by:
lim ju boo BSc, MD, Postgrad Dip Nutrition, MSc, PhD (Med), FRSPH, FRSM
Someone on the radio was talking about nutrition and about foods like fish, meat and eggs that are high in proteins that supports growth in children
But she did not mention anything - not a single word about the quality of the proteins. She only mentioned about the quantity of proteins in different foods. What she mentioned was far from being adequate.
Understanding the Biological Value of Protein: A Comparison with the Chemical Score
Proteins are the building blocks of life, essential for the structure, function, and regulation of the body's cells, tissues, and organs. Central to evaluating the nutritional quality of protein sources is the concept of biological value (BV). BV measures how efficiently the body utilizes the absorbed protein for growth and maintenance. Alongside BV is the chemical score, another method for assessing protein quality. While both offer insights into the usefulness of proteins in human nutrition, they differ fundamentally in their basis, methodology, and implications.
What Is Biological Value (BV)?
Biological value is defined as the proportion of absorbed protein that is retained by the body for its own purposes, such as tissue repair, enzyme synthesis, and immune defense. It reflects how well the amino acid composition of a protein matches the body's requirements.
Mathematically:
BV considers:
1. Digestibility: How well a protein is broken down and absorbed.
2. Utilization: How effectively the absorbed amino acids are used for protein synthesis.A BV of 100 (as seen in egg protein) implies perfect utilization, 100% of the absorbed amino acids are used by the body.
Other high-BV proteins include milk (around 93), fish (about 76–83), and meat (around 70–80). Plant proteins typically have lower BV due to limiting essential amino acids and lower digestibility.
What Is the Chemical Score?
Chemical score is a simpler method of evaluating protein quality, focusing solely on the amino acid profile. It compares the amount of the most limiting essential amino acid in the test protein to that in a reference protein (usually egg or human milk protein).
Mathematically:
For example, if lysine is the limiting amino acid in wheat protein and is present at only 40% of that in the reference protein, the chemical score is 40. This method does not account for digestibility or amino acid losses, making it less comprehensive than BV.
Key Differences Between Biological Value and Chemical Score
| Aspect | Biological Value (BV) | Chemical Score |
|---|---|---|
Basis | Retention and utilization of absorbed nitrogen | Limiting amino acid content |
| Focus | Functional use in the body | Amino acid profile comparison |
Accounts for Digestibility | Yes | No |
Practicality | More biologically meaningful, but harder to measure | Simpler, easier for preliminary assessment |
| Limitation | May vary with individual metabolism and health status | Ignores absorption, digestibility, and metabolic utilization |
| Ideal Use | For understanding real impact on human protein needs | For quick screening of protein quality in raw materials |
Why Both Measures Matter?
While BV gives a more holistic view of a protein’s usability by the body, chemical score is a convenient, cost-effective method to detect which amino acids are lacking in a given protein source.
When combined with other indices like Protein Digestibility Corrected Amino Acid Score (PDCAAS) or the Digestible Indispensable Amino Acid Score (DIAAS), a more complete nutritional assessment emerges.
For example:
1. Soya protein may have a high chemical score (rich in essential amino acids) and a relatively high BV, making it a good plant-based substitute.
2. Gelatin, despite being 100% digestible, has a chemical score of 0 for tryptophan and thus a very low BV, highlighting the importance of both measures. In fact gelatin is the worse form of protein that has zero nitrogen-retention value in the body.Understanding the biological value and chemical score of proteins enables nutritionists, medical scientists, and food technologists to determine the adequacy of dietary proteins in supporting human health. While the chemical score alerts us to limiting amino acids, biological value reveals how well a protein serves the body’s actual needs. When combined, these tools guide optimal dietary planning, food formulation, and clinical nutrition practices, ensuring that protein intake contributes effectively to bodily function and well-being.
Let me now explain in my view why eggs and milk possess the highest biological value (BV) of protein.
Eggs and milk are universally recognized as having the highest biological value (BV) among all naturally occurring proteins. The reason lies not only in their amino acid composition but also in their biological role in nurturing early life. Here are my reasons.
1. Designed by Nature for Complete Development
Eggs: The egg white (albumen) and yolk together contain all the essential nutrients required to support the entire development of a chick from a single cell to a fully formed hatchling in 21 days, without any external nourishment. This includes a complete profile of essential amino acids in precise proportions, ensuring efficient protein synthesis throughout embryonic development.
Milk: Similarly, milk is the first and only food for a newborn mammal. It must supply all the nutrients, especially high-quality protein, necessary for rapid growth, immune support, and the development of organs and tissues. The infant’s reliance on milk alone necessitates that its protein be not only highly digestible but also perfectly balanced for human utilization.These life-critical roles strongly suggest evolutionary optimization for maximal nutritional efficacy, hence the highest BV.
