Diseases & Conditions



Undernutrition is a deficiency of calories or of one or more essential nutrients.

Undernutrition is usually thought of as a deficiency primarily of calories (that is, overall food consumption) or of protein. Deficiencies of vitamins and minerals are usually considered separate disorders. However, when calories are deficient, vitamins and minerals are likely to be also. Undernutrition, which is often used interchangeably with malnutrition, is actually a type of malnutrition. Malnutrition is an imbalance between the nutrients the body needs and the nutrients it gets. Thus, malnutrition also includes overnutrition (consumption of too many calories or too much of any specific nutrient—protein, fat, vitamin, mineral, or other dietary supplement).

In developed countries, undernutrition is usually far less common than overnutrition. However, undernutrition does occur, especially in people who are very poor, such as the homeless, and in those who have psychiatric disorders. Also, people who are very ill may be unable to eat enough food because they have lost their appetite or because their body's need for nutrients is greatly increased. Infants, children, and adolescents are at risk of undernutrition because they are growing and thus need a lot of calories and nutrients.

Undernutrition also occurs in older people. About 1 of 7 older people who live in the community consume fewer than 1,000 calories a day—not enough for adequate nutrition. As many as half of older people in hospitals and long-term care facilities do not consume enough calories.

Did You Know...

About 1 of 7 older people who live in the community and about half of older people in long-term care facilities have undernutrition. Drinking too much alcohol can cause undernutrition.

When not enough calories are consumed, the body first breaks down its own fat and uses it for calories—much like burning the furniture to keep a house warm. After fat stores are used up, the body may break down its other tissues, such as muscle and tissues in internal organs, leading to serious problems, including death.

A severe deficiency of protein and calories (called protein-energy undernutrition or protein-energy malnutrition) results when people do not consume enough protein and calories for a long time.

In developing countries, protein-energy undernutrition often occurs in children. It contributes to death in more than half of children who die (for example, by increasing the risk of developing life-threatening infections and, if they develop, increasing their severity). However, this disorder can affect anyone, regardless of age, if food supplies are inadequate. Protein-energy undernutrition has two main forms:

Marasmus: Marasmus is a severe deficiency of calories and protein. It tends to develop in infants and very young children. It typically results in weight loss and dehydration. Breastfeeding usually protects against marasmus.

Starvation is the most extreme form of marasmus (and undernutrition). It results from a partial or total lack of essential nutrients for a long time.

Kwashiorkor: Kwashiorkor is a severe deficiency more of protein than of calories. Kwashiorkor is less common than marasmus. The term is derived from an African word meaning “first child-second child” because a first-born child often develops kwashiorkor when the second child is born and replaces the first-born child at the mother's breast. Because children tend to develop kwashiorkor after they are weaned, they are usually older than those who have marasmus. Kwashiorkor tends to be confined to certain areas of the world where staple foods and foods used to wean babies are deficient in protein even though they provide enough calories as carbohydrates. Examples of such foods are yams, cassava, rice, sweet potatoes, and green bananas. However, anyone can develop kwashiorkor if their diet consists mainly of carbohydrates. People with kwashiorkor retain fluid, making them appear puffy and swollen. If kwashiorkor is severe, the abdomen may protrude.

How Starvation Affects the Body



Digestive system

Decreased production of stomach acid

Shrinking of the stomach

Frequent, often fatal, diarrhea

Cardiovascular system (heart and blood vessels)

Reduced heart size, reduced amount of blood pumped, slow heart rate, and low blood pressure

Ultimately, heart failure

Respiratory system

Slow breathing and reduced lung capacity

Ultimately, respiratory failure

Reproductive system

Reduced size of ovaries and testes

Loss of sex drive (libido)

Cessation of menstrual periods

Nervous system

Apathy and irritability

In children, mental retardation (sometimes)

Mental dysfunction, particularly in older people


Reduced muscle size and strength, imparing the ability to exercise or work



Metabolism (body processes to convert food into energy or to synthesize needed substances)

Low body temperature (hypothermia)

Fluid accumulation in the arms, legs, and abdomen

Disappearance of fat

Skin and hair

Thin, dry, inelastic skin

Dry, sparse hair that falls out easily

Immune system

Impaired ability to fight infections and repair wounds


Undernutrition may result from the following:

Lack of access to food Disorders or drugs that interfere with the intake, metabolism or absorption of nutrients A greatly increased need for calories

Taking certain drugs may contribute to undernutrition. Many drugs decrease appetite. Examples are drugs used to treat high blood pressure (such as diuretics), heart failure (such as digoxin), or cancer (such as cisplatin). Some drugs cause nausea, which decreases appetite. Others (such as thyroxine and theophylline) increase metabolism, and still others may interfere with the absorption of certain nutrients in the intestine. Also, stopping certain drugs (such as antianxiety drugs and antipsychotics) or alcohol may lead to weight loss.

