What is the Protein Efficiency Ratio (PER)?
The Protein Efficiency Ratio (PER) is a historical but still relevant method for assessing the quality of a protein source. It quantifies how efficiently a protein is used for growth by measuring the weight gain of a test subject relative to the amount of protein consumed. The assay is conducted under standardized conditions to ensure consistent and comparable results. These controlled conditions typically involve a four-week feeding trial with a group of young, growing rats, which are highly sensitive to protein quality variations.
The Standardized PER Bioassay
The official PER bioassay, defined by organizations like the Association of Official Analytical Chemists (AOAC), follows a strict protocol:
- Test Subjects: Young, weanling rats (typically 20-23 days old) are used as they are in a period of rapid growth.
- Diet: The rats are fed a diet containing a low but sufficient level of protein (around 10% of dry weight) to prevent growth from being limited by other nutrients.
- Reference Group: A control group is fed a reference protein, usually casein, which has a well-established PER value (historically 2.5).
- Measurement: Over a 28-day period, researchers meticulously measure the weight gain of each rat and the total amount of protein consumed.
- Calculation: At the conclusion of the test, the weight gain is divided by the protein intake to determine the raw PER for each protein.
This standardized approach is crucial for producing reliable data for nutritional evaluations and for meeting regulatory requirements in some countries.
The Step-by-Step Calculation Process
The calculation for the protein efficiency ratio involves two main steps: calculating the raw PER for both the test and reference groups, and then adjusting the test PER relative to the standard casein value.
Step 1: Calculate the Raw PER
The basic formula for calculating the raw PER is straightforward:
$PER = \frac{\text{Weight Gain (g)}}{\text{Protein Consumed (g)}}$
To apply this, follow these steps:
- Feed the Test Group: Feed a group of weanling rats a diet containing the protein being tested (e.g., soy protein). Record their total weight gain in grams over the test period.
- Measure Protein Consumption: Track and measure the total protein consumed by the test group during the same period.
- Calculate Raw PER: Divide the total weight gain by the total protein consumed. This gives you the raw PER for the test protein.
Example: If the test group of rats gained a total of 120 grams and consumed 50 grams of the test protein, the raw PER would be $120 / 50 = 2.4$.
Step 2: Adjust the Test PER
Since the PER value can be influenced by many factors, including the specific batch of casein used, the result is standardized against a reference casein with a consistent value (2.5).
$Adjusted PER = \frac{\text{PER of Test Food}}{\text{PER of Casein Control}} \times 2.5$
- Calculate Casein Control PER: In the same experiment, a control group is fed a diet with reference casein. Calculate its raw PER using the same method.
- Adjust the Test PER: Use the adjusted formula to compare your test protein's raw PER to the control group's raw PER, normalizing the result to the standard 2.5 value.
Example: Using the previous result of a raw PER of 2.4 for the test protein and assuming the casein control group's raw PER was 2.6, the adjusted PER would be $(2.4 / 2.6) \times 2.5 \approx 2.31$.
Comparison: PER vs. Modern Protein Quality Metrics
While PER has been a long-standing standard, newer methods are now considered more accurate for human nutrition. The limitations of PER, such as its inability to distinguish between lean mass and fat gain, led to the development of methods like the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and the Digestible Indispensable Amino Acid Score (DIAAS).
| Feature | Protein Efficiency Ratio (PER) | Protein Digestibility Corrected Amino Acid Score (PDCAAS) | Digestible Indispensable Amino Acid Score (DIAAS) |
|---|---|---|---|
| Basis of Measurement | Animal (rat) growth bioassay. | Amino acid composition and fecal digestibility. | Individual amino acid composition and ileal digestibility. |
| Primary Limitation | Doesn't differentiate between lean mass and fat gain; may not correlate perfectly to human needs. | Overestimates digestibility due to measuring at the fecal level; caps scores at 1.0, not allowing for differentiation of high-quality proteins. | Requires more invasive ileal measurements, posing ethical and practical challenges for human testing. |
| Regulatory Use | Used for infant formulas in the US and food protein claims in Canada. | Previously the gold standard; still widely used in food labeling worldwide. | Recommended by the FAO since 2013 as a more accurate standard for human protein quality assessment. |
| Key Strength | Simple and cost-effective, providing a basic comparison of protein sources. | Provides a value based on human amino acid requirements and digestibility. | Provides the most accurate measure of protein digestibility and bioavailability for humans. |
Interpreting the Adjusted PER Value
The adjusted PER value provides a comparative score. A higher adjusted PER indicates that the protein source is more efficient at supporting growth in the test subjects. For example, animal proteins like egg, casein, and whey typically have higher PER values than plant-based proteins such as wheat. This difference is largely due to the varying balance of essential amino acids.
Limitations and Modern Context
The PER method's main limitations stem from its reliance on rat growth data, which does not always directly translate to human nutritional needs. The different amino acid requirements and metabolic rates between humans and rats can lead to discrepancies. For instance, PER tends to overestimate the value of some animal proteins and underestimate some vegetable proteins relative to their human efficacy.
Despite these shortcomings, PER is not entirely obsolete. Its use in specific regulatory contexts underscores its historic importance in food science and nutrition labeling. Food scientists and regulatory bodies continue to utilize a variety of methods, including PER, PDCAAS, and DIAAS, to get a comprehensive picture of protein quality.
Conclusion
Calculating the protein efficiency ratio (PER) is a standardized, two-step process that compares the weight gain of a test subject to its protein intake, with the final result adjusted against a casein reference. While superseded by newer methods like DIAAS for comprehensive human nutritional analysis, PER remains a foundational concept in food science and a required metric for certain food product regulations in North America. Its historical and ongoing use highlights the complex evolution of protein quality assessment, moving from simple growth measurements to more sophisticated, human-specific digestibility and amino acid scoring methods. Understanding how to calculate PER provides valuable context for interpreting modern protein quality standards.
