What is the significance of protein efficiency ratio?

For decades, the protein efficiency ratio (PER) was a standard method for evaluating protein quality, widely adopted by regulatory bodies. This measure assesses how effectively a dietary protein contributes to body weight gain in a growing test subject, typically weanling rats.

Quick Summary

This article explores the protein efficiency ratio, its traditional use in measuring protein quality based on animal growth, and its eventual replacement by more accurate assessment methods.

Key Points

• Definition: The protein efficiency ratio (PER) measures the effectiveness of a protein by calculating weight gain relative to protein intake in growing test animals.

• Historical Context: For much of the 20th century, PER was a primary method used by regulators and the food industry to assess protein quality, especially for infant formulas.

• Standard Reference: PER values are typically compared to casein, a milk protein, which is assigned a reference value of 2.5.

• Key Limitations: The method has several drawbacks, including its reliance on animal models, its inability to differentiate between lean and fat mass gain, and its focus solely on growth requirements.

• Modern Replacements: Due to its limitations, PER has been largely replaced by more advanced and accurate methods like the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and the Digestible Indispensable Amino Acid Score (DIAAS).

• Ongoing Use: While no longer a universal standard, PER is still used for certain regulatory purposes in some regions, such as by Health Canada, and in specific animal feed evaluations.

What is the Protein Efficiency Ratio (PER)?

The protein efficiency ratio, or PER, is a biological method for evaluating the quality of a protein by measuring how well it promotes growth. The technique involves feeding a test group of weanling rats a diet containing a specific protein source at a standardized concentration, usually 10% of the total diet. Over a designated period, typically 28 days, researchers meticulously record the animals' weight gain and total protein consumption. The PER is calculated using a straightforward formula:

$PER = (Body~weight~gain~in~grams) / (Protein~intake~in~grams)$

This simple ratio provides a numerical value that reflects the efficiency with which a protein is used for growth. The resulting PER is then compared against a standard reference protein, most commonly casein, which has a reference value of 2.5. A protein with a PER value exceeding 2.5 is traditionally considered an excellent protein source.

The Historical Context and Significance

From its development in the early 20th century through the late 1980s, the PER was a widely utilized and respected method for assessing protein quality. It served as a foundational tool for regulatory agencies and the food industry for several key purposes:

• Regulatory Compliance: In countries like Canada, the PER remains an official method for evaluating protein quality in foods. The U.S. Food and Drug Administration (FDA) historically used PER and still references it in specific regulations, particularly concerning infant formulas.
• Infant Formula Testing: Manufacturers used PER bioassays to demonstrate that new infant formulas possessed adequate biological protein quality to support proper growth. This was a critical quality control procedure to ensure the formula was nutritionally sufficient.
• Product Development: For decades, food manufacturers used PER data to assess and compare different protein sources, informing the development of new products and formulations intended to provide concentrated, high-quality protein.
• Animal Husbandry: In agricultural and animal feed studies, PER provided a practical metric for evaluating how different dietary proteins contributed to weight gain in livestock or test subjects.

Limitations and Modern Alternatives

Despite its historical importance, the PER method has significant limitations that have led to its decline as the primary global standard.

• Relevance to Humans: The primary criticism is that PER relies on rodent studies, and the results do not always correlate accurately with human protein needs and metabolism. Rats and humans have different dietary amino acid requirements, particularly concerning sulfur-containing amino acids.
• Inability to Differentiate Tissue Types: PER only measures overall weight gain and cannot distinguish between lean body mass (muscle) and fat mass. A diet could potentially cause weight gain through fat accumulation, giving a misleading impression of protein effectiveness.
• Ignoring Maintenance Needs: The bioassay is designed for rapidly growing animals, and its focus is on growth potential, not the protein required for maintenance in non-growing animals or humans.
• Lack of Proportionality: The PER value is not proportional to protein quality. A protein with a PER of 2.0 is not necessarily twice as valuable as one with a PER of 1.0. This makes direct comparisons difficult.
• Digestibility and Bioavailability: The PER doesn't account for the digestibility of the protein or the bioavailability of its amino acids, crucial factors in determining true nutritional value.

