One of the most commonly asked questions on MuscleTalk is 'How much protein can I absorb in one sitting?' Unfortunately this is another 'How long is a piece of string?' question, as there are so many factors to consider, and in reality, there is no maximum amount. Often the poster is referring to how much protein can be utilised in a time period I guess, but this again depends on a multitude of factors and I would never like to put a figure on it.
This article sets out to give readers some basic background as to how proteins and carbohydrates are digested and absorbed, as being armed with this information will help you plan your nutrition in order to maximise gains. I am not going to name all the chemicals, enzymes and substrates involved in the processes as this is beyond the scope of this article; if you want to know this any basic physiology textbook will give you the information. Also this article will not debate the fate of these nutrients post absorption.
Digestion of food begins in the mouth and continues until all nutrients have been absorbed in the intestines. A number of digestive enzymes are involved in protein and carbohydrate digestion which result in short chain structures of the nutrients (oligomers) or the basic unit of each (monomers). The simplest unit of proteins are amino acids of which there are 20-odd different types. Two amino acids linked together are called dipeptides, three are called tripeptides, a few amino acids in a chain are called oligopeptides and long chains of them are called polypeptides.
The simplest units of carbohydrates are monosaccharides, two of which together are called disaccharides, a few in a chain are oligosaccharides and long chains of monosaccharides are called polysaccharides.
Many articles will have you believe that proteins and carbohydrates can only be absorbed from the intestinal lumen in their simplest form, i.e. as amino acids and monosaccharides, but this is not the case. Amino acids and monosaccharides are absorbed in their basic monomer form by an active sodium dependant transport process, however di- and oligopeptides and di- and oligosaccharides can be taken up in their short chain form and then further broken down to free amino acids and monosaccharides when inside the cells of the intestine rather than in the lumen. The process of this is not precisely known but is definitely unrelated to a sodium transport system and thought to be either cell enzyme-related or dependent on an ion gradient. Thus there are two unrelated systems in operation to absorb protein and carbohydrates.
OK, I can hear you saying: 'Enough boring science, James, how does this affect my bodybuilding nutrition in practical terms?' You may be aware of the concept of amino acids competing for receptor uptake when being absorbed. If the protein available at intestinal level is both monomer and oligomer forms then absorption will be maximised. If you are eating a combination of food sources then both processes will be optimised naturally due to digestion processes. Also 'peptide' supplement formulas using proteins from different sources will have an advantage here as some of the peptides will be fully digested to amino acids before absorption and some will still be peptides.
Whey protein is semi-elemental, i.e. it is naturally partially digested, and made up of amino acids and short chain peptides. So, whey consumed with nothing else except water, will be digested and absorbed very quickly, which can be advantageous when demand is high (post workout and first thing in the morning). However, at other times whey on its own may pass through the digestive system too quickly and be fully broken down to amino acids by the time it reaches the intestines level hence absorption will not be maximised. For this reason other protein blends are preferred (such as those containing casein or egg) or, even better, eat protein from food sources and maybe compliment the meal/snack with just 10g of whey to bump this up.
As explained above, protein and carbohydrate are absorbed by the same two methods, but also there is a degree of synergy of absorption, i.e. the presence of both in the small intestine compliments each other's absorption.
For optimal results I would suggest only whey in water first thing in the morning and post workout, and at other times a combination of food sources of protein or a mixed protein formula consumed with carbohydrates.