What is the Protein Efficiency Ratio?
The protein efficiency ratio (PER) is a measurement of protein quality that assesses how effectively a protein source supports the growth of a test animal. The higher the PER value, the better the quality of the protein is considered to be. A good quality protein is one that contains all the essential amino acids in the right proportions needed for bodily functions, particularly growth. The PER bioassay was developed in 1919 and gained widespread use in the food industry and regulations. It is most suitable for evaluating the needs of growing organisms, which is why it was historically used for infant formula testing. While its prominence has waned, understanding the PER is crucial for grasping the evolution of protein quality assessment.
The Method of Calculating PER
The calculation for the protein efficiency ratio is a straightforward formula that uses animal feeding data.
PER = (Weight Gain in grams) / (Protein Consumed in grams)
To conduct a PER assay, food scientists must follow a standardized protocol, often outlined by organizations like the Association of Official Analytical Chemists (AOAC). The typical procedure involves:
- Test subjects: A group of weanling rats (around 21 days old) is used, as they are in a period of rapid growth.
- Diet: The rats are fed a diet with a fixed percentage of protein (typically 10% of the total diet) from the test protein source.
- Duration: The feeding trial usually lasts for a period of several weeks, such as three or four weeks.
- Measurements: The rats' body weight gain and their total protein intake are carefully recorded during the trial.
- Standard comparison: The calculated PER for the test protein is then compared to a reference protein, usually casein, which is assigned a standardized PER of 2.5. A corrected PER is often reported to account for variations between trials.
Limitations of the PER Method
Despite its long history, the PER method has significant limitations that have led to its replacement by more sophisticated techniques for human nutrition labeling.
- Animal-based: The most significant drawback is that it relies on studies conducted on rats, which have different amino acid requirements than humans. Results from animal studies may not accurately reflect how a protein is utilized in the human body.
- Measures growth only: PER is primarily a measure of growth and is not suitable for assessing the protein needs for maintenance in adult organisms.
- Cannot differentiate weight gain: The test cannot distinguish between weight gained as lean body mass (muscle) versus fat.
- Doesn't consider individual amino acids: PER provides a general quality score but does not give information about the specific amino acid profile of the protein, nor does it identify which amino acid might be limiting for growth.
- No correction for digestibility: The original PER method does not account for the digestibility of a protein, which significantly affects its bioavailability.
The Modern Landscape: PER vs. Other Metrics
Due to its limitations, the PER has been superseded by more modern and accurate methods for evaluating protein quality. The most notable successor is the Protein Digestibility Corrected Amino Acid Score (PDCAAS), which was adopted by the U.S. Food and Drug Administration (FDA) in 1991. More recently, the Digestible Indispensable Amino Acid Score (DIAAS) has been proposed as an even more accurate replacement.
Comparison of Protein Quality Metrics
| Metric | Basis of Evaluation | Key Advantage | Key Disadvantage | Primary Use | Example Score |
|---|---|---|---|---|---|
| Protein Efficiency Ratio (PER) | Animal (rat) growth relative to protein intake | Simple calculation and equipment | Not directly applicable to humans; ignores digestibility | Historical, infant formula regulations (Canada) | Casein = 2.5 |
| Protein Digestibility Corrected Amino Acid Score (PDCAAS) | Amino acid score corrected for fecal digestibility | More accurate for human needs than PER; industry standard (formerly) | Capped at 1.0; does not account for anti-nutritional factors | Food labeling (pre-2013) | Egg protein = 1.0 |
| Digestible Indispensable Amino Acid Score (DIAAS) | True ileal digestibility of individual amino acids | More accurate measure of bioavailability and digestibility | Complex to measure; less widely implemented currently | Proposed modern standard; research | Whey protein = 1.09 |
Conclusion
In summary, the protein efficiency ratio is a historical measure of protein quality that uses animal growth as its benchmark. While it provided a valuable starting point for nutritional evaluation in the early to mid-20th century, its inherent limitations—particularly its reliance on animal data and inability to correct for digestibility—led to its decline in favor of more sophisticated and human-centric scoring systems like PDCAAS and DIAAS. For example, the FDA's guidance on using the PER for infant formula bioassays highlights its specific applications in modern contexts, demonstrating its continued niche relevance. For consumers, understanding that modern food labels rely on different, more accurate metrics is essential for making informed dietary choices about protein sources.
