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Protein intake is something that is of concern to almost every bodybuilder out there, but few stop to consider how much of this protein is being absorbed. If protein is being poorly absorbed, not only can this mean less of a result than expected, but wasted money on nutrition. How can protein absorption be optimised?

Protein Absorption and Digestion

Proteins have to be broken down to short peptides or single amino acids before they can be absorbed by the body. The digestion process typically starts in the stomach, where Hydrochloric acid (HCl) denatures the protein and cleaves a pro-enzyme called pepsinogen in the stomach to form an enzyme called pepsin, which breaks down the denatured chains of amino acids into long peptides.

The long peptides then pass into the small intestine where they are further broken down by enzymes such as Trypsin and Chymotrypsin. Once the protein fragments are three amino acids long, or smaller, they are absorbed by the small intestine.

Protein Absorption Rate

There are many variables that can affect the rate of protein absorption. Some of the most important are:

Type of protein – many of us have heard of fast acting proteins, like hydrolysed whey, and slow acting proteins, like micellar casein. Hydrolysed whey contains short peptides because it has already been hydrolysed, or broken down, during the manufacturing process, whereas micellar casein forms a mass in the stomach, which is digested slowly over a long period of time. Additionally, some proteins are easier for the body to absorb than others. A study showed more amino acids pass through the digestive system without being absorbed when soy protein is consumed, in comparison to an equivalent dose of whey (1).

Other nutrients – What you eat at the same time as your protein can affect absorption. The average time on an empty stomach for viscous fluids, like a protein shake, to pass through to the end of the small intestine, where protein absorption ceases, is about about 90 minutes (2). Eating protein as part of a large meal can slow passage time, increasing absorption as larger amounts of food take longer to break down and move through the stomach and small intestine than smaller amounts. In addition, single amino acids, produced when protein breaks down, can compete for absorption with monosaccharide sugars.

Conditions in the digestive system – Conditions such as low stomach acid, which may be caused by the use of antacids, can slow the digestion process or make it less efficient. Acid plays numerous roles in digestion, hydrolysing protein, which is the first step in breakdown, cleaving pepsinogen into pepsin which is the active form of this protein cleaving enzyme, and the arrival of acidic material into the small intestine stimulates the release of pancreatic juices, the next step in digestion (3). Additionally, he digestive system secretes hormones, and can slow down or speed up digestion in response to the nutrients present in the stomach or intestine (4).

What Helps Protein Absorption?

Slowing down the rate of passage through the intestine – As discussed above, the rate of protein absorption into the body is subject to a number of variables, and various types of protein have different rates of absorption into the body. At the top end is whey, from which 8-10g amino acids/hour are absorbed, to egg, at 1.3g/hour. Although the body compensates for this, slowing the rate of passage through the digestive system can give the body more time to utilise all the protein in a source of nutrition. This can be done by eating protein as part of a meal, or adding a bulking agent such as oats to a liquid nutrition source like a whey shake (4).

Optimising protein breakdown – The role of acid and digestive enzymes in the efficient breakdown and absorption of protein cannot be understated. Some manufacturers are adding extra proteolytic enzymes to protein formulations and this has shown some success in enhancing protein absorption (5). Ensuring medications like antacids are not taken concurrently with protein may enhance effeciency of protein breakdown.

Mix of proteins – Eating a mix of fast and slow absorbing proteins can ensure digestion and absorption of protein takes place over a long time. The body does have a limited rate of uptake, and utilisation of protein within the body is also subject to many factors (4). Using a blended protein is a common solution, and many of the top selling protein supplements are made from a protein mixture.

Protein Absorption Myth

The idea that the body cannot process more than 20-30g of protein in a single intake is widely circulated myth. This notion is based on the transit time and absorption rate of protein in the digestive system. This figure does sound like it has a solid basis, but it has been proven false on many occcasions.

In one notable study which directly tested this myth, subjects were fed 80% of their daily protein intake either as a single 60g meal, or split over four meals of 15g. In both cases, identical levels of protein were absorbed and utilised (6). A similar study compared protein absorption in a group receiving nutrition over the full day to a group that fasted intermittently and consumed their nutrition over a short time frame. No changes to protein metabolism were detected (7).

While steps can be taken to maximise protein absorption, there is no evidence to suggest a strict upper limit on the amount that can be taken in at one time, and a lot of evidence suggesting most people are able to digest and effectively metabolise large servings of protein. It is important to note that everybody is different, and some people may metabolise nutrients at a different rate to others. The most important thing to do is to find a way to eat protein that works, and gets results for you.

(1) Gaudichon C, Bos C, Morens C, Petzke KJ, Mariotti F, Everwand J, Benamouzig R, Daré S, Tomé D, Metges CC. Ileal losses of nitrogen and amino acids in humans and their importance to the assessment of amino acid requirements. Gastroenterology. 2002 Jul;123(1):50-9.
(2) Mahé S, Roos N, Benamouzig R, Davin L, Luengo C, Gagnon L, Gaussergès N, Rautureau J, Tomé D. Gastrojejunal kinetics and the digestion of [15N]beta-lactoglobulin and casein in humans: the influence of the nature and quantity of the protein. Am J Clin Nutr. 1996 Apr;63(4):546-52.
(3) Freeman HJ, Sleisenger MH, Kim YS. Human protein digestion and absorption: normal mechanisms and protein-energy malnutrition. Clin Gastroenterol. 1983 May;12(2):357-78.
(4) Bilsborough S, Mann N. A review of issues of dietary protein intake in humans. Int J Sport Nutr Exerc Metab. 2006 Apr;16(2):129-52.
(5) Julius Oben, Shil C Kothari, Mark L Anderson. An open label study to determine the effects of an oral proteolytic enzyme system on whey protein concentrate metabolism in healthy males. Journal of the International Society of Sports Nutrition 2008, 5:10
(6) Arnal MA, Mosoni L, Boirie Y, Houlier ML, Morin L, Verdier E, Ritz P, Antoine JM, Prugnaud J, Beaufrère B, Mirand PP. Protein feeding pattern does not affect protein retention in young women. J Nutr. 2000 Jul;130(7):1700-4.
(7) Maarten R Soeters, Nicolette M Lammers, Peter F Dubbelhuis, Mariëtte Ackermans, Cora F Jonkers-Schuitema, Eric Fliers,Hans P Sauerwein, Johannes M Aerts, Mireille J Serlie. Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism. Am J Clin Nutr November 2009 vol. 90 no. 5 1244-1251

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