The Digestive Barrier to MPS

 

The length of time from protein consumption before the new amino acids can reach the bloodstream depends on a number of parameters, and the time taken has a direct influence on the potential for muscle protein synthesis.

Assuming lean meat consumption, the first step is for the body to begin to digest the food mechanically.  Initial chewing increases the surface area of the food leaving it accessible by various digestive enzymes in the stomach in what is termed the cephalic phase.  For solid protein sources such as meat, fish and some vegetables, this phase is vital.  Consuming food causes the stomach to begin secreting an enzyme called pepsinogen which is then converted into pepsin by the low pH of the stomach.  The churning action of the stomach and the pepsin begin the process of separating muscle tissue or other fibrous protein sources into long, complex chains of proteins called polypeptides.

A typical meal will require at least two hours of digestion before the consumed food is reduced to chyme, a semi-fluid mass of partially digested food.  Once in this state, small portions of chyme will be passed at regular intervals of around 20 minutes into the duodenum, the first section of the small intestine [44].   Delivery of amino acids to the blood is still some time away when this stage is reached.

The time scales for liquid protein consumption are significantly shorter as, although requiring some digestion in the stomach, liquid protein sources pass into the duodenum faster as there is no need for mechanical digestion [45].  However, the maximal capacity of the small intestines is only 120ml, meaning any liquid in excess of this volume will not immediately begin the final processes of digestion [46] [47].

Once in the duodenum, the chyme or liquid protein source must be further digested.  Protein now consists of long chains of amino acids, and these must be broken down further before they can be released into the bloodstream.  The arrival of chyme into the duodenum triggers the release of various digestive enzymes including intestinal gastrin, trypsin and chymotrypsin, all of which conspire to break the bonds between amino acids, turning the complex long-chain proteins into shorter and shorter polypeptides.

When these polypeptides are short enough, they pass into the jejunum, the next section of small intestine and the region from which the majority of protein is absorbed by the body.

The villi of the jejunum are equipped with inbuilt enzymes, embedded into their cell membranes.  These work to further break down the peptides, converting them into free amino acids or very small peptides less than four amino acids in length.  At this stage, the amino acids and peptides are ready for absorption.

The villi of the jejunum feature four types of transporter, one for each type of amino acid (acidic, basic, polar and non-polar amino acids).  These transporters bind the amino acids and then change shape, depositing the amino acid inside the villi cells, before returning to their original position facing into the intestines.  The free amino acids are then pumped out into the bloodstream.

In addition to these four transporter proteins, there is an additional transporter protein called PepT1 which is able to transport short peptides (two or three amino acids in length) into the cells where they are broken down into single amino acids.

This entire digestive process takes a significant portion of time.  For solid foods such as meat and fish, the very first amino acids do not begin to enter the bloodstream until a minimum of two hours [48] after consumption, with some sources suggesting the process takes as long as four hours [49].  For proteins already in liquid form, the time for amino acid absorption is reduced because the need for mechanical digestion is removed, but the proteins themselves still need to be digested.  The most popular forms of liquid proteins, whey and casein protein, are derived from milk.  A side-effect of being derived from milk is that these protein sources contain 5-7% milk fat alongside the protein.  For many athletes, this fat content is undesirable, so manufacturers process the protein, using a technique called ultrafiltration.  Here, any molecules smaller than 20,000Da* are disposed of as waste as a direct result of the filtration method.

[*Da = Dalton, a measure of weight that is used for determining the size of proteins.  The hormone insulin, a small protein used in metabolising sugars, has a weight of 5,808Da, whilst Titin, a very large protein found in muscle has a weight of 3,906,488Da.]

This results in ‘whey protein isolate’ and, whilst possessing very low levels of fat, it comes with drawbacks.  Removing all molecules smaller than 20,000Da means that all the small and most easily digestible protein is lost [50].  Amino acids have an average molecular weight of 110Da, which means that, after ultrafiltration, even the smallest size protein in a whey or casein protein isolate would be approximately 180 amino acids in length [51].  Yet, the largest peptide that can be absorbed by the villi of the jejunum is just three amino acids in size.  This need for further digestion in the duodenum helps to explain why whey and casein protein ingestion can only result in an increased blood amino acid level after an average of one hour (most reports showing between 45 and 120 minutes) [52-55].

As previously discussed, the window for peak MPS occurs immediately after exercise, peaking between one and three hours with the MPS potential declining over 48 hours.  The presence of significant amino acids in the blood during this window allows for high levels of MPS, boosting recovery and allowing for strength gains.  Maximising the potential of the MPS window is therefore a priority for every athlete wishing to initiate rapid muscle recovery and thus achieve the largest possible strength and performance gains.

Solid foods consumed immediately post exercise require significant amounts of digestion.  As such, amino acids will not be present in the blood until the window of maximum MPS potential is declining.  Even when using digestible liquid proteins, such as whey, the valuable window of maximum MPS has begun to decline before the amino acids become bioavailable.  Furthermore, the one hour period immediately post exercise when the muscles receive the most blood and are actively transporting new amino acids to aid muscle recovery [34] [35] is completely unreachable through conventional dietary means.  This indicates that the full potential of the anabolic window cannot be reached by post exercise ingestion using conventional protein sources.