Protein and amino acids - a primer
Sometimes it feels like an advanced degree is necessary to make use of dietary recommendations and nutrition research. The more knowledge we acquire, the easier decision-making will be, but some fundamental concepts can go a long way towards aiding sensible choices about products we purchase and consume.
The food we eat is made up of, in varying amounts, macronutrients (protein, carbohydrates, and fats) and micronutrients (an array of vitamins, minerals and other important/essential nutrients). There are, of course, other significant, undigestible components (dietary fiber), as well as an abundance of water and special-case entities such as alcohols and food additives.
Proteins are chains of amino acids that can be found in such food sources as meats, eggs, nuts, grains, legumes, and dairy products (eg, milk and cheese). Protein is digested and degraded once consumed, and the resulting amino acids (and their derivatives) are then absorbed by the gastrointestinal tract. Of all the chemically possible amino acids, 20 are used by the human body to either synthesize (new) proteins and other biomolecules, or are oxidized to urea and carbon dioxide as a source of energy. These twenty are referred to as the standard amino acids, as they are are encoded, in living things, by the standard genetic code. Of these twenty, 12 are non-essential, in that (in adult humans), they can be synthesized from other compounds (in children, some of the synthesis pathways of some of the non-essential amino acids are not fully developed…meaning they are, in fact, semi-essential in the early stages of life); the remaining 8 are the essential amino acids (ie, they cannot be synthesized by the human body and must therefore be ingested).
The essential amino acids are: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (of these, leucine, isoleucine and valine are known as the branched-chain amino acids…they have particular significance when it comes to exercise as they make up approximately 1/3 of skeletal muscle in the human body, and play an important role in protein synthesis [Blomstrand E et al, 2006 - PubMed link]).
The non-essential amino acids are: alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, tyrosine, arginine, and histidine.
The minimum amount of protein an average adult requires (the Recommended Dietary Allowance [RDA]) is quoted as being 0.8g protein per kg bodyweight per day. The published Acceptable Macronutrient Distribution Range (AMDR) for protein, however is 10-35% of the daily energy intake (for a sedentary reference male)…or a protein intake of 0.95-3.3g protein/kg bodyweight/day (Wolfe RR, Miller SL, 2008 - PubMed link). Wolfe and Miller point out that “…if the reference man were to consume the RDA of protein, it would constitute only 8.5% of his energy intake, which is below the lowest recommended percentage of the AMDR”! It is generally accepted that, for strength athletes, a moderate (1.40g protein/kg bodyweight/day) or high-protein diet will result in whole body protein synthesis (WBPS), while the minimum amount described by the RDA will not (Tarnopolsky MA et al, 1992 - PubMed link). Levels of 1.5-2.0g protein/kg bodyweight/day have been suggested for this group (Lemon PW, 1991 - PubMed link). Requirements are also increased during growth/development, pregnancy, breastfeeding, or when recovering from trauma/surgery.
Persons involved in intense athletic endeavors, including bodybuilders, will often consume protein in amounts exceeding this moderate protein intake level. The lay media, in particular, has put forward the notion that “too much protein stresses the kidney”. The literature most often quoted in association with this claim is based primarily on animal models and patients with co-existing renal disease…the summarized, but considered, response to this is that there is insufficient “research demonstrating a link between protein intake and the initiation or progression of renal disease in healthy individuals” (Martin WF, Armstrong LE, Rodriguez NR, 2005 - PubMed link, full text link). Indeed, the Institute of Medicine concludes that there is inadequate scientific evidence for recommendations of an upper limit of protein intake, and instead provides the AMDR for protein indicated above - 10-35% of the daily energy intake (0.95-3.3g protein/kg bodyweight/day).
A few comments on protein sources are important to round out an introductory discussion. A complete (or whole) protein is one that contains all of the essential amino acids in the proportions required by the human body. Most animal (including dairy and seafood) sources, and some plant sources, of protein contain an appropriate complement of the essential amino acids. Most plant/vegetable sources of protein are deficient in one or more of the essential amino acids (often lysine or methionine), and, if an individual’s diet is deficient in, or does not contain, animal-sourced protein, this must be compensated for by consuming a complementary source to achieve a balanced amino acid intake (eg, beans & rice - or, more generally legumes & seeds/nuts/grains). This protein combining is sometimes referred to as having to take place in each meal, but it now seems to be generally considered adequate to balance amino acid intake by consuming multiple vegetable sources of protein throughout the day. Vegetable sources that are complete without having to resort to some variation on protein combining include soybeans, quinoa, and spirulina.