2. Perfectly Balanced Essential Amino Acids
Eggs and milk contain all nine essential amino acids in the ideal ratios required by the human body. This eliminates any bottleneck in protein synthesis that typically arises from a limiting amino acid, as is common in many plant and even some animal proteins.
For example:
Lysine, often limited in cereal proteins, is abundant in both egg and milk.
Methionine, which can be deficient in legumes, is well-represented in egg proteins.This complete amino acid profile means that almost all of the absorbed protein is retained and used, resulting in:
Egg BV approximately 100
Milk BV approximately 93–97 (depending on the form: whole, casein, or whey)3. High Digestibility and Bioavailability
BV takes into account not only the amino acid composition but also how much of the protein is absorbed and used by the body.
Egg and milk proteins are easily digested, with minimal loss during absorption.
Their proteins are soluble, enzyme-accessible, and lack inhibitors that might hinder digestion, unlike some plant proteins which contain antinutritional factors (e.g., phytates, tannins, or enzyme inhibitors).This means:
"What goes in, gets used efficiently."
4. Minimal Nitrogen Waste
Nitrogen retention is a key component of BV. When a protein has a balanced amino acid profile, the body can use it without needing to deaminate and excrete excess amino acids. This is the case for eggs and milk.
Less nitrogen is lost in urea or feces.
The net protein utilization is high, contributing to their superior BV.5. Evolutionary Nutritional Blueprint
From an evolutionary standpoint, both eggs and milk are primary life-support systems:
The egg is a self-contained, pre-programmed life capsule.
Milk is an adaptive biological fluid, modulated to suit the infant’s developmental stage (e.g., colostrum vs. mature milk).Thus, their composition, including proteins, is fine-tuned by natural selection to be optimally usable, not only for the target species (chick, calf, baby), but also for humans consuming them.
In short, eggs and milk have the highest biological value of protein because they are biologically designed to support the entirety of early development. Their proteins are:
Rich in all essential amino acids
Highly digestible and absorbable. It is efficiently used by the body with minimal waste. It matched to the body’s metabolic and anabolic needsThe developing chick and the nursing infant captures the profound biological intent behind these foods. They are not just sources of protein; they are life's first foods, masterfully composed to give life its best start.
Limiting Amino Acids: Understanding Incomplete Proteins and the Power of Complementation
Proteins are made up of amino acids, the fundamental units required by the human body for growth, tissue repair, enzyme activity, and immune function. Among the 20 amino acids, nine are essential, meaning they must be obtained from the diet because the human body cannot synthesize them.
However, not all protein sources are created equal. Many plant-based proteins lack one or more essential amino acids in sufficient quantities, which gives rise to the concept of the limiting amino acid. This limitation can impair the body's ability to build and maintain proteins efficiently, unless dietary strategies are used to overcome it.
What Is a Limiting Amino Acid?
A limiting amino acid is the essential amino acid present in the lowest quantity relative to the body's requirement in a given food protein. When a particular essential amino acid is deficient, it limits the body’s ability to utilize the other amino acids, even if they are present in adequate amounts. Think of protein synthesis as an assembly line. If one part is missing or insufficient, the entire production slows or halts.
For example:
Lysine is the limiting amino acid in wheat, rice, and corn. Methionine and cysteine are limiting in legumes such as beans and lentils. Tryptophan may be limiting in maize. Threonine can be limited in some grains.
In cassava roots, the limiting amino acids are methionine and lysine. However, the overall protein quality in cassava is acceptable, but these specific amino acids are present in lower quantities compared to the needs of the body. Other studies have also identified sulfur amino acids (methionine and cysteine) as limiting, along with leucine and lysine.
In maize the protein is zein and in maize the
limiting amino acids are lysine and tryptophan.
Why Are Limiting Amino Acids Important?
1. Nutritional Quality: A food with all essential amino acids in the correct proportions (e.g., egg) has a high biological value. If one amino acid is limiting, the overall utility of the protein decreases.
2. Protein Efficiency: Even if total protein intake is adequate, a deficiency in one essential amino acid impairs the body's ability to synthesize proteins effectively.3. Growth and Health: In populations relying heavily on plant-based diets without proper complementation, limiting amino acids can contribute to malnutrition, stunting, or weakened immune function.
Complementary Proteins: Nature’s Nutritional Puzzle
Fortunately, nature provides a solution with complementary proteins. When two or more plant-based foods with different limiting amino acids are consumed together, their amino acid profiles can complement each other, forming a complete protein.
