What is Protein Efficiency Ratio (PER)?
The Protein Efficiency Ratio (PER) is a measurement of protein quality based on the weight gain of a test subject in relation to its protein intake. This method was historically used to evaluate the quality of protein in food, particularly for nutrition labeling in the United States until 1993, and is still used in Canada today. The formula for PER is straightforward: it divides the weight gain of a test group by the amount of protein they consumed over a specified period.
How PER is calculated
The typical procedure for calculating PER involves feeding a group of young rats a diet containing a specified percentage (e.g., 10%) of the protein being tested. A control group is fed a reference protein, usually casein, which has a standard PER value. The weight gained and protein consumed by the test group are monitored over a few weeks. The PER value for the test protein is then reported relative to the casein control.
Limitations of the PER method
Despite its simplicity, the PER method has several notable limitations:
- Animal-Based: PER is measured in growing rats, and the growth needs of rats do not directly correlate with the maintenance and repair requirements of adult humans.
- Growth vs. Maintenance: The method only measures growth and cannot accurately assess the protein requirements for the maintenance of body tissues in non-growing adults.
- Body Composition: It cannot differentiate between weight gained as fat versus weight gained as lean body mass.
What is Biological Value (BV)?
The Biological Value (BV) is a more refined measure of protein quality that assesses how efficiently the body utilizes absorbed protein. It quantifies the proportion of absorbed nitrogen that is retained and incorporated into the body's tissues, rather than being excreted. A protein source with a high BV is rich in the essential amino acids needed for protein synthesis, allowing the body to use it more effectively.
How BV is measured
To measure BV, a subject is first placed on a protein-free diet to determine the baseline nitrogen excretion. This allows for an estimation of nitrogen lost from normal metabolic processes. The subject is then given a test protein, and the nitrogen intake and excretion (fecal and urinary) are carefully measured. The formula calculates the percentage of absorbed nitrogen that is retained by the body.
Factors affecting BV
The BV of a protein can be influenced by several factors:
- Amino Acid Profile: The composition of essential amino acids is the primary determinant. If a protein lacks or is low in a specific essential amino acid, its BV will be limited by that "limiting amino acid".
- Preparation: Cooking or processing methods can alter a protein's structure, affecting its digestibility and potentially reducing its BV.
- Nutrient Context: Other foods, minerals, and vitamins present in the diet can impact the utilization of a specific protein.
PER vs. BV: A comparison of protein quality metrics
Choosing the right protein is not just about quantity; it's also about quality. The Protein Efficiency Ratio and Biological Value offer different perspectives, as summarized in this comparison:
| Feature | Protein Efficiency Ratio (PER) | Biological Value (BV) |
|---|---|---|
| Methodology | Based on weight gain per gram of protein consumed in growing rats. | Based on the proportion of absorbed nitrogen retained by the body in humans or animals. |
| Measurement Focus | Measures growth and maintenance combined, but not separately. | Measures the efficiency of protein utilization for synthesis and retention. |
| Relevance to Humans | Lower relevance due to testing on rats and focus on growth, which doesn't directly apply to human maintenance needs. | Higher relevance as it measures efficiency for human metabolic processes, though still subject to experimental conditions. |
| Considerations | Does not account for digestibility or the specific amino acid profile needed by humans. | Primarily considers the amino acid profile and utilization efficiency once the protein is absorbed. |
| Primary Use | Historically for regulatory purposes and still used in some regions like Canada. | Considered a valuable, though limited, measure of protein quality based on amino acid composition. |
The evolution of protein quality assessment
The limitations of PER and BV led to the development of more advanced scoring methods. The Protein Digestibility Corrected Amino Acid Score (PDCAAS) was introduced in 1989 and adopted by the FDA in 1993 to address the shortcomings of older methods. PDCAAS considers both the essential amino acid content and the protein's digestibility, scoring proteins on a scale up to 1.0. For example, high-quality proteins like eggs, casein, and soy protein isolate often score near 1.0, while proteins with limiting amino acids score lower.
However, PDCAAS has its own issues, such as capping high-quality scores at 1.0 and using less precise fecal digestibility measurements. This prompted the development of the more accurate Digestible Indispensable Amino Acid Score (DIAAS) in 2013, which measures absorption at the ileal level and provides an uncapped score for a more refined comparison of high-quality proteins.
Conclusion
The Protein Efficiency Ratio and Biological Value represent foundational concepts in nutrition science, offering distinct ways to measure protein quality. While PER, based on animal growth, is less relevant for human nutrition, BV provides a more direct metric of how the body utilizes absorbed protein. The evolution of these methods, from early bioassays to the modern DIAAS, highlights the continuous effort to understand and accurately assess the nutritional value of different protein sources. Ultimately, a balanced diet rich in a variety of protein sources is the most effective strategy for ensuring all essential amino acid needs are met.
