Malnutrition is defined in three ways: by a weight for height indicator with a reference population, or mid-upper arm circumference (MUAC), or by the presence of edema (a bloated appearance to the feet and face).

If dietary deficiencies are persistent, children will stop growing and become stunted (low height for one's age). This is referred to as chronic malnutrition. If they experience weight loss or "wasting" (low weight for one's height), they are described as suffering from acute malnutrition. Both of these presentations of malnutrition may be further classified as moderate or severe.

Severe acute malnutrition includes two main clinical forms - severe wasting (called marasmus) and nutritional oedema (known as kwashiorkor). It is the clinical analysis that determines if treatment will be in hospital or with therapeutic RUF at home. Médecins Sans Frontières experience in Niger has been that most children do not have complications and can therefore follow therapeutic RUF treatment at home. Severe acute malnutrition has a case fatality rate of up to 21% without effective intervention. But any child with malnutrition is at an increased risk of developing complications leading to severe illness and death.