Current Progressive Dairy digital edition

What is a functional protein?

Jim Quigley Published on 01 March 2010

Originally published: March 1, 2011, in Progressive Dairyman magazine, this article has received more than 4,300 pageviews, with a majority of the online traffic coming from a Google search for “functional proteins.” to jump to the article.

In this article, Jim Quigley explained that proteins that have biological activity can be referred to as functional proteins. Immunoglobulins found in colostrum are an example of beneficial functional proteins.



“Our traditional concept of protein digestion has changed,” Quigley wrote. “We now recognize that proteins in the intestine can retain their functional characteristics and affect the animal – either positively or negatively.”

Incorporating functional proteins when feeding milk-fed calves, either through colostrum or through plasma proteins, can enhance calves’ immunity.

Quigley said, “… the addition of functional proteins to the diet can reduce disease, reduce the use of antibiotics and improve profitability of the overall operation.”

See Quigley's related article, "What’s new(er) in colostrum research? "

Proteins are long chains of amino acids put together in a specific sequence to perform a particular function in the body. The amino acid sequence of proteins, and the subsequent three-dimensional structure of the protein, are incredibly complex and responsible for all their actions in the body.


Nutritionists usually consider protein structure to be relatively unimportant. Indeed, we generally measure crude protein (the total amount of nitrogen in a feed multiplied by 6.25 to estimate overall protein) without considering protein structure. The traditional view is that through the actions of digestion – acids, enzymes, etc. – all proteins are digested to individual amino acids in the intestine, and those amino acids are subsequently absorbed into the body to be resynthesized into animal protein. Of course, some proteins are more digestible than others, and we sometimes use the digestible protein concept in formulating rations for animals.

However, evidence clearly indicates that proteins have important biological effects in the animal. That is, when consumed, these proteins “do something” In other words, they perform some biological function in the body. Proteins that have biological activity can be referred to as “functional proteins.” There are many examples of functional proteins in animal nutrition.

For example, nutritionists have long known that raw soybeans contain a protein called trypsin inhibitor. Soybean trypsin inhibitor binds to the enzyme called trypsin, thereby impairing the animal’s ability to digest protein. This disrupts digestion and can cause reduced growth and efficiency and can cause scours. Soy products are usually processed by heating or chemical extraction to disrupt the three-dimensional structure of the enzyme, thereby making it unable to inhibit the trypsin enzyme. The animal can then properly digest the proteins it consumes. If the conventional theory of digestion held, then the trypsin inhibitor would be digested to its component amino acids and would not negatively affect digestion in pigs, calves and poultry.

Another example of a functional protein is a protein in egg whites called avidin. This functional protein binds biotin with great specificity and tenacity. Animals fed diets containing large amounts of egg white may become deficient in biotin, even though biotin is added to their diet. Heating (cooking) disrupts the three-dimensional structure of avidin so that it can no longer bind biotin. It’s a good reason not to eat raw eggs!

Not all functional proteins have negative effects. Some functional proteins provide great benefit to the animal, particularly during periods of stress. Immunoglobulins (Ig), found in plasma proteins, colostrum and eggs, can positively impact the animal’s immune system, improving health, reducing the effects of pathogenic bacteria and viruses and improving recovery following an intestinal challenge.

To understand the effects of functional proteins on calf health, it’s important to understand the interaction of the intestine and the immune system. Though we often think of the intestine as simply the site of digestion and absorption of nutrients, the intestine is actually one of the most important immune organs in the body. The intestine contains more immune tissue than any other organ in the body. This makes sense, considering the length of the intestine (the intestine in a newborn calf is 10 to 12 meters, or about 33 to 39 feet in length) and the constant exposure of the intestine to foreign antigens and potential pathogens. The intestine is lined with sophisticated monitoring systems and the ability to mount an immune response when an infection occurs.


Feeding Ig can help reduce the risk of scours in young, milk-fed calves. Numerous studies with bovine colostrum, plasma proteins and egg powder have documented that Ig are not completely digested (they retain their function) and cause an immune response in the intestine. When calves are exposed to a challenge with E. coli, Salmonella or other pathogen, the Ig in the diet can bind to the pathogen, inactivating it or interfering with its ability to bind to the intestinal wall. Then, the organism passes harmlessly through the intestine into the feces. It’s important to remember that the effect of the Ig is not due to absorption of Ig into the bloodstream but a local effect in the intestine. Indeed, the body itself secretes Ig (both IgA and IgG) into the intestine as protection against invasion from intestinal bacteria, protozoa and viruses.

Researchers in Europe, Japan and the U.S. have shown that oral supplementation of functional proteins in plasma, colostrum and egg powder can reduce the risk of scours in milk-fed calves. Trials with calves have been conducted with both calf milk replacer and whole milk. Studies have looked at specific challenges (cryptosporidium, E. coli, others) and practical on-farm feeding trials. Generally, feeding two or more grams of functional protein from colostrum or plasma protein will elicit a measurable response; however, the actual amount required in a calf depends on the specificity of the Ig and degree of challenge. Results from these studies have reported reduced death loss, lower scours, more rapid daily gains and reduced shedding of organisms (in the case of cryptosporidium, reduced shedding of oocysts).

Colostrum and plasma proteins have an advantage in that they provide Ig against a wide range of potential pathogens, whereas egg powders contain Ig against a specific pathogen. If the pathogen on the farm differs from the one used to produce the egg Ig, the egg powder will provide little benefit.

The intestinal immune system is also closely linked to the immune system in the rest of the body. Immune cells migrate between the intestine and the rest of the body, transferring the “immune information” they carry with them. So, if we are able to influence the intestinal immune system with functional proteins, we can also affect the immune system in the rest of the body. Researchers in Spain recently reported that rats fed plasma proteins orally had improved immune response when they were exposed to inflammation in the lung. Other researchers have reported that prolonged feeding of bovine colostrum can reduce the risk of respiratory infections in humans. When considered together, these studies show an interaction of functional proteins fed orally and the immune system – both intestinal and respiratory. Supplementation of the diet with functional proteins, such as Ig, can improve overall immunity and reduce morbidity and mortality in young animals.

Our traditional concept of protein digestion has changed. We now recognize that proteins in the intestine can retain their functional characteristics and affect the animal – either positively or negatively. Functional proteins like those in plasma proteins and colostrum can affect the intestinal immune system and immunity in the rest of the body, thereby improving overall animal health and productivity. Responses are particularly valuable in animals – such as milk-fed calves – with under-developed immune systems. In these animals, addition of functional proteins to the diet can reduce disease, reduce the use of antibiotics and improve profitability of the overall operation. PD

Jim Quigley
  • Jim Quigley

  • Vice President & Director of Calf Operations
  • APC Inc.
  • Email Jim Quigley