Protein: From plate to muscles Part 1

What happens to the protein we eat?  Early on in my education I had visions of dietary protein going straight from my stomach to my muscles.  Hence, the more protein you eat, the more muscle you’ll gain.

Unfortunately, it doesn’t really pan out that way.  For those under-consuming protein chronically, more protein usually does result in more muscle, improved recovery, increased satiety and anecdotally, “feeling good”.

In those already consuming an optimal amount, how much should we stress over complete vs incomplete proteins?  Protein timing? Can we encourage better recovery and growth with protein that exceeds our day to day needs?

What happens to the protein (and thus individual amino acids) when we consume and digest them is fascinating and often unexpected. 

Take a walk with me through the process of protein digestion and absorption to find out.

Digestion

So you’ve eaten 8oz of lean ground turkey in hopes of blasting your muscles with loads of amino acids (the individual components that make up a whole protein). Perhaps this is after a workout and your expectations are that all of that protein is going to be sucked up by your muscles for repair and growth.

Before any of those amino acids reach your muscles however, they take quite a few pit stops.

We rely on digestion to break down whole or long-chain proteins into much smaller parts.  To actively transport protein through the intestinal wall and into the bloodstream, we need to break proteins down into individual amino acids.  You’ve probably heard of some of them: taurine (If you’ve ever had a Red Bull), tryptophan (often discussed around the Thanksgiving table) or perhaps leucine (if you are interested in branched chain amino acid supplements).

Amino acids rely on active transport across the intestinal membrane and thus we cannot force large amounts of amino acids into our bloodstream for the most part because there is a rate-limiting step.  Some di and tri-peptides (two or three amino acids in length) can be transported across, but normally we’re relying on individual amino acids at a time. If long chains of amino acids were allowed through the intestinal membrane unchecked, our body would mount an immune response.  For some individuals with gut issues, this unchecked transport of larger proteins entering the bloodstream can result in an allergy response.

A whopping near-50% of amino acids you ingest and digest are actually absorbed by the gut itself.  Most of the glutamine found in protein is absorbed by the gut for repair and energy but around half of the total amino acids are taken up by the small intestine. 

Luke’s Notes:

Interestingly enough, these amino acids absorbed by the gut help to buffer the amount of amino acids entering the bloodstream.  Just like glucose or fatty acids, our physiology pretty tightly regulates how many amino acids are floating around our bloodstream.  Instead of a massive influx, the small intestine absorbs about half the ingested amino acids to release while we sleep, between meals and to balance individual amino acid concentrations in the blood.

Liver

With about half of the amino acids ingested actually making it into the blood stream, these are transported via the portal vein into the liver.  So, we’re STILL not at the muscle!

The liver oxidizes (burns for energy) a large portion of these amino acids.  The liver is an all around kick-ass organ.  Not only is the liver regulating the metabolism of amino acids and their release into the bloodstream, but it creates new ones too.

​​​​​​​

About half of the amino acids entering the liver are oxidized.  This leaves us currently with only about 25% of the amino acids we’ve ingested to be used.

One way the liver is especially kick-ass is it’s ability to create new amino acids, glucose, fatty acids and ketones from ingested amino acids.

An amino acid has a few components. One is the amino group; this can be broken off and used for energy OR used to create new amino acids.  If you’ve heard of non-essential amino acids, these are the ones your liver can create by pairing that free amino group to another compound.

What’s left after either of these two scenarios are two things: one is ammonium. This is the nitrogen carrying component of protein and is converted into urea which is excreted through the urine.  It’s one reason a diet excessively high in protein can result in smelly pee or sweat.

The second left-over component is the backbone of the amino acid: the carbon skeleton. The liver can use this for energy OR it can be used to synthesize glucose, fatty acids or ketones.

You can see just from these past couple of processes alone that dietary protein is used for much more than building muscle!

The Bloodstream

With the liver using a large portion of amino acids for synthesis of new proteins or burned for energy, there isn’t much left entering the bloodstream. Somewhere around 10-12% of the total amino acids ingested are released into the bloodstream after all their previous pit-stops.

Once in the bloodstream, the amino acids aren’t used solely for muscle.  Other organs like the heart, brain and kidneys use those amino acids for repair as well.

