Structure & Bioavailability of Protein
Structure of Protein and How it's Digested and Utilized
Although many athletes and health-conscious consumers know about the benefits of consuming complete protein for a balanced diet, few really understand protein bioavailability, or the measure of how much a protein is utilized in the body to build muscle and repair tissue for optimum performance.
Unlike what most think, the bioavailability of protein is not just a function of the essential amino acids that are present and how they relate to the body’s requirements. It's also dictated by the structure of the protein. Some proteins are more amenable to being broken down (digested) than others. To better understand this point, it is necessary to discuss briefly how proteins are assembled from amino acids and the various shapes those proteins take.
There are some 20 different amino acids which are commonly identified. Each and every protein is made from these 20 amino acids put together in varying order and in varying amounts and combinations, thus providing the possibility of almost limitless combinations. That said, most proteins are large molecules that may contain several hundred to many thousand amino acids arranged in branches and chains.
The assembly of amino acids into proteins is actually determined and directed by information encoded in your genes. Each protein has its own unique amino acid sequence as specified by the gene encoding that particular protein. Protein synthesis takes place inside cellular cytoplasm and can actually reach a rate as high as the joining of up to 20 amino acids per second in a given cell. In fact, the assembly of amino acids is responsible for more than just the creation of protein. It is also responsible for the creation of peptides and polypeptides, which can be thought of as "short" or "incomplete" proteins. It should not be surprising then that the definitions of proteins, polypeptides, and peptides somewhat overlap. The protein designation, however, is generally used to refer to longer, complete biological molecules in a stable structure.
The size of a synthesized protein can be measured by the number of amino acids it contains and by its total molecular mass. Some proteins may contain just a few hundred amino acids strung together, but the largest can contain close to 30,000 amino acids all chained together.
So how does a protein composed of a chain of 27,000 amino acids achieve stability? And the answer is through structure. These long chains fold in on themselves to form stable structures.
Most proteins fold into unique 3-dimensional structures. The shape into which a protein naturally folds is known as its native state -- although proteins may shift between several related structures during the course of performing their biological functions.
There are many reasons biochemists try to determine the various structures of any given protein. Most notably, those structures give clues as to the function of the protein in the human body. But for our purposes, there's one primary reason for being aware of protein structure. Protein structure plays a major role in determining how readily it can be broken down into its constituent amino acids during the digestive process. In other words, it plays a major role in determining the bioavailability of the protein and its propensity to stimulate allergic responses.
How what you eat with protein affects your health
When found in nature, protein never comes by itself. Whether from animal or vegetable sources, protein comes in the presence of various fats and carbohydrates. In most cases, their presence is a non issue. But in a small number of cases, these "extras" play a major factor in determining the digestibility of the protein. For example, some accompanying nutrients can inhibit proteolytic enzymes that would normally break down the protein, or can suppress the release of stomach acid necessary for the digestion of the protein, or simply cover the protein so that enzymes and stomach acid cannot reach it.
Determining protein bioavailability to optimize nutrition
Protein bioavailability is the sum total of the three factors we mentioned above:
- The mix of amino acids in the protein -- or in the combination of proteins eaten during the day. Remember, the shortage of an essential amino acid provides a limiting factor on how much of the overall protein can be utilized by the body.
- The structure and size of the protein molecule. The larger and more tightly folded the molecule, the less able the body is to break it down. Large proteins that frequently undergo incomplete digestion include those found in wheat, corn, dairy, and soy. It is not coincidental that these foods are identified by the FDA as being highly allergenic. (We will discuss protein allergies more in our next newsletter.)
- The other foods (or components in the protein source itself) that inhibit the breakdown of the protein.
Protein utilization can be measured
There are several tests for measuring protein utilization, or bioavailability, although they're a bit like the story of the blind men describing an elephant -- each one gives an incomplete picture. The blind man who feels the trunk says an elephant is like a snake. The one who feels its legs says an elephant is like a tree. The one who feels the ears says an elephant is like a giant fan. And the one who feels its body says an elephant is like a massive wall. Each one provides useful information; but each one also provides an incomplete picture.
The Kjeldahl method is the standard for measuring the total protein concentration in food. It provides the number that you normally see on nutrition labels on the side of food packages. Unfortunately, it tells you nothing about how much of that protein actually gets used by the body -- which in some cases can be very little.
Biological value (BV) measures how much of the protein that you eat gets incorporated into your body tissue. It does so by measuring how much of the nitrogen in the protein you eat is absorbed by the body and then how much is excreted. The assumption is that the difference is what got incorporated into your body protein. Its weakness is that BV varies greatly depending on how food is prepared and according to what other foods were eaten in the recent diet that can alter nitrogen measurements. Although the following three methods all provide better measures of protein utilization, BV is still commonly used by nutritionists out of force of habit.
Net protein utilization (NPU) is the ratio of amino acids converted to proteins to the ratio of amino acids supplied in the protein source. Experimentally, this value is calculated by determining the amount of dietary protein you are consuming and then measuring how much nitrogen is excreted. It is significantly affected by the limiting amino acids (as discussed earlier) in the particular food.
Protein Efficiency Ratio (PER) is based on the weight gain of a test subject divided by its intake of a particular food protein during the test period. Theoretically, it is a biological assay of the quality of a particular protein, measured as the gain in weight of an animal per gram of a particular protein eaten. At one time, this was the industry standard, but unfortunately PER is based upon the amino acid requirements of growing rats, which differ noticeably from that of humans.
Protein digestibility corrected amino acid score (PDCAAS) evaluates protein quality based on the amino acid requirements of humans. This is now the preferred standard. Nevertheless, it too has holes.
- PDCAAS takes no account of where proteins have been digested and cannot account for proteins that are absorbed by bacteria in the digestive tract.
- PDCAAS is calculated solely on the basis of single protein consumption and therefore once again does not calculate the changes in protein utilization resulting from the intake of complementary protein sources.
Improving protein utilization for optimum health
Now let's take what we've covered so far and see if we can extract some benefit from it that will help us improve our ability to improve the utilization of the protein we eat.
Consume more than one type of protein
As we've discussed, protein utilization is defined, to a large degree, by the limiting protein in the diet. Even complete proteins (those containing all of the essential amino acids) can still be out of balance so as to limit maximum utilization. Although dairy and egg tend to be well balanced and largely avoid this problem, they present other allergenic issues such as gas, bloating, and cramping. Meat, chicken, and fish, on the other hand, can benefit from the consumption of other proteins that help balance them out. And soy, most definitely.
Bottom line, the key to high protein utilization is to be cautious of the type of protein you are ingesting. To learn about the different types of proteins and what is the most beneficial for overall health, check out our Protein Types page.