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Overview of Undernutrition


John E. Morley

, MB, BCh, Saint Louis University School of Medicine

Last full review/revision Jul 2021| Content last modified Jul 2021
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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, or increased nutritional requirements (as occurs in cancer or infection). Undernutrition progresses in stages; it may develop slowly when it is due to anorexia or very rapidly, as sometimes occurs when it is due to rapidly progressive cancer-related cachexia. 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 (eg, albumin).

The number of undernourished people in the world has been increasing since 2014. In The State of Food Security and Nutrition in the World 2020, the United Nations Food and Agriculture Organization (FAO) reported that almost 690 million people, or 8.9 percent of the global population, were undernourished in 2019, which is 60 million more people than in 2014. Most live in countries with high rates of food insecurity. The FAO report anticipates that the COVID-19 pandemic may increase the number of people who are undernourished by an additional 83 to 132 million in 2020.

Risk Factors for Undernutrition

Undernutrition is associated with many disorders and circumstances, including poverty and social deprivation.

Risk is also greater at certain times (ie, during infancy, early childhood, adolescence, pregnancy, breastfeeding, and old age).

Infancy and childhood

Infants and children are particularly susceptible to undernutrition because of their high demand for energy and essential nutrients. Because vitamin K does not readily cross the placenta, neonates may be deficient, so they are given a single injection of vitamin K within 1 hour of birth to prevent hemorrhagic disease of the newborn, a life-threatening disorder. Infants fed only breast milk, which is typically low in vitamin D, are given supplemental vitamin D; they can develop vitamin B12 deficiency if the mother is vegan.

Inadequately fed infants and children are at risk of protein-energy undernutrition (PEU—previously called protein-energy malnutrition) and deficiencies of iron, folate (folic acid), vitamins A and C, copper, and zinc.

During adolescence, nutritional requirements increase because the growth rate accelerates. Anorexia nervosa may affect adolescent girls in particular.

Pregnancy and breastfeeding

Requirements for nutrients increase during pregnancy and breastfeeding. Aberrations of diet, including pica (consumption of nonnutritive substances, such as clay and charcoal), may occur during pregnancy. Anemia due to iron deficiency is common, as is anemia due to folate deficiency, especially among women who have taken oral contraceptives. Vitamin D deficiency is common during late pregnancy, predisposing the child to decreased bone mass.

Old age

Aging—even when disease or dietary deficiency is absent—leads to sarcopenia (progressive loss of lean body mass), starting after age 40 and eventually amounting to a muscle loss of about 10 kg (22 lb) in men and 5 kg (11 lb) in women. Undernutrition contributes to sarcopenia, and sarcopenia accounts for many of the complications of undernutrition (eg, decreased nitrogen balance, increased susceptibility to infections).

Causes of sarcopenia include the following:

  • Decreased physical activity

  • Decreased food intake

  • Increased levels of cytokines (particularly interleukin-6)

  • Decreased levels of growth hormone and mechano growth factor ( insulin-like growth factor-3)

  • In men, decreasing androgen levels

Aging decreases basal metabolic rate (due mainly to decreased fat-free mass), total body weight, height, and skeletal mass; from about age 40 to age 65, mean body fat (as a percentage of body weight) increases to about 30% (from 20%) in men and to 40% (from 27%) in women.

From age 20 to 80, food intake decreases, especially in men.

Anorexia due to aging itself has many causes, including

  • Reduced adaptive relaxation of the stomach’s fundus

  • Increased release and activity of cholecystokinin (which produces satiation)

  • Increased leptin (an anorectic hormone produced by fat cells)

  • Diminished taste and smell, which can decrease eating pleasure but usually decrease food intake only slightly

  • Depression (a common cause)

  • Loneliness

  • Inability to shop or prepare meals

  • Some chronic disorders

  • Use of certain drugs

Occasionally, anorexia nervosa (sometimes called anorexia tardive in older patients), paranoia, or mania interferes with eating. Dental problems limit the ability to chew and subsequently to digest foods. Swallowing difficulties (eg, due to strokes, other neurologic disorders, esophageal candidiasis, or xerostomia) are common. Poverty or functional impairment limits access to nutrients.

Institutionalized older patients are at particular risk of protein-energy undernutrition. They are often confused and may be unable to express hunger or preferences for foods. They may be physically unable to feed themselves. Chewing or swallowing may be very slow, making it tedious for another person to feed them enough food.

In older patients, particularly those who are institutionalized, inadequate intake and often decreased absorption or synthesis of vitamin D, increased demand for vitamin D, and inadequate exposure to sunshine contribute to vitamin D deficiency and osteomalacia.

