Understanding the WHO Classification of Protein Energy Malnutrition

November 20, 2025 •

4 min read

According to the WHO, nearly half of all deaths among children under five are linked to undernutrition, highlighting the critical importance of an accurate and standardized diagnosis. The WHO classification of protein energy malnutrition provides a globally recognized framework to identify, assess, and manage this serious public health issue.

Quick Summary

The WHO classifies protein energy malnutrition using anthropometric measurements such as wasting, stunting, and underweight. It also identifies severe forms like kwashiorkor and marasmus based on clinical signs, guiding effective treatment strategies.

Key Points

Evolution of Classification: Early systems like Gomez and Wellcome have been superseded by the more robust WHO approach, which uses standardized growth charts and z-scores.
Key Indicators: The WHO relies on three anthropometric measures—wasting (weight-for-height), stunting (height-for-age), and underweight (weight-for-age)—to classify undernutrition.
Severe Acute Malnutrition (SAM): SAM is diagnosed by a weight-for-height z-score < -3, a mid-upper arm circumference (MUAC) < 115 mm, or the presence of bilateral pitting edema.
Kwashiorkor vs. Marasmus: Kwashiorkor is characterized by edema due to protein deficiency, while marasmus results from an overall calorie deficit, leading to severe wasting.
Systematic Management: The WHO provides a systematic, 10-step protocol for managing complicated SAM, focusing on initial stabilization and subsequent rehabilitation.
Global Impact: Standardized WHO classification is crucial for global health monitoring, guiding interventions, and reducing the high mortality rates associated with undernutrition.

The Evolution of Malnutrition Classification

Historically, the classification of protein-energy malnutrition (PEM) has evolved significantly. Early systems like the Gomez classification, developed in the 1950s, relied solely on weight-for-age, dividing malnutrition into first, second, and third degrees based on the percentage of expected weight. While useful for assessing a population's nutritional status, it failed to differentiate between acute (wasting) and chronic (stunting) forms. The Wellcome classification, another early system, improved upon this by incorporating the presence or absence of edema, creating categories for Kwashiorkor, Marasmus, and Marasmic Kwashiorkor based on weight-for-age percentiles.

The World Health Organization's (WHO) current approach represents a more robust and comprehensive framework, incorporating the use of z-scores and standardized growth charts. The 2006 WHO Child Growth Standards provided a new international reference, acknowledging that healthy children, regardless of ethnicity, have similar growth patterns when raised under optimal conditions. This shift moved away from older, potentially biased reference populations and towards a more universal standard for assessment.

Core WHO Indicators: Wasting, Stunting, and Underweight

The modern WHO classification of undernutrition relies on three key anthropometric indicators, each measured as a z-score relative to the WHO Child Growth Standards:

Wasting (low weight-for-height): Indicates recent and severe weight loss, often due to acute food shortage or infectious diseases like diarrhea. Wasting is a sign of acute malnutrition and is associated with a significantly increased risk of death in young children.
Stunting (low height-for-age): Reflects chronic or recurrent undernutrition, leading to a failure to reach physical and cognitive potential. It results from long-term poor socioeconomic conditions, inadequate maternal nutrition, or repeated illness.
Underweight (low weight-for-age): A composite measure that reflects both wasting and stunting. An underweight child may be stunted, wasted, or both.

These indicators are typically expressed as z-scores, which measure how many standard deviations a child's measurement is from the median of the reference population. A z-score of less than -2 indicates moderate undernutrition, while a score less than -3 signifies a severe state.

Severe Acute Malnutrition (SAM)

The WHO classifies Severe Acute Malnutrition (SAM) based on specific and highly sensitive criteria to identify children at the highest risk of mortality. SAM is defined by one or more of the following:

Weight-for-height z-score (WHZ) < -3 SD: A very low weight relative to the child's height.
Mid-upper arm circumference (MUAC) < 115 mm: A measurement around the arm that serves as a rapid screening tool for severe wasting, especially in resource-limited settings.
Bilateral pitting edema: Swelling, typically in the feet and lower legs, caused by fluid retention. The presence of this clinical sign alone is sufficient for a SAM diagnosis, as it indicates Kwashiorkor.

