via a process known as deamination in the liver, which produces ammonia as a byproduct. Different keto acids can be converted into different components which are part of the metabolism so they can help make energy, once keto acids form these components they can later be reversed back into keto acids then amino acids. Ammonia is toxic in large quantities, this is why it travels in the bloodstream to the liver, where it is converted to urea, which then travels to the kidneys to be excreted. Those with liver complications can develop renal damage or renal failure if they consume a diet very high in protein. Excess amino acids and excess carbohydrates work with one another in the liver to turn into fatty acids, which can be stored in adipose tissue stores, but this process is inefficient and slow.

Protein intake recommendations

The muscle full concept proposes that muscle protein synthesis doesn’t increase any further from a single meal beyond a certain point, and instead, any amino acids circulating in the blood will be used for energy, to suppress muscle breakdown or for alternate bodily compounds. When an individual consumes a meal with a large amount of protein muscle protein synthesis is stimulated within 1 hour but then returns to baseline within 3 hours, even though amino acid availability is still elevated, this may advocate for slow digesting protein or mixing protein with some healthy fats which will slow digestion this way muscles can put more protein towards muscle protein synthesis. Areta and colleagues performed a study to find which was the best way to distribute protein consumption when it comes to muscle growth, 24 resistance-trained men were randomly assigned to 1 of 3 groups, a pulse-feeding group which consumed 10g of protein 8 times per day, every 1.5 hours, an intermediate feeding group which consumed 20g of protein 4 times per day, every 3 hours, and a bolus feeding group which consumed 40g of protein 2 times per day, every 6 hours. The intermediate-feeding group was superior to the other 2 groups when it comes to stimulating protein synthesis, this is likely because the pulse-feeding group was not consuming enough protein per meal to allow for leucine to maximally activate muscle protein synthesis, whilst the bolus-feeding group was likely not consuming enough meals per day to maintain elevated muscle protein synthesis levels. However, the results of this test can not fully be linked to a typical realistic diet, for one the participants were consuming whey protein which has an absorption rate of 10g per hour, for comparison the absorption rate of eggs is 3g per hour, so if they was consuming the protein in a typical meal which took longer to enter the body then the results would likely be altered, as this would prolong aminoacidemia, so the amount of amino acids in the bloodstream at one point will never reach the same level as it does with the whey protein, it therefore may take more protein to provide a leucine trigger, also the participants also only consumed 80g of protein per day which would cause a negative protein balance, considering they was partaking in resistance training. Perhaps a more reliable study is the one performed by Mamerow and colleagues over a 2-week period where 8 healthy subjects were put into 1 of 2 groups, one group had their protein intake distributed evenly through their breakfast lunch and dinner, whilst the other group had ⅔ of there protein intake consumed at dinner whilst the rest of their protein intake was distributed evenly between their breakfast and lunch, each participant consumer 1.6g of protein per kg of body weight each day, which is enough for maximal anabolism. The group which had their protein intake evenly distributed throughout the day had 25% greater muscle protein synthesis than the other group. These 2 studies provide evidence towards distributing protein intake evenly throughout the day for maximal muscle growth, but these benefits are likely minimal if there are any at all, and there is also evidence suggesting that having a diet where an individual’s protein intake comes primarily from 1 meal is superior to evenly distributing protein intake throughout the day. Bear in mind that more elderly individuals need to consume roughly double the amount of high-quality protein to reach the leucine threshold, so they will likely need to consume more protein per meal, this may advocate for elderly people to consume fewer meals each with more protein in than young people. There is also some evidence that the body becomes more efficient at utilising a minimal amount of meals in the day with high protein, as fewer amino acids are used for energy and more are put towards muscle protein synthesis, but this is not yet a certainty. Providing all the information that we know the best advice we can give is to consume 0.4-0.55g of protein per kg of body per meal, and you should consume 4 meals throughout the day with at least a 2 hour window in between each meal, this will allow you to reach 1.6-2.2g of protein per kg of body weight each day, you also may be able to consume more protein in the meal following resistance training, it is also worth mixing this meal with some healthy carbohydrates which will intensify the anabolic effects of protein by significantly elevating protein synthesis, however there is no concrete evidence of an anabolic window, but hypothetically it does make sense that consuming a high protein meal shortly after resistance training will cause a super compensatory response to stimulate hypertrophy, but as long as you consume a high protein meal within 2 hours before or after resistance training anabolism is likely maximised, as muscle protein synthesis will peak 2.5-3 hours after resistance training and remains elevated for at least 24 hours, when no protein is consumed whatsoever. There is also no need to worry about not getting in protein through the hours when you are asleep as total daily protein intake appears to be far more important than the timing of its intake.