Drinking too much alcohol, which has calories but little nutritional value, decreases the appetite. Because alcohol damages the liver, it can also interfere with the absorption and use of nutrients. Smoking dulls taste and smell, making food less appealing. Smoking also seems to cause other changes in the body that contribute to a low body weight. For example, smoking stimulates the sympathetic nervous system, which increases the body's use of energy.

In older people, many factors, including age-related changes in the body, work together to cause undernutrition.

Causes of Undernutrition

Lack of access to food Poverty Famine Inability to obtain food (for example, due to lack of transportation or physical impairment) Voluntary restriction of calories (as for a strict reducing diet or a fast) Disorders that interfere with the intake, metabolism, or absorption of nutrients Vomiting Diarrhea AIDS Cancer Diabetes Kidney failure Malabsorption disorders Inflammatory bowel disorders (such as Crohn's disease and ulcerative colitis) Liver disorders Anorexia nervosa Depression Alcoholism Drug abuse Drugs that interfere with the intake, metabolism, or absorption of nutrients Drugs used to treat anxiety, high blood pressure, heart failure, an underactive thyroid gland, asthma, and cancer Conditions that greatly increase the need for calories Injury, such as burns Surgery An overactive thyroid gland (hyperthyroidism) Infections that are widespread or severe High fever Demanding exercise, such as rehabilitation or training for athletic competition Pregnancy and breastfeeding Growth and development in infants, children, and adolescents


The most obvious sign of a calorie deficiency is loss of body fat (adipose tissue).

If the calorie deficiency is severe, adults can lose up to half of their body weight, and children can lose even more. Bones protrude, and the skin becomes thin, dry, inelastic, pale, and cold. Eventually, fat in the face is lost, causing the cheeks to look hollow and the eyes to seem sunken. The hair becomes dry and sparse, falling out easily. Severe wasting away of muscle and fat tissue is called cachexia. Cachexia is thought to result from excess production of substances called cytokines, which are produced by the immune system in response to a disorder, such as cancer or AIDS.

Other symptoms include fatigue, an inability to stay warm, diarrhea, loss of appetite, irritability, and apathy, sometimes leading to unresponsiveness (stupor). People feel weak, unable to do their normal activities. The number of some types of white blood cells decreases, resembling what happens in people who have AIDS. As a result, the immune system is weakened, increasing the risk of infections. If the calorie deficiency continues for a long time, liver, heart, and respiratory failure may develop. Total starvation (when no food is consumed) is fatal in 8 to 12 weeks.

In children who are severely undernourished, behavioral development may be markedly slow, and mental retardation may occur. Undernutrition, even when treated, may have long-lasting effects in children. Impairments in mental function and digestive problems may persist, sometimes throughout life. With treatment, most adults recover fully.


Doctors can usually diagnose severe, long-standing undernutrition based on the person's appearance. They also ask questions about diet, weight loss, the ability to shop for and prepare food, the presence of other disorders, and the use of drugs. These questions may help confirm the diagnosis, particularly when undernutrition is less obvious, and identify a cause. Identifying the cause is particularly important in children.

Blood tests may be done to measure the level of albumin (which decreases when people do not consume enough protein) and the number of certain types of white blood cells. A physical examination, x-rays, and skin tests may be done to determine the severity and effects of undernutrition. If doctors suspect the cause is another disorder, other tests may be done to help identify the cause.

Spotlight on Aging

Undernutrition in older people is serious because it increases the risk and severity of fractures, problems after surgery, pressure sores, and infections.

Older people are at risk of undernutrition for many reasons:

Age-related changes in the body: In the aging body, production of and sensitivity to hormones (such as growth hormone, insulin, and androgens) change. As a result, the percentage of fat in the body increases. How the body produces and uses energy also changes. Older people tend to feel full sooner and have less of an appetite. Thus, they may eat less. They may also eat less because the ability to taste and smell decreases, reducing the enjoyment of food. The ability to absorb some nutrients is reduced.

