Proteins are the most structurally complex of all macronutrients and among the most complex molecules known. The
three-dimensional ball and stick model below of a protein called hexokinase gives some idea of the staggering
intricacy of protein. Remember that every hexokinase molecule in the human body--and there are trillions upon
trillions of them--has EXACTLY the same structure as this. By the way, hexokinase is a protein enzyme that
catalyzes the first step of glycolysis-the breakdown of glucose to create energy (the products of this step,
glucose-6-phosphate and adenosine diphosphate are shown, to scale, free-floating in the upper right.)
Essential Amino Acids
Humans cannot make and therefore absolutely require adequate dietary intake of eight amino acids. These
essential amino acids are phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, and
lysine. Plant proteins are often defficient in lysine which makes that amino acid a special concern for vegan
Daily Requirement of Protein
Because some proteins are relatively defficient in certain essential amino acids, it is not possible to issue a
blanket statement about daily protein need; it depends upon the quality of the protein. Whole egg is often
considered the "gold standard" for protein quality and assuming that all of one's protein is egg, the daily
need is fairly low, about 45 grams for women and 55 grams for men. That is roughly 1½ and 2 ounces
respectively. Remember too those amounts reflect daily needs for pure protein absent any other compounds
including water. Since the weight of an egg is mostly water, people would need to eat more than 1½ to 2
ounces of eggs to satisfy their daily needs.
The body has no way to store protein or amino acids. They are either used to satisfy the body's daily needs
or they are "burned" into some other form of energy and their waste products are excreted. Body builders
often believe that very high levels of protein intake will cause muscle to grow faster but there is no
scientific evidence that this is true.
Is Consumption of Excess Protein then Totally Pointless?
No. As described elswhere in this website, relatively high levels of dietary protein seem to suppress appetite
and because of the high thermic effect of protein, it is a macronutrient of low effective caloric density. In
other words, a lot of protein holds relatively few calories.
Is Consumption of Excess Protein Safe?
For healthy people, excess protein intake is probably harmless to a point. For people with kidney disease,
it's another story. Because amino acids all have "amino" or nitrogen groups attached to them, the
end-products of protein breakdown are always nitrogenous compounds like urea that are excreted in urine
after being "filtered" out by the kidney. If someone sufferes from kidney disease, this extra "load" can
further weaken or damage their kidneys. While there is no hard evidence that moderate excesses in protein
intake are dangerous to healthy people, common sense dictates caution. Some body builders claim to eat
over 200 grams of protein daily and if this is true, it is likely only a matter of time until people are harmed.
About 100 grams per day seems a reasonable upper limit for healthy people.
Pharmacologic Effects of Protein and Amino Acids
Some amino acids like tyrosine and tryptophan are needed and used by the body for the production of
neurotransmitter chemicals like dopamine and serotonin (respectively). This raises the question whether
higher than normal intake of these amino acids can "push" the body to produce more neurotansmitter
chemicals to acheive effects like appetite suppression (via dopamine) or sleep induction (via serotonin).
While some research suggests that indeed it is possible, the effect is slight and there remains the open
question of wether it is a good idea to "load up" on pure amino acids (since they are never found pure in
Keep in mind that by showing hexokinase, I have deliberately chosen a complex protein. Proteins like this that catalyze
chemical reactions are called enzymes and although there are tens of thousands of different enzymes controlling a
living cell, enzymes account for just a tiny fraction of the protein we eat in food. In meat, the main proteins are
muscle proteins called actin and myosin and these molecules are simpler in structure than enzymes.
The building blocks of all protein are small molecules called amino acids. Every living thing on the planet uses exactly 22
or fewer amino acids in making its proteins. These amino acids are shownin the chart on the right (except for
pyrrolysine). The key to the incredible complexity of protein lies in the sequence of amino acids
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