Undernutrition is a form of malnutrition. (Malnutrition also includes overnutrition).

Undernutrition can result from:

• inadequate ingestion of nutrients,
• malabsorption,
• impaired metabolism,
• loss of nutrients due to diarrhea,
• increased nutritional requirements.

Undernutrition progresses in stages: it may develop slowly when it is due to anorexia or very rapidly. First, nutrient levels in blood and tissues change, followed by intracellular changes in biochemical functions and structure. Ultimately, symptoms and signs appear. Diagnosis is by history, physical examination, body composition analysis, and sometimes laboratory tests. 

Undernutrition from micronutrient deficiencies, or "hidden hunger", affects over 2 billion people globally and can lead to reduced growth and cognitive development, birth defects, blindness, and overall poor health. Vitamin A deficiency, iron deficiency anaemia and iodine deficiency disorders are among the most common forms of micronutrient malnutrition.