Modern Methods of Protein Quality Assessment

Recognizing the limitations of PER, the scientific community and regulatory bodies have developed more sophisticated methods for assessing protein quality.

• Protein Digestibility Corrected Amino Acid Score (PDCAAS): In 1993, the FAO/WHO recommended PDCAAS to replace PER as the preferred method. This score compares the amino acid profile of a food protein with a reference profile and corrects it for protein digestibility. However, even PDCAAS has been criticized for certain shortcomings.
• Digestible Indispensable Amino Acid Score (DIAAS): Proposed by FAO/WHO in 2013, DIAAS is the most recent and precise method. It is considered superior to PDCAAS because it measures amino acid digestibility at the end of the small intestine (ileum) rather than the entire digestive tract, providing a more accurate reflection of amino acid absorption.

Comparison of Protein Quality Measures

Feature	Protein Efficiency Ratio (PER)	Protein Digestibility Corrected Amino Acid Score (PDCAAS)	Digestible Indispensable Amino Acid Score (DIAAS)
Method	Animal-based (rat bioassay)	Chemical analysis, fecal digestibility	Chemical analysis, ileal digestibility
Measurement	Weight gain per gram of protein consumed	Amino acid score corrected for digestibility	Amino acid score corrected for ileal digestibility
Standard	Casein (PER = 2.5)	Reference amino acid profile for age group	Reference amino acid profile for age group
Main Advantage	Simple equipment and procedure required	Considered an improvement over PER, simple	Most accurate and modern method, accounts for ileal digestibility
Main Limitation	Not proportional, uses growing animals, cannot differentiate fat/lean mass, poor human correlation	Truncated scores (max 1.0), fecal digestibility measure, less accurate than DIAAS	Depends on research data, newer method

Conclusion

The protein efficiency ratio holds significant historical importance as an early and influential method for assessing protein quality. Its legacy lies in establishing the initial criteria for protein evaluation, particularly within the food industry and in the regulation of specialized products like infant formulas. However, the PER's fundamental limitations—including its reliance on animal models and its inability to provide a nuanced understanding of protein utilization—have rendered it obsolete for modern, high-precision nutritional science. While it still holds a place in specific regulatory contexts like in Canada, the shift towards more advanced and accurate methods like PDCAAS and particularly DIAAS represents a major evolution in nutritional science, offering a more complete and reliable assessment of protein quality for human health.

Visit the FDA website for specific guidance regarding protein quality factors in infant formula regulation.

Frequently Asked Questions

How is the protein efficiency ratio (PER) calculated?

The PER is calculated by dividing the weight gain of a test subject (typically a growing rat) by its total protein intake over a standardized period.

Why is the Protein Digestibility Corrected Amino Acid Score (PDCAAS) considered superior to PER?

PDCAAS is considered an improvement over PER because it is based on human amino acid requirements and accounts for protein digestibility, providing a more relevant measure for human nutrition.

What is the main drawback of using rats to measure protein quality?

Rats have different amino acid requirements than humans, meaning a protein that promotes efficient growth in rats may not have the same effect or be as nutritionally valuable for humans.

Does PER distinguish between fat and muscle gain?

No, PER only measures total weight gain and does not differentiate between weight gained as fat and weight gained as lean body mass.

Where is PER still used in nutritional evaluation?

PER is still used for regulatory purposes in certain countries, like Canada, and is also applied in specific applications such as testing animal feeds.

What protein is used as the standard for comparison in PER testing?

Casein, a protein found in milk, is traditionally used as the standard reference protein in PER testing, with a reference value of 2.5.

What is DIAAS and why is it a superior method for assessing protein quality?

DIAAS stands for Digestible Indispensable Amino Acid Score and is the most advanced protein quality measure. It is superior to PDCAAS because it measures amino acid digestibility at the ileum, the point of maximum absorption, rather than at the fecal level.