The Clinical Forms of SAM: Kwashiorkor and Marasmus

Within SAM, two major clinical presentations are recognized: Marasmus and Kwashiorkor. While both represent severe undernutrition, their manifestations and underlying pathophysiology differ significantly. Marasmus is a result of prolonged energy and protein deficiency, leading to severe wasting and an emaciated appearance. Kwashiorkor, conversely, is characterized by edema, often masking the true extent of wasting, and is historically linked to a predominantly carbohydrate diet with insufficient protein. A third, more severe category, Marasmic-Kwashiorkor, is a combination of both conditions.

Comparison of Severe Malnutrition Types


Feature	Kwashiorkor	Marasmus	Marasmic Kwashiorkor
Primary Deficiency	Protein deficiency, with potentially adequate energy intake.	Overall calorie and macronutrient deficiency.	Combined deficiency of both protein and calories.
Appearance	Bilateral pitting edema (swelling), especially in the feet, face, and abdomen.	Emaciated, shriveled, and wasted appearance with visible ribs.	Features of both kwashiorkor and marasmus, with both edema and severe wasting.
Subcutaneous Fat	Relatively preserved, initially.	Markedly absent, leading to loose, wrinkled skin.	Significantly diminished.
Weight	Can appear deceptively high due to edema; less than 60% of expected weight-for-age.	Very low, often less than 60% of expected weight-for-age.	Below 60% of expected weight-for-age, with edema.
Hair Changes	Sparse, brittle, and discolored (flag sign).	Sparse and dry.	Combination of both hair changes.
Mental State	Apathetic, miserable, and irritable when disturbed.	Alert, but often irritable and anxious.	Mixed presentation of apathy and irritability.

Management of Severe Malnutrition

WHO guidelines provide a systematic, 10-step approach to managing complicated SAM, divided into initial stabilization and rehabilitation phases. The initial phase focuses on addressing immediate life-threatening issues such as hypoglycemia, hypothermia, dehydration, and electrolyte imbalances. Broad-spectrum antibiotics are administered due to the suppressed immune system. The second phase, rehabilitation, emphasizes catch-up growth through high-energy therapeutic feeding formulas and psychosocial support. The management of uncomplicated SAM is typically handled on an outpatient basis using ready-to-use therapeutic food (RUTF) and regular follow-up.

Conclusion

The WHO's comprehensive classification system for protein energy malnutrition, now more broadly termed undernutrition, has provided a crucial framework for global health interventions. By moving beyond early classifications and standardizing anthropometric indicators and clinical criteria, it has enabled more precise diagnosis, particularly for severe forms like Kwashiorkor and Marasmus. The system's application is essential for identifying at-risk populations and implementing evidence-based management protocols, ultimately reducing morbidity and mortality associated with undernutrition worldwide.

For more detailed information on WHO guidelines for malnutrition management, visit the official resource page at WHO.int.

Frequently Asked Questions

The three main anthropometric indicators are wasting (low weight-for-height), stunting (low height-for-age), and underweight (low weight-for-age). These are used to assess undernutrition in children based on z-scores relative to WHO growth standards.

The key difference is the clinical presentation. Kwashiorkor is characterized by bilateral pitting edema (swelling) due to severe protein deficiency, while marasmus is a deficiency of both calories and protein, resulting in severe muscle and fat wasting without edema.

Bilateral pitting edema is a critical sign of Kwashiorkor, a form of SAM. Its presence indicates a severe underlying condition that requires immediate medical attention, even if other anthropometric measures seem less severe.

The 2006 WHO Child Growth Standards are based on healthy, breastfed children from diverse ethnic backgrounds. They provide a universal benchmark, demonstrating that all children have the potential for similar growth, rather than relying on potentially biased regional or historical data.

Initial steps for complicated SAM involve treating immediate life-threatening conditions. This includes managing hypoglycemia, hypothermia, dehydration (using a low-sodium solution like ReSoMal), and administering broad-spectrum antibiotics to address potential infections.

MUAC is a simple, effective screening tool for acute malnutrition, especially in low-resource settings. A MUAC of less than 115 mm is a reliable indicator of severe wasting and is part of the SAM diagnostic criteria.

The WHO's broader definition of malnutrition now includes not only undernutrition (wasting, stunting, underweight) but also micronutrient-related malnutrition (deficiencies or excess) and overweight and obesity.