The Protein Efficiency Ratio (PER) is a classical index used to evaluate the quality of dietary proteins based on their ability to support growth. It is defined as the ratio of body weight gain (in grams) over a specified period to the amount of protein consumed (in grams) during the same period.

Mathematically, it reflects a simple biological outcome: growth response per unit protein intake

In experimental nutrition, PER has historically been widely used, particularly in animal studies, to compare protein sources under standardized conditions. The PER values of common foods show a decreasing order approximately as follows: fish muscles, beef muscles, beef liver and kidneys, whole egg, milk, soybeans, oats, and wheat.

This ranking broadly reflects differences in protein quality, particularly amino acid composition and digestibility. However, in modern nutritional science, PER is considered a limited metric. It does not account for human amino acid requirements in detail, nor does it adequately incorporate true digestibility in humans. As a result, it has largely been replaced by more advanced methods such as:

• PDCAAS (Protein Digestibility-Corrected Amino Acid Score)
• DIAAS (Digestible Indispensable Amino Acid Score)

These modern systems provide a more accurate and physiologically relevant assessment of protein quality by integrating amino acid composition with human digestibility data.

   Biological Value and Classification of Proteins

Traditionally, proteins have been classified as “first-class” and “second-class” proteins based on their ability to maintain nitrogen balance in the body.

Animal proteins are referred to as first-class proteins because they are capable of maintaining nitrogen balance even when they constitute the sole source of dietary nitrogen. This is due to their ability to provide all essential amino acids in adequate amounts.

In contemporary terminology, the term “first-class protein” has been largely replaced by:

• High Biological Value (HBV) proteins
• Complete proteins

Conversely, most plant proteins were traditionally labeled as second-class proteins or low biological value proteins, because many of them cannot independently maintain nitrogen balance when used as the sole protein source. This limitation arises primarily from the absence or low concentration of one or more essential amino acids.

However, modern nutritional science provides a more nuanced understanding. Plant proteins are not inherently inferior; rather, they often have limiting amino acids and may require combination with other plant sources to achieve completeness. For example, cereals are typically low in lysine, whereas legumes are often limited in methionine. When combined appropriately (such as rice with lentils), they complement each other to form a nutritionally complete protein profile.

Another important modern consideration is that protein quality is not determined solely by amino acid composition. It is also influenced by:

• Digestibility
• Presence of anti-nutritional factors (e.g., phytates, tannins)
• Food processing and cooking methods
• Bioavailability of amino acids

These factors significantly affect how efficiently dietary protein is utilized by the human body.

   Protein Quality and Biological Value

The superiority of animal proteins over plant proteins is often attributed not only to their complete amino acid profile but also to the fact that their essential amino acids are present in proportions closely aligned with human physiological requirements.

It was previously suggested that an excess intake of certain amino acids may exert toxic effects. Modern understanding refines this view: amino acids in excess are not typically toxic under normal dietary conditions, but they may contribute to metabolic imbalance or increased nitrogen load, which must be processed by the liver and kidneys.

Thus, protein quality is determined by a combination of:

• Amino acid completeness
• Amino acid balance (proportional adequacy)
• Digestibility and absorption efficiency

A protein is considered to have high biological value if it satisfies the following conditions:

1. It contains all essential amino acids in sufficient quantities.
2. These amino acids are present in optimal physiological proportions.
3. The protein is readily digestible and efficiently absorbed.

Most animal proteins meet these criteria and therefore possess high biological value. However, there are notable exceptions:

• Gelatin – deficient in tryptophan and therefore incomplete
• Keratin – poorly digestible and not nutritionally useful
• Hemoglobin – although it contains amino acids, it is not a practical dietary protein source due to limited digestibility and physiological relevance

   Plant Proteins and Nutritional Value

The earlier classification of plant proteins as uniformly “low value” is an oversimplification. Many plant sources such as corn germ, wheat germ, yeast, and soybeans provide substantial amounts of amino acids and can contribute significantly to dietary protein intake.
Plant proteins are better understood in terms of complementarity rather than deficiency. When consumed in diverse combinations, they can collectively provide a complete amino acid profile comparable to that of animal proteins.