I mentioned previously that the body has pretty tight control over the levels of amino acids in the blood.  Even after a very high protein diet (in some research using 3g per kilogram or 1.5g per pound) the blood levels of amino acids might only reach about 30% increase over baseline, and this increase doesn’t last long.

Blood Levels of Amino Acids

The body doesn’t just rely on incoming protein/amino acids for repair, growth and energy. We have something called the “free amino acid pool”.  This consists of all the free amino acids present in tissue (like heart and muscles) and free amino acids in the bloodstream.

We don’t JUST have amino acids entering our bloodstream and tissue from our diet.  In fact, we have amino acids constantly being broken down and freed up by many tissues all over our body.  This creates the “pool”: amino acids entering this pool from the diet along with amino acids entering the pool from the breakdown of protein in tissue.  This creates a total free pool of amino acids that can be transported around the body for use in various tissues or simply to be oxidized for energy.

The free pool, as you may have guessed, is pretty small.  The total pool is estimated to contain around 100g amino acids (not very much) with the bloodstream containing only about 5g at any one time.

Just like with glucose, we have many various tissues that can store, release and burn amino acids for energy but we do not, at any one time have much floating around the bloodstream.

But, why?

If we need protein and amino acids for muscle growth and repair, why can’t we just jack up our blood amino acid concentrations through tons of meat and protein shakes to force muscle growth?

The body has various mechanisms at play to keep this from happening.  One is that increases of dietary amino acids correlate with an increase in amino acid oxidation.  Essentially, the more you eat, the more you burn.  Similar to carbohydrates, there is a pretty linear line you could draw between how much you eat and how much you burn for energy.  Once you have met the optimal needs for protein, ingesting more means you’ll just burn the excess off for other energy needs. Hence, you can’t “force” muscle growth through eating more protein.

One thing I did not mentioned previously is that when the liver releases amino acids into the bloodstream for use by various tissues, these amino acids are mostly the branched-chain amino acids: leucine, iso-leucine and valine.

You may have heard of these as being “anabolic”. These are amino acids that are preferentially absorbed and utilized by muscle tissue.  When researchers infuse amino acids directly into the bloodstream, about 75% are absorbed and used by the liver and gut.  When BCAAs are infused, skeletal muscle with absorb about 60-70%. So we obviously have better absorption and utilization rates of BCAAs by muscle tissue.

If we circle back to the gut, we can’t “force” amino acids into the bloodstream just by eating more steak and chicken.  For certain proteins like whey and individual amino acids, we can bypass the rate-limiting step a bit. If you consumed a whey protein isolate or BCAA supplement, you can in fact bypass much of the gut absorption but the rapid rise in blood amino acid concentrations means more amino acids oxidized for energy.

So even with rapid rises in blood amino acid levels from consuming a lot of whey or amino acid supplements, we’re going to end up burning more of them for energy rather than getting them directly into muscle tissue.

To discuss how we get the muscle to absorb more protein AND some ways we can kickstart that repair/growth process, some back next week!

But before we go, lets recap the fate of the protein we eat!

The Various Fates of Protein

• Ingested proteins are broken down primarily into individual amino acids, the gut selectively transports these
• About 50% of ingested amino acids are absorbed by the gut; almost all glutamine is absorbed by the gut
• Stored amino acids in the gut are used to buffer blood amino acid levels between meals and at night
• Amino acids enter the liver through the portal vein
• Over half of these amino acids in the liver are oxidized for energy or used to synthesize new proteins
• The remaining 10% of the total ingested amino acids can now enter the bloodstream - most of these are the Branched Chain Amino Acids (BCAA)
• Amino acids entering the bloodstream from dietary sources pool together with amino acids released from tissue to form the "free amino acid pool"
• Only very small amounts of amino acids form the pool at any one time: about 100g total with only about 5g of that being in the bloodstream
• You can induce larger rises in blood amino acid concentrations by consuming whey or amino acid supplements but amino acid oxidation (use for energy) also increases
• Most protein/amino acids ingested from diet are used for various repair by organs, burned for energy or used to create new proteins. Only a very small amount are actually used for muscle growth and repair.
• Ingesting protein in amounts over the optimal needs results in more protein being simply burned for energy, you cannot "force" muscle growth through more protein