Malnutrition is defined as any nutritional imbalance (and as such includes overnutrition), but in common use it refers to states of nutritional deficiency of calories, protein, or other nutrients necessary for tissue repair and maintenance.

Malnutrition can occur due to inadequate nutrient intake (inadequate access to nutrients or inadequate intake despite availability, as in anorexia nervosa), increased nutritional demands that outstrip intake (infection, cancer, traumatic injury and burns, other etiologies for inflammation), inadequate or impaired bowel absorption and transport, and altered nutrient utilization by the body.

While inadequate nutritional intake is the most common etiology of malnutrition in the developing setting and remains a significant and often underrecognized issue in the elderly, impaired utilization and increased demand due to inflammation are a more common etiology in the developed world.

Regardless of etiology, malnutrition is associated with both increased morbidity and mortality, and its presence is associated with increased frequency and length of hospitalization and higher health care costs. Drug-related nutrient deficiency risk increases with the patient's number of medications and length of time used.

In older adults, depression, cognitive impairment, and dementia may impact nutrition. Common late-stage dementia problems include dehydration, weight loss, and the inability to swallow, chew, or eat. 

No single parameter is definitive for adult malnutrition. Consensus guidelines from the American Society for Parenteral and Enteral Nutrition (ASPEN) define malnutrition as the presence of 2 of the following 6 findings:

1. Insufficient energy intake
2. Weight loss
3. Loss of muscle mass
4. Loss of subcutaneous fat
5. Localized or generalized fluid accumulation that may mask weight loss
6. Diminished functional status measured by hand-grip strength

The Global Leadership Initiative on Malnutrition (GLIM) framework, used for the syndromic diagnosis of malnutrition, is based on assessing 3 phenotypic (weight loss, low body-mass index, and low muscle mass) and 2 etiologic criteria (low intake [or maldigestion or malabsorption] and inflammation), with at least 1 criterion from each group being present.

Specific deficiencies of or impaired utilization of specific micronutrients are also considered malnutrition but are addressed individually and are not within the scope of this topic.

Related topics: protein-energy malnutrition, kwashiorkor, marasmus, bulimia, malabsorption syndrome, failure to thrive and neglect, vitamin and mineral deficiencies (vitamin A, vitamin B2, vitamin B6, vitamin B12, niacin, vitamin C, vitamin D, vitamin E, vitamin K, thiamine, folate, copper, iodine, zinc [acquired and hereditary], iron)