Reference
Protein Efficiency Ratio (PER)
**Protein Efficiency Ratio (PER)** is a measure of protein quality defined as the body weight gained, in grams, per gram of protein consumed. It is determined in growing laboratory rats fed a test protein under controlled conditions, and it was for decades the standard regulatory metric for evaluating dietary protein.
How PER is measured
PER is an animal-growth assay. Weanling rats are fed a diet in which the only protein source is the test protein, typically at a fixed proportion of the diet, for a set period. Researchers record total food intake and weight gain, then divide weight gain by the grams of protein eaten. A higher ratio indicates that more of the consumed protein was used for growth.
Because results vary between laboratories and between animal batches, PER values are conventionally reported relative to a casein reference run in the same experiment. This standardization step was an early acknowledgement that the assay’s absolute numbers were difficult to reproduce.
Why PER was superseded
PER has two structural problems. First, it measures the protein requirements of a rapidly growing rat, whose amino acid needs — particularly for sulfur amino acids — differ from those of an adult human. A protein that supports rat growth efficiently is not necessarily matched to human maintenance needs. Second, the assay measures only growth, not the separate contributions of digestibility and amino acid composition.
For these reasons, expert bodies moved to human-referenced methods. The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) became the preferred method of the World Health Organization for evaluating protein quality, scoring a protein against human amino acid requirements and correcting for digestibility. DIAAS (Digestible Indispensable Amino Acid Score) was later proposed by the FAO in March 2013 to replace PDCAAS as the protein-quality standard, refining digestibility measurement at the level of individual amino acids.
Relevance to plant proteins
Older PER-based rankings tended to favor animal proteins, partly because the rat-growth model rewards the amino acid pattern those proteins supply. Newer human-referenced scores give a more nuanced picture: potato protein isolate, for example, achieves a PDCAAS among the highest of vegetable protein sources. When comparing labels, the metric a brand chooses to cite matters — a point covered in the protein powder buyer’s guide.
Related terms