Amino acids and proteins both have an important place in muscle building and fitness supplementation. They both serve many functions including, most importantly, maintaining the positive nitrogen balance that keeps the body in catabolic state. If amino acids are the building blocks of protein, and protein is made from amino acids, surely consuming protein is the same as consuming amino acids?
In one sense this is the case – they are both made from the same raw material. On the other hand, there differences, some of which may come as a surprise.
Even though proteins are eventually broken down into amino acids in the body, the main biological difference between protein and amino acids is in the way they are absorbed from the digestive system.
After protein is eaten, the acid and digestive enzymes in the stomach and intestine break it down. There are many factors that influence the rate of breakdown, including the presence of carbohydrates and fibre, level of pancreatic and intestinal secretions, and protein structure (1). Between proteins, this can be a very variable process, for example, hydrolysed whey is absorbed into the body in under an hour, while micellar casein may take up to eight hours before it is fully absorbed.
The intestine very rarely absorbs any protein fragment greater than four amino acids long, but di-and tri- peptides (chains of amino acids two and three amino acids long), along with single amino acids, are able to cross the cell membrane and enter the body.
Once broken down, di- and tri- peptides are taken in quickly by a specific transporter in the intestinal cell wall, while single amino acids are transported into the body using a slower system, where they compete with each other, and with sugar molecules to enter the body (2). It sounds strange, but single amino acids are absorbed less efficiently than proteins, and in some cases, more slowly.
For the reasons above, amino powders are not generally used as a sole source of nitrogen. Many amino acids have activity secondary to their role as a nitrogen source, and this is the main reason people take amino acid supplements. It is very common to use both protein and amino acid supplements.
Some well known examples are BCAAs (Leucine, Isoleucine and Valine) which can enhance both recovery and immune function (5). Leucine by itself can stimulate the secretion of insulin and other metabolic hormones (5).
Glutamine (4) transports nitrogen throughout the body and plays a role in immune system health. Tryptophan can promote sleep and enhance mood (6), and Arginine can enhance pump and may increase lean mass and endurance (9).
There are also a number of amino acids of importance to bodybuilders that do not appear in protein, like beta alanine, which can buffer the acidity in working muscle cells, helping to attenuate fatigue (7) and taurine, which may help the body better use glucose and increase strength (8).
Amino Acids in Whey Protein
Each type of protein is made up of amino acids in different proportions. The amino acid profile of a protein affects the bioavailability, the quality and the way in which is is used, and is something to take into account when selecting a supplement.
Whey is known as the "king of proteins" and for good reason. Whey contains all of the nine essential amino acids that the body cannot make, and these comprise over 60% of the total amino acids present.
Whey contains more than 20g/100g of BCAAs, which means it is an excellent protein for stimulating muscle growth and aiding recovery.
Amino Acids in Casein Protein
Whey and casein are the two major proteins present in milk. Whey accounts for 20% and casein makes up 80%. Casein is also a very high quality protein, containing all of the essential amino acids, and it has a similar amino acid profile to whey, with comparable levels of BCAAs. It is slightly lower in anabolic leucine than whey, and is digested at a much slower rate. Casein's advantage is the slow release of amino acids the body, allowing the maintenance of a positive nitrogen balance over a long period of time. Casein and whey are similar, excellent quality proteins rich in essential amino acids which complement each other well.
(1) Ten Have GA, Engelen MP, Luiking YC, Deutz NE. Absorption kinetics of amino acids, peptides, and intact proteins. Int J Sport Nutr Exerc Metab. 2007 Aug;17 Suppl:S23-36.
(2) Frenhani PB, Burini RC. Mechanisms of absorption of amino acids and oligopeptides. Control and implications in human diet therapy Arq Gastroenterol. 1999 Oct-Dec;36(4):227-37.
(3) Millward DJ. Knowledge gained from studies of leucine consumption in animals and humans. J Nutr. 2012 Dec;142(12):2212S-2219S
(4) Adeva MM, Souto G, Blanco N, Donapetry C. Ammonium metabolism in humans. Metabolism. 2012 Nov;61(11):1495-511.
(5) Negro M, Giardina S, Marzani B, Marzatico F. Branched-chain amino acid supplementation does not enhance athletic performance but affects muscle recovery and the immune system. J Sports Med Phys Fitness. 2008 Sep;48(3):347-51.
(6) Fernstrom JD. Effects and side effects associated with the non-nutritional use of tryptophan by humans. J Nutr. 2012 Dec;142(12):2236S-2244S.
(7)Hoffman JR, Emerson NS, Stout JR. β-Alanine supplementation. Curr Sports Med Rep. 2012 Jul-Aug;11(4):189-95.
(8) Essentials of Sports Nutrition and Supplements, Jose Antonio, PhD, Douglas Kalman, Phd, RD, Jeffrey R. Stout, PhD, Mike Greenwood, PhD, Darryn S. Willoughby, PhD, G. Gregory Haff PhD., p. 507
(9) Elam et al (1989), Effects of arginine and ornithine on strength, lean body mass and urinary hydroxyproline in adult males. J Sports Med Phys Fitness, 29: 52-56.