Continuing from Wednesday’s article on What are good sources of protein? – Protein Quality, I want to talk a little bit about the amino acid profile of proteins and how that impacts on the answer to the question What are good sources of protein.
I’m going to actually divide this into two parts to keep it from getting too long. In Part 1, I’ll discuss some basic concepts and look at how the amino acid profile of various proteins relates to supporting basic bodily function. In Part 2, which I’ll run on Monday, I’ll discuss the possibility that athletes have specific amino acid requirements above and beyond what’s necessary to support basic function.
What are Amino Acids?
Now, as I’ve mentioned but not gone into any great detail in this series, amino acids are simply the building blocks of protein. Depending on which reference source you use, there are 18-22 different amino acids that occur in the human food supply. Whole food proteins are simply long chains of these amino acids bonded together. Typically whole food proteins are extremely long chains of amino acids, as I discussed in What are good sources of protein? – Digestibility, these long chains are cut into smaller and smaller chunks during digestion until only single amino acids and chains of 2-3 amino acids are actually absorbed.
I’d note that individual amino acids are often sold for either health or sports performance purposes. Readers may be familiar with the amino acid L-tryptophan which is often sold as a sleep aid. L-Tryptophan converts to serotonin in the brain which is involved in sleep. Take L-tryptophan on an empty stomach and you get drowsy because of increased brain serotonin levels.
In the athletic realm, all kinds of products are available. The branched-chain amino acids (BCAA) L-leucine, L-isoleucine and L-valine have been pushed for years to athletes; recently there has been a big push for isolated leucine for a number of reasons that I’ll touch on in Part 2.
Another example is L-carnitine, an amino acid involved in fat metabolism that has been sold as a fat loss aid for years (it doesn’t work by the way). I, myself, have recommended the amino acid L-tyrosine (which converts in the brain to adrenaline and noradrenaline) as part of a stimulant cocktail to improve performance.
You may be wondering what the ‘L-‘ means above; it refers to the chemical structure of the amino acid (to be technical, it’s an organic chemistry notation that stands for levorotary). There are also ‘D-‘ amino acids (the ‘D’ stands for dextrorotary). The human body only uses the ‘L-‘ form of amino acids; the ‘D-‘ form can actually be toxic.
Essential vs. Non-essential Amino Acids
I should note that the amino acids are usually subdivided into essential amino acids and inessential or non-essential amino acids. It’s important to note that both are absolutely essential for life, the term inessential/non-essential simply means that those amino acids don’t need to be obtain from the diet; the body can make them. The essential amino acids can only come from the diet; hence they are ‘essential’.y
I should also note things aren’t actually quite this simple, some amino acids which are inessential under normal conditions can become essential under others; glutamine is perhaps the most well known example. Under normal conditions, glutamine is inessential, the body can make what it needs. However, under conditions of massive stress (such as blunt force trauma, burn injuries or sepsis), the body can’t make as much glutamine as it needs; glutamine becomes conditionally essential under those conditions.
And while there are a few other odd exceptions to the essential/inessential distinction, they tend to be rare and not very relevant under most conditions, so I won’t talk about them.
Why do Amino Acids matter?
Now, as mentioned in What are good sources of protein? – Digestibility, after being broken down in the gut and intestine, proteins then appear in the bloodstream as amino acids. These are then used in the body for various processes such as the synthesis of new proteins.
Your heart, liver and many other organs are made of protein, skeletal muscle contains about 20% protein (most of it is actually water), your hair and skin is made of protein, there are numerous enzymes and liver proteins made in your body every day; all are synthesized from incoming amino acids from the diet.
Recall from What are good sources of protein – Speed of Digestion Part 1, that the tissues in the human body are in a constant state of turnover, which is the combination of breakdown and re-synthesis. So skeletal muscle is being broken down and remade, so is hair, skin, etc. Of course, since no process in the body processed with 100% efficiency, some of the broken down amino acids are lost.
That is, fundamentally, the basis for human protein requirements; the amino acids lost in the process of breakdown and re-synthesis have to be replaced from the diet. Otherwise, there will be a gradual loss of protein tissues in the body (as occurs in complete starvation). Lose enough body protein (about 40%) and you die.