Examples of Complementary Food Pairings
| Primary Food | Limiting Amino Acid | Complementary Food | Rich in |
|---|---|---|---|
Rice | Lysine | Beans or lentils | Lysine |
| Wheat | Lysine | Chickpeas or soybeans | Lysine |
| Corn (maize) | Tryptophan & lysine | Black beans or peanuts | Tryptophan & lysine |
| Legumes (beans) | Methionine & cysteine | Grains (e.g., brown rice) | Methionine & cysteine |
These combinations don’t need to be eaten in the same meal—as long as they are consumed within the same day, the body can draw from the amino acid pool to complete protein synthesis.
Role of Soybeans, Legumes, and Pulses
Among plant proteins, soybeans stand out as complete proteins, containing all nine essential amino acids in reasonably balanced amounts. Thus, soy can:
1. Bridge amino acid gaps in other plant foods.2. Act as a substitute for animal protein in vegetarian and vegan diets.
3. Be incorporated into meals via tofu, tempeh, soy milk, or textured vegetable protein.
Legumes and pulses such as lentils, chickpeas, black beans, and peas are rich in lysine, which is typically lacking in cereals. Conversely, cereals can supply methionine and cysteine, which are limiting in legumes. When combined, they form a nutritionally complete protein source.
What are Practical Applications
1. Traditional Diets Reflect Complementation
Indian dhal with rice, Mexican tortillas with beans, and Chinese congee with tofu are examples of traditional diets that unknowingly follow the principles of protein complementation.
2. Vegetarian and Vegan Nutrition
Careful planning ensures that plant-based diets meet all essential amino acid requirements by combining diverse protein sources.
3. Public Health and Food Security
In regions with limited access to animal proteins, promoting complementary plant protein strategies helps prevent protein-energy malnutrition.
Limiting amino acids are an important concept in understanding protein quality, especially in plant-based diets. While individual foods may lack certain essential amino acids, strategic combinations of plant proteins can overcome these limitations. By incorporating soybeans, legumes, pulses, and grains in thoughtful combinations, one can achieve a complete and balanced amino acid intake, ensuring robust health even without reliance on animal protein sources.
Clinical Protein Malnutrition:
Let me now write something about kwashiorkor a protein deficiency in infants, and what are my recommendation to prevent and to treat kwashiorkor - a specialized area in medicine.
This disease touches upon two of the most tragic and preventable conditions afflicting vulnerable children in many parts of the developing world: kwashiorkor and marasmus. These conditions are not just medical problems, they are reflections of social, economic, and nutritional inequities.
Kwashiorkor and Marasmus: Understanding and Preventing Severe Childhood Malnutrition
Malnutrition remains a pressing global health concern, particularly among infants and young children in low-income regions. Two major forms of severe protein-energy malnutrition (PEM) are kwashiorkor and marasmus. Though often coexisting, they represent distinct pathophysiological responses to nutritional deficiencies.
1. Kwashiorkor – The Disease of Displaced Children
Definition:
Kwashiorkor is a form of protein deficiency malnutrition, primarily affecting children who are weaned early and fed a diet high in carbohydrates but deficient in protein.
Origin of the Name
The term kwashiorkor comes from the Ga language of Ghana, meaning "the disease the older child gets when the next baby is born." - disease of second deprived child.
This refers to the situation where the first child is weaned off breast milk prematurely when the mother begins nursing a newborn, and is then fed a starchy but protein-poor diet.
Clinical Features
Oedema (swelling in legs, feet, and face)
Fatty liver (due to impaired lipoprotein synthesis)
Distended abdomen
Dermatitis with flaky-paint skin lesions
Sparse, brittle hair with reddish or blonde discoloration (flag sign)
Irritability, apathy, and failure to thrive
Impaired immunity, increasing susceptibility to infections
Pathophysiology
Low plasma albumin causes fluid to leak into interstitial spaces, leading to edema.
Lack of amino acids disrupts liver function, especially fat metabolism, leading to hepatic steatosis.2. Marasmus – The Wasting Disease
Definition
Marasmus results from caloric deficiency, a total lack of sufficient energy intake, including both protein and carbohydrates.
Clinical Features
Severe wasting and emaciation
Loss of subcutaneous fat ("skin and bones" appearance)Muscle wasting
Sunken eyes and drawn facial appearance
No edema (unlike kwashiorkor)
Persistent hunger and alertness (in early stages)
Pathophysiology
The body adapts to the extreme energy deficit by breaking down fat and muscle for energy. Growth halts, and the child becomes extremely thin with reduced resistance to infections.
