“Essential” Amino Acids

I remember Professor Didem mentioning early in our introduction to amino acids, that certain amino acids are endogenous in humans, while there are some that must be consumed from other sources. I’ve heard this before as well, and I’ve begun wondering about what makes these endogenous amino acids (often called “nonessential” amino acids) different from the “essential” amino acids. So I decided to do a bit of research!

There are 9 essential amino acids (phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine), and an additional three for infants and growing children (cystein, tyrosine, and arginine). Interestingly enough, all of the adult amino acids could be supplied by eating eggs or soy protein (as in, eggs and soy protein both contain all of these essential amino acids).

While there is no obvious pattern among these amino acids, except that all of the aromatic amino acids are essential for children, the main difference is that the nonessential amino acids can be synthesised from other biological substrates, while the other amino acids cannot. For example, alanine can be synthesized from many other amino acids. To complicate this a bit more, a few of the essential amino acids are easily converted to each other (ex.phenylalanine and tyrosine), so only a diet including one of these amino acids is truly essential.

Since the body does not store amino acids for later use, as it does with lipids and carbohydrates, lacking only one amino acid could result in muscle degradation, or degradation of other protein structure; experiments on healthy male graduate students (of course they were grad students :]) also showed that lack of certain essential amino acids resulted in symptoms of nervousness, exhaustion, and dizziness (http://www.jbc.org/content/193/2/605.full.pdf). So, lesson learned – eat your protein (esp. eggs)!