Disorders and medical procedures

Diabetes, some chronic disorders that affect the gastrointestinal tract, intestinal resection, and certain other gastrointestinal surgical procedures tend to impair absorption of fat-soluble vitamins, vitamin B12, calcium, and iron. Gluten enteropathy, pancreatic insufficiency, or other disorders can result in malabsorption. Decreased absorption possibly contributes to iron deficiency and osteoporosis.

Liver disorders impair storage of vitamins A and B12 and interfere with metabolism of protein and energy sources. Renal insufficiency predisposes to protein, iron, and vitamin D deficiencies.

Anorexia causes some patients with cancer or depression and many with AIDS to consume inadequate amounts of food.

Infections, trauma, hyperthyroidism, extensive burns, and prolonged fever increase metabolic demands. Any condition that increases cytokines may be accompanied by muscle loss, lipolysis, low albumin levels, and anorexia.

Vegetarian diets

Iron deficiency can occur in ovo-lacto vegetarians (although such a diet can be compatible with good health). Vegans may develop vitamin B12 deficiency unless they consume yeast extracts or Asian-style fermented foods. Their intake of calcium, iron, and zinc also tends to be low.

A fruit-only diet is not recommended because it is deficient in protein, sodium, and many micronutrients.

Fad diets

Some fad diets result in vitamin, mineral, and protein deficiencies; cardiac, renal, and metabolic disorders; and sometimes death. Very low calorie diets (< 400 kcal/day) cannot sustain health for long.

Drugs and nutritional supplements

Many drugs (eg, appetite suppressants, digoxin) decrease appetite; others impair nutrient absorption or metabolism. Some drugs (eg, stimulants) have catabolic effects. Certain drugs can impair absorption of many nutrients; eg, anticonvulsants can impair absorption of vitamins.

Alcohol or drug dependency

Patients with alcohol or drug dependency may neglect their nutritional needs. Absorption and metabolism of nutrients may also be impaired. IV drug addicts typically become undernourished, as do people with alcohol use disorder who consume 1 quart of hard liquor/day. Alcohol use disorder can cause deficiencies of magnesium, zinc, and certain vitamins, including thiamin.

Symptoms and Signs of Undernutrition

Symptoms vary depending on the cause and type of undernutrition (eg, protein-energy undernutrition, vitamin deficiency).

Evaluation of Undernutrition

Diagnosis of undernutrition is based on results of medical and diet histories, physical examination, body composition analysis, and selected laboratory tests. Explicit consensus criteria have been proposed but are not yet universally adopted (1).


History should include questions about

  • Dietary intake—see Mini Nutritional Assessment (MNA®), a nutrition screening and assessment tool that can identify patients age ≥ 65 who are malnourished or at risk of malnutrition (2)

  • Recent changes in weight

  • Risk factors for undernutrition, including drug and alcohol use

Unintentional loss of 10% of usual body weight during a 3-month period indicates a high probability of undernutrition. Social history should include questions about whether money is available for food and whether the patient can shop and cook.

Review of systems should focus on symptoms of nutritional deficiencies (see table Symptoms and Signs of Nutritional Deficiency). For example, impaired night vision may indicate vitamin A deficiency.


Symptoms and Signs of Nutritional Deficiency


Symptom or Sign


General appearance





Many vitamins, zinc, essential fatty acids

Rash in sun-exposed areas

Niacin (pellagra)

Easy bruising

Hair and nails

Thinning or loss of hair

Premature whitening of hair

Spooning (upcurling) of nails


Impaired night vision

Corneal keratomalacia (corneal drying and clouding)

Vitamin A


Cheilosis and glossitis

Riboflavin, niacin, pyridoxine, iron

Bleeding gums

Vitamin C, riboflavin





Paresthesias or numbness in a stocking-glove distribution

Thiamin (beriberi)


Cognitive and sensory deficits

Thiamin, niacin, pyridoxine, vitamin B12


Thiamin, niacin, vitamin B12


Wasting of muscle


Bone deformities (eg, bowlegs, knocked knees, curved spine)

Vitamin D, calcium

Bone tenderness

Vitamin D

Joint pain or swelling

Vitamin C



Protein, niacin, folate, vitamin B12

Diarrhea and dysgeusia


Dysphagia or odynophagia (due to Plummer-Vinson syndrome)




Physical examination

Physical examination should include

  • Measurement of height and weight

  • Inspection of body fat distribution

  • Anthropometric measurements of lean body mass

Body mass index (BMI = weight[kg]/height[m]2) adjusts weight for height. If weight is < 80% of what is predicted for the patient’s height or if BMI is ≤ 18, undernutrition should be suspected. Although these findings are useful in diagnosing undernutrition and are acceptably sensitive, they lack specificity.

