Protein quality: Real nutrition versus public perception

Many petfoods today are promoted for the ingredients they do or do not contain rather than their nutritional performance, disingenuously playing off of the consumers’ perception about the ingredients rather than their real nutritional value.

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Many petfoods today are promoted for the ingredients they do or do not contain rather than their nutritional performance, disingenuously playing off of the consumers’ perception about the ingredients rather than their real nutritional value. Since proteins are the dietary cornerstone for carnivores, they may be the most exploited in this manner. Whether it be “yuck” factors with organ meats, misunderstood regulatory definitions for terms like “by-product” or elitism with novel or exotic species, there are important nutritional decisions being made based solely on the ingredient name and not on any measured nutritional parameter.

Perhaps because consumers and retailers don’t understand what nutritional protein quality is relative to an animal’s needs, they don’t have anything else to use for decision-making. However, if they understood their purchasing decisions based on measured ingredient quality parameters it is very probable commercial petfoods would change dramatically.

Protein quality to the  nutritionist is a measure of how well the amino acid composition and the availability of the dietary protein matches the animal’s requirements. It’s a pretty simple idea on the surface, but one that can be clouded quickly by a number of factors including dilution with non-essential amino acids, connective or structural proteins that decrease overall digestibility and availability, and complexation or destruction of the protein due to indigestible carbohydrates, anti-nutritional factors and (or) heat damage during processing.

While we can formulate to overcome some of these insufficiencies with complementary ingredients, or even supplement with synthetic amino acids, there is need to evaluate individual ingredients for their contribution to the whole. Selecting the right methodology can be all-important; the goal is to identify a method that is quick, inexpensive and provides valuable insight. Initially it might be used for research, but ultimately the best method would be used for quality control, daily decision-making, communication and even marketing.

Methods used to determine  protein quality include analysis for amino acid composition, digestibility or bioavailability experiments, nitrogen balance studies and various performance assays. Each has its merit and some can be combined to enhance interpretation.

At the most elementary, some ingredients are evaluated by an in vitro enzymatic assay to determine degradability (e.g., pepsin digestibility test). This can be effective if one has a very routine set of ingredient samples and the strength of the enzyme is specified. It doesn’t necessarily tell us whether or not the amino acids in the ingredient will match the animal’s needs, but it does give some quick idea regarding the magnitude of digestibility within an ingredient class. Comparing the amino acid composition of an ingredient to the animal’s requirements, a so-called “index” value, can tell us where a key amino acid might be limiting. However, this approach may overestimate the quality by crediting an amino acid that has been made unavailable by processing (e.g., heat damage of lysine due to the so-called “browning” or Maillard reaction).

To compensate, one can conduct a protein digestibility corrected amino acid score (PDCAAs, pronounced PeeDeeKays by some) where amino acid composition is weighted according to overall protein digestibility. The general convention is to use rats to determine the digestibility of the ingredient. This may still overestimate the availability of a specific amino acid bound in the matrix. A digestibility test in the target species, i.e., dog or cat, can help but is time consuming, expensive and can’t directly determine the value of a single ingredient. The cecectomized rooster digestibility assay is used in some research as a surrogate. This elegant procedure produces a true energy and amino acid digestibility value for a single ingredient using an animal model, but it doesn’t necessarily provide a reference to the animal’s ability to utilize the amino acids that were digested.

That requires one to perform a balance study with the target species and collect urine to ascertain biological value or net protein utilization. This procedure is an extension of the digestibility study and does not measure an ingredient directly; it becomes too costly to perform for routine analysis. That is where a performance assay like the protein efficiency ratio (PER) or net protein ratio (NPR) can be effective. The PER and NPR measures animal gain per unit protein intake when fed a restricted diet for a brief period of time. This can be done rapidly with farm (chicks) or lab (rats) species and yields very robust data that is easy to compare to standards.

Most petfoods exceed the  animal’s protein and amino acid requirements by a significant margin and this seems to be increasing with each new product entry. So why should petfood makers care about the real quality of the protein they use in their products? In short, proteins are expensive and wasting them has implications on animal health.

To explain, protein digestibility at the small intestine is around 80%-85% of that consumed for most petfoods. That means some 15%-20% of whatever protein in the diet, along with endogenous secretions, becomes part of the indigestible materials mix entering the colon. These materials become substrate for the colonic bacteria.

Rather than fiber-like indigestible carbohydrates that promote beneficial lactic acid fermentation, the indigestible proteins shift the population of the microorganisms to more clostridial and pathogenic species. The spent amino acids are degraded during fermentation into putrefactive compounds like skatole, indole, putrescine, cadaverine and ammonia. Some of these biogenic amines are thought to be harmful, contribute to fecal odor and may change the osmotic balance in the colon resulting in inconsistent and runny stools. Thus, anything that reduces protein utilization before the colon should be considered a negative. This is exactly what an ingredient protein quality evaluation can tell us.

Most companies today don’t monitor the quality of their proteins on a routine basis. Frankly, most are caught-up in the “No this and No that” game being pushed by various consumer groups. Should they elect to re-cast their attention to what is really important to animal nutrition, we would soon find by-products more accepted, organ meats given higher value and some of the vegetable proteins used more judiciously in an effort to focus the diet on proper quality for the animal, rather than misaligned consumer perceptions.

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