Comparison of Kwashiorkor and Marasmus
| Feature | Kwashiorkor | Marasmus |
|---|---|---|
| Cause | Protein deficiency | Calorie (energy) deficiency |
| Edema | Present | Absent |
| Body appearance | Swollen belly, edema | Severe wasting, thin limbs |
| Mental state | Apathy, irritability | Alert (initially), later lethargy |
| Fat and muscle loss | Moderate | Severe |
| Hair changes | Common | Less common |
| Fatty liver | Present | Absent |
Prevention of Kwashiorkor and Marasmus
1. Promote Exclusive Breastfeeding
For at least the first 6 months of life, with continued breastfeeding up to 2 years or more alongside complementary feeding.
2. Educate on Balanced Weaning Diets
Introduce protein-rich complementary foods such as:
Mashed eggs, milk, legumes, soya, meat, and fish
Fortified cereals with added proteinLocally available high-protein foods (e.g., groundnuts, lentils)
Promote community gardens, school feeding programs, and maternal support.
Encourage nutrition education for caregivers, particularly mothers.4. Immunization and Clean Water
Prevent infections that can exacerbate malnutrition.
Promote sanitation to reduce diarrheal diseases which increase nutrient loss.Treatment of Kwashiorkor and Marasmus
Treatment follows WHO’s guidelines for severe acute malnutrition (SAM):
Initial Phase: Stabilization
Rehydrate with low-sodium oral rehydration solutions.
Treat infections empirically with antibioticsProvide therapeutic milk formulas (F-75: low protein, low sodium, low fat)
Correct micronutrient deficiencies (zinc, vitamin A, folate, iron)
Rehabilitation Phase
Gradual refeeding with high-energy and high-protein diet (F-100 formula or ready-to-use therapeutic foods like Plumpy’Nut).
Encourage return to normal family foods with added oils and protein sourcesCatch-up Growth
Monitor weight gain: Target >10g/kg/day
Long-term developmental and psychological supportHope through Nutrition
Kwashiorkor and marasmus are both preventable and treatable conditions. They represent a biological cry for help, a signal that a child has been denied the basic right to nourishment. With proper awareness, community engagement, and compassionate healthcare, these children can not only survive but thrive.
My Recommendations:
Promote local nutrition: Use accessible, protein-rich local foods like legumes, groundnuts, soy, and fish.
Integrate food with culture: Encourage caregivers to combine cereal and pulse diets (rice + lentils, maize + beans).Educate on maternal nutrition to ensure healthy breast milk and reduce low birth weight babies.
Summary of Nutritional Topics Discussed
1. Biological Value (BV) of Proteins
Biological Value refers to how efficiently the body utilizes the protein absorbed from food for growth and repair.
Eggs (BV approximately, 100) and milk have the highest BV among natural foods due to their complete amino acid profiles, ideal for supporting early life (e.g., chick in an egg; infant nourished by milk).2. Difference Between BV and Chemical Score
Biological Value considers both absorption and utilization of amino acids in the body.
Chemical Score measures the presence of essential amino acids relative to human needs but does not account for digestion or metabolism.3. Limiting Amino Acids and Complementation
Some plant-based proteins (e.g., cereals like rice or wheat) lack one or more essential amino acids (limiting amino acids like lysine or methionine).
Protein complementation (e.g., combining cereals with legumes) allows one food to supply the limiting amino acid missing in the other.Examples: Rice + lentils, Maize + beans, Bread + peanut butter.
4. Kwashiorkor
A form of protein deficiency malnutrition with features like edema, fatty liver, dermatitis, and growth failure.
Common in weaned children fed a high-carb but protein-poor diet.Caused by low serum albumin and amino acid deficiency.
5. Marasmus
A result of total calorie deficiency, including both proteins and energy.
Characterized by severe wasting, muscle loss, and no edema.Children appear very thin, with prominent bones and sunken faces.
6. Prevention & Treatment
Exclusive breastfeeding for 6 months, then protein-rich complementary feeding.
Nutritional education for mothers, locally sourced high-protein foods, clean water, and sanitation.Therapeutic feeding with formulas (F-75, F-100), or ready-to-use therapeutic foods (RUTFs like Plumpy’Nut).
Summary Conclusion:
Protein is not merely a nutrient, it is the fabric of life. The tragedies of kwashiorkor and marasmus are preventable when science is united with compassion, cultural wisdom, and community engagement.
A caring pursuit of truth and education echoes the mission of the most esteemed healers and humanitarians
Nutrition is right on the top of the list and the most important factor to consider in healthcare, not just use drugs as a short-cut to treat a disease.
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