What is Digestibility?
Digestibility has a specific definition when it comes to dietary proteins. As such it is more technically and accurately referred to as true protein digestibility. Unlike many confuse, it does not refer to how well you as an individual tolerate a type of dietary protein based on your individual health. It refers to how much of the ingested protein is actually available for absorption by the body. For example, if you consumed 20 grams of protein X that was 100% digestible then your body would digest (break into individual amino acids) all 20 grams of that protein. If protein X was only 80% digestible then your body would only digest 16 grams of the protein (80% of 20). The remaining 4 grams are excreted and unused by the body.
Digestibility is one of several ways to assess a protein’s quality. And quality in food/nutrition science world refers to how well a dietary protein is used by the body to build new tissue. So in other words, the better digested a protein, the higher quality it is likely to be.
What Affects the Digestibility of a Food/Protein?
The ability for the body to fully digest the protein within a food can be affected by several things including:
- Inherent fibers. Foods like beans, legumes and grains contain inherent fibers which although incredibly beneficial to the digestive system, do block some of the protein from being fully broken down and absorbed.
- Anti-nutritional factors. As the name implies, these factors inhibit the absorption of nutrients. Trypsin inhibitors, commonly found in legumes and cereals for example, blocks trypsin (a protein-breaking enzyme) from working and thus limits protein absorption.
- Food processing. Processing techniques can significantly alter the absorption of proteins. For example, the removal of starch and fiber from peas to make pea protein can render the protein highly digestible.
- Other foods consumed in the meal. Much like inherent fibers, consuming high fiber vegetables within a meal can render some of the protein in meat for example, less digestible.
Digestibility of Plant vs Animal Proteins
In general, most animal-based proteins like eggs and beef are typically highly digestible, meaning that at minimum, 90% of the protein within them is digested by the body. The same goes for dairy-based proteins. Human breast milk is known to be 100% digestible, which makes it a perfect food for growing infants.
Whole plant proteins on the other hand, are typically lower in digestibility than animal proteins because they are more likely to contain the fibers, anti-nutritional factors, etc. which negatively impact protein break down. For example, black beans are 75% digestible, so if you ate a cup of them (15g protein) your body would only be digesting 11.25 grams of protein. (But it’s not necessarily a bad thing because fiber is so good for you!) In any case, this is one of the primary reasons vegans and vegetarians have a slightly higher protein requirement than their omnivorous counterparts. Because they need to make up for the lower digestibility when consuming whole plant proteins.
What about Plant Protein Powders?
Though many assume that plant protein powders (being derived from plant foods) are also poorly digested, it’s not actually true. At least not for Growing Naturals rice or pea proteins. As stated above, food processing can significantly alter the digestibility of a protein. By removing most of the starch and fiber in the brown rice and peas–the resulting rice and pea protein powders are 100% digestible. This means every gram of protein you consume is available for absorption by your body to help build muscle and more.
How was digestibility tested?
There are several ways (methods) to test the digestibility of a protein. The tried and true method is in vivo and involves elaborate animal and or human testing and measuring the amount of nitrogen excreted either in the feces or in the ileum (end of the small intestine). As you may have guessed, we did not go with this option and measured digestibility with an in vitro enzyme assay instead, at a 3rd party laboratory.
The assay is a patented method for estimating in vivo protein digestibility. Each protein was first put through a human digestion simulation, thereby breaking it into its constituent amino acids. The amino acids were then reacted with a special compound in order to determine its overall digestibility.
By: Scarlett Full, in-house Registered Dietitian