Now, since the body is actually using specific amino acids for these various processes, it’s actually a little more accurate to say that the body has specific ‘amino acid requirements’ rather than ‘protein requirements’ per se. I’d note that there is also a general ‘nitrogen requirement’ (that can only be met with dietary protein) but I don’t want to get into that level of detail.
As a final note, I want to mention that the tissues in the human body that use proteins all use them in varying proportions and amounts. That is, the amino acid profile of say, your liver, or a specific enzyme may not be the same as skeletal muscle, hair or bone. Basically, the tissue you’re focusing on will determine what the ideal amino acid profile ‘might be’. I’ll come back to this.
Back to Protein Quality
Now, as I mentioned in What are good sources of protein – Protein Quality, one of the determinations of protein quality has to do with how well or poorly a given protein fulfills the amino acid requirements of the body and the above discussion basically explains why. Every day your body loses some amino acids which have to be replaced. One determinant of a protein’s quality is how well it matches the body’s need for those specific amino acids.
Now, I should mention again that most of the work on protein quality deals with the issue of general health, especially in those people who are not getting sufficient protein, protein from high quality sources, and who aren’t eating much in the first place. That is, the research is aimed at folks in third world countries.
The goal is to find ways of improving overall health and bodily function in people who are starving to death. And the focus is basically on keeping them healthy overall, that is meeting the amino acid requirements of the whole body in terms of keeping the basic stuff functioning well (or at least passably). Issues such as optimizing athletic performance or increasing muscle mass are not the focus.
Not only does this mean it has questionable relevance to those of us lucky enough to live in a modern world where protein and food is generally very available, it also means that it isn’t aimed at athletes or individuals involved in training (which tends to be the group I focus on). It’s conceivable (and, of course, supplement companies pander to this idea) that athletes or individuals in hard training might have specific requirements for amino acids.
That is to say, it’s conceivable that someone involved in a strength/power sport (powerlifting, bodybuilding, etc.) might require a different amino acid profile to support the growth of skeletal muscle; an endurance athlete might conceivably need a specific amino acid profile to support the synthesis of mitochondria (the powerhouse of the cell) or enzymes involved in energy production. This topic is drastically under studied.
But, simply (and of course this is discussed in great detail in The Protein Book), amino acid requirements can be sub-divided into the amino acid requirements needed to support basic health and bodily function (what most research deals with) and the amino acid requirements (if any) to optimize athletic performance.
Meeting Basic Bodily Requirements
Now, for reasons I’m not going to get to, the amino acid requirements for 2-5 year old children are actually used to examine whether or not a specific protein is sufficient. That is, any dietary protein which has an amino acid profile that meets or exceeds the requirements for 2-5 year old children is considered sufficient to support the basic needs of adults.
I’d note that, in keeping with the section on essential/inessential amino acid discussion above, the real focus is on whether or not a given protein can meet the essential amino acid requirements of the body. Assuming sufficient protein is being consumed in the first place, the inessential amino acid profile isn’t that relevant.
And as I show in Table 2 on Page 56 of The Protein Book (which I’m not going to reproduce here), basically all high quality proteins, and this even includes soy protein, can meet the basic amino acid needs of an adult human being. Human milk, cows milk, eggs, beef, whey and soy all contain amino acids far in excess of the requirements for 2-5 year old children; by extension this means that they can readily meet the requirements for adults.
This is in keeping with the discussion of the PDCAAS from What are good sources of protein – Protein Quality showing that proteins such as soy (which were typically thought of as low quality) are more than sufficient to meet adult human essential amino acid requirements. Assuming adeequate dietary protein is being eaten in the first place (and this is basically never an issue in the modern world), all proteins easily meet human protein requirements.
Which doesn’t make them all identical or equivalent mind you; there may be reasons (such as the presence or absence of other nutrients such as iron, zinc, or calcium, or the fatty acid profile) to choose one protein over another. But from the standpoint of amino acid profile, there isn’t much of a functional difference between proteins (I’d note, rather tangentially, that recent work has suggested that fish protein per se seems to have benefits on insulin sensitivity, possibily due to the high taurine content).
Which, as noted above, doesn’t really address the issue of athletes and possible differences in requirements. But that will have to wait for Part 2.