A key concept in this context is that of the limiting amino acid, defined as the essential amino acid present in the lowest proportion relative to human requirements. This amino acid determines the overall efficiency of protein utilization. In most cereals, lysine is limiting, while in many legumes, methionine is the limiting amino acid.

Modern nutrition emphasizes that protein adequacy can be achieved through dietary diversity rather than reliance on a single protein source.

   Protein Complementation and Food Fortification

Significant improvements in plant protein quality can be achieved through protein complementation, where different plant proteins are combined to compensate for individual amino acid deficiencies.

Historical nutritional programs, such as the INCAP formulation (Institute of Nutrition of Central America and Panama), attempted to create balanced plant-based protein mixtures. A typical formulation included corn, sesame meal, cottonseed meal, yeast, and grass protein, and was reported to achieve a biological value comparable to casein under experimental conditions.

While such formulations were important in the development of nutritional science, modern approaches have evolved toward:

• Corn–soy blends
• Legume–cereal combinations in daily diets
• Industrial fortification with isolated amino acids

In India and other developing regions, combinations of peanut meal, Bengal gram, black gram, and sesame meal have been explored to enhance protein quality. Fish meal has also been used historically, although its use in human diets is now limited due to regulatory, safety, and acceptability concerns.

Advances in biotechnology have enabled the production of essential amino acids such as lysine through microbial fermentation. These amino acids are now widely used to fortify plant-based foods and improve their nutritional quality.

Older claims regarding chemical synthesis of amino acids from coal are obsolete and not representative of modern industrial practices.

   Improvement of Protein Quality in Crops

One of the most important developments in agricultural nutrition is the improvement of protein quality through plant breeding and biotechnology.

Traditional cereal crops have been modified to increase both total protein content and essential amino acid balance. Examples include:

• High-protein mutant strains of wheat and maize
• Quality Protein Maize (QPM), enriched with lysine and tryptophan

These biofortified crops represent a significant advancement in addressing global protein deficiency, especially in populations that rely heavily on cereal-based diets.

Modern agricultural science employs:

• Selective breeding
• Mutation breeding
• Genetic engineering and molecular techniques

to enhance both yield and nutritional quality of staple crops.

   Recommended Dietary Allowances of Protein

Protein requirements vary with age, physiological state, and metabolic demand. The approximate recommended intakes are as follows:

• New-born infant: ~1.5–2.0 g/kg body weight
• Infant (1 year): ~1.2–1.6 g/kg body weight
• Children (8–10 years): ~1.0–1.2 g/kg body weight
• Males (14–22 years): ~0.9–1.0 g/kg body weight
• Adult males (22–75 years): ~0.8 g/kg body weight
• Adult females (14–75 years): ~0.8 g/kg body weight

These values represent general guidelines based on modern international recommendations (WHO/FAO and related bodies). For most healthy adults, approximately 0.8 g/kg/day is sufficient to maintain nitrogen balance under normal sedentary conditions.

However, requirements increase significantly during:

• Growth and development
• Pregnancy and lactation
• Recovery from illness or injury
• Heavy physical work or endurance training

In such situations, intake may rise to 1.0–1.5 g/kg/day or higher, depending on physiological stress and energy expenditure.

It is also important to recognize that protein utilization is closely linked to total energy intake. In conditions of calorie deficiency, protein may be diverted toward energy production rather than tissue building, thereby reducing its nutritional efficiency.

   Minimum Protein Requirement and Physiological Demand

Earlier estimates suggested a fixed minimum protein requirement of approximately 35 grams per day for an average adult. This value is now considered outdated and overly simplified.

Modern understanding defines protein requirements relative to body weight, with an approximate minimum of 0.8 g/kg/day for adults, rather than a fixed absolute quantity.

Individuals engaged in physically demanding labor or intense exercise require proportionally higher protein intake. However, contemporary nutrition science also emphasizes that adequate total caloric intake is equally essential to ensure efficient utilization of dietary protein for growth, repair, and maintenance of body tissues.