The mid upper arm muscle area estimates lean body mass. This area is derived from the triceps skinfold thickness (TSF) and mid upper arm circumference. Both are measured at the same site, with the patient’s right arm in a relaxed position. The average mid upper arm circumference is about 34.1 cm for men and 31.9 cm for women (3). The formula for calculating the mid upper arm muscle area in cm2 is as follows:


This formula corrects the upper arm area for fat and bone. Average values for the mid upper arm muscle area are 54 ± 11 cm2 for men and 30 ± 7 cm2 for women. A value < 75% of this standard (depending on age) indicates depletion of lean body mass (see table Mid Upper Arm Muscle Area in Adults). This measurement may be affected by physical activity, genetic factors, and age-related muscle loss.


Mid Upper Arm Muscle Area in Adults

Percentage of Standard (%)

Men (cm2)

Women (cm2)

Muscle Mass

100 ± 20*

54 ± 11

30 ± 7














* Mean mid upper arm muscle mass ± 1 standard deviation.

From the National Health and Nutrition Examination Surveys I and II.

Physical examination should focus on signs of specific nutritional deficiencies. Signs of protein-energy undernutrition (eg, edema, muscle wasting, skin changes) should be sought. Examination should also focus on signs of conditions that could predispose to nutritional deficiencies, such as dental problems. Mental status should be assessed because depression and cognitive impairment can lead to weight loss.

The following assessment tools may be useful:

  • The widely used Subjective Global Assessment (SGA) uses information from the patient history (eg, weight loss, change in intake, gastrointestinal symptoms), physical examination findings (eg, loss of muscle and subcutaneous fat, edema, ascites), and the clinician’s judgment of the patient’s nutritional status.

  • The Mini Nutritional Assessment (MNA®) has been validated and is widely used, especially for older patients.

  • The Simplified Nutrition Assessment Questionnaire (SNAQ), a simple, validated method of predicting future weight loss, may be used.

Simplified nutrition assessment questionnaire (SNAQ)

Simplified nutrition assessment questionnaire (SNAQ)


The extent of laboratory testing needed is unclear and may depend on the patient’s circumstances. If the cause is obvious and correctable (eg, a wilderness survival situation), testing is probably of little benefit. Other patients may require more detailed evaluation.

Serum albumin measurement is the laboratory test most often used. Decreases in albumin and other proteins (eg, prealbumin [transthyretin], transferrin, retinol-binding protein) may indicate protein deficiency or protein-energy undernutrition (PEU). As undernutrition progresses, albumin decreases slowly; prealbumin, transferrin, and retinol-binding protein decrease rapidly. Albumin measurement is inexpensive and predicts morbidity and mortality better than measurement of the other proteins. However, the correlation of albumin with morbidity and mortality may be related to nonnutritional as well as nutritional factors. Inflammation produces cytokines that cause albumin and other nutritional protein markers to extravasate, decreasing serum levels. Because prealbumin, transferrin, and retinol-binding protein decrease more rapidly during starvation than does albumin, their measurements are sometimes used to diagnose or assess the severity of acute starvation. However, whether they are more sensitive or specific than albumin is unclear.

Total lymphocyte count, which often decreases as undernutrition progresses, may be determined. Undernutrition causes a marked decline in CD4+ T lymphocytes, so this count may not be useful in patients who have AIDS.

Skin tests using antigens can detect impaired cell-mediated immunity in PEU and in some other disorders of undernutrition.

Other laboratory tests, such as measuring vitamin and mineral levels, are used selectively to diagnose specific deficiencies.

Evaluation references

  • 1. Cederholm T, Jensen GL, Correia MITD, et al: GLIM [Global Leadership Initiative on Malnutrition] criteria for the diagnosis of malnutrition: a consensus report from the global clinical nutrition community. Clin Nutr 38(1):1-9, 2019. doi: 10.1016/j.clnu.2018.08.002

  • 2. Guigoz Y, Vellas B: Nutritional assessment in older adults: MNA® 25 years of a screening tool and a reference standard for care and research; What next? J Nutr Health Aging 25(4):528-583, 2021. doi: 10.1007/s12603-021-1601-y

  • 3. Fryar CD, Gu Q, Ogden CL, Flegal KM: Anthropometric reference data for children and adults: United States, 2011–2014. National Center for Health Statistics. Vital Health Stat 3 (39), 2016.

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