Some older people produce less saliva, resulting in dental problems and difficulty swallowing.

Disorders: Many disorders that contribute to undernutrition are common among older people. Depression can cause loss of appetite. A stroke or tremors may make chewing, swallowing, or preparing food difficult. Arthritis or other physical impairments, which reduce the ability to move, may make shopping for and preparing food more difficult. Malabsorption disorders interfere with the absorption of nutrients. Cancer can reduce the appetite and increase the body's need for calories. People with dementia may forget to eat and so lose weight. People with advanced dementia cannot feed themselves and may resist attempts by others to feed them. Dental problems (such as ill-fitting dentures or gum disease) may make chewing and thus digesting food more difficult. Anorexia nervosa that has been present for a long time may be made worse by an event late in life, such as death of a partner or fear of aging.

Drugs: Many of the drugs used to treat disorders common among older people (such as depression, cancer, heart failure, and high blood pressure) can contribute to undernutrition. Drugs can increase the body's need for nutrients, change how the body uses nutrients, or decrease the appetite. Some drugs have side effects that interfere with eating, such as nausea, diarrhea, and constipation.

Living situation: Older people who live alone may be less motivated to prepare and eat meals. They may have limited funds, causing them to buy cheap, less nutritious food or less total food. They may be physically unable or afraid to go out to buy food or may not have transportation to a grocery store.

Older people who live in institutions have even more obstacles to adequate nutrition.

They may be confused and unable to say when they are hungry or what they would like to eat. They may be unable to choose foods they like. They may be unable to feed themselves. If they eat slowly, especially if they need to be fed by a staff member, the staff member may not allow enough time to feed them adequately.

Older people who are hospitalized sometimes have the same problems.

Prevention and treatment: Older people can be encouraged to eat more, and food can be made more appealing. For example, strongly flavored or favorite foods, rather than low-salt or low-fat foods, can be served. Older people who need help with feeding should be given more help. Depression and other disorders, if present, should be treated. For older people living in institutions, making the dining room more attractive and giving them more time to eat may enable them to eat more.


For most people, treatment involves gradually increasing the number of calories consumed. Eating several small, nutritious meals each day is the best way. For people who have been starving, foods are reintroduced carefully. People who have difficulty digesting solid food may need liquid supplements. If undernutrition is severe, people may need to be hospitalized. Multivitamin supplements are also given.

Nutrients are given by mouth whenever possible. If they cannot be given by mouth, nutrients may be given through a tube inserted into the digestive tract or into a vein (intravenously).

Tube Feeding: This method may be used to feed people whose digestive tract is functioning normally but who cannot eat enough to meet their nutritional needs (such as people with severe burns) or who cannot swallow (such as some people who have had a stroke). For tube feeding, a thin plastic tube (a nasogastric tube) is passed through the nose and down the throat until it reaches the stomach or small intestine. If tube feeding is needed for a long time, the tube can be inserted directly into the stomach or small intestine through a small incision in the abdomen.

Tube Feeding

Food given through a tube (enteral nutrition) should contain all the nutrients a person needs. Special solutions, including some for people with specific needs (such as restriction of fluid intake), are available. Or, solid foods may be processed and given through a nasogastric tube. Tube feedings may be given slowly and continuously or in a larger amount (called a bolus) every few hours.

Tube feeding causes many problems, and the problems may be life threatening: Inhalation (aspiration) of food into the lungs: For older people, aspiration is the most common problem caused by tube feeding. Aspiration of food can lead to pneumonia. Food is less likely to be aspirated when the head of the bed is elevated for 1 to 2 hours after tube feeding, reducing the risk of spitting food up (regurgitation), and when the solution is given slowly. Diarrhea and abdominal discomfort: Changing the solution or giving it more slowly may lessen these problems. Irritation of tissues: The tube may irritate and erode tissues of the nose, throat, or esophagus. If tissues become irritated, the feeding tube can usually be removed, and feedings can be continued using a different type of tube.

Intravenous Feeding: This method is used when the digestive tract cannot adequately absorb nutrients (for example, in people with a malabsorption disorder). It is also used when the digestive tract must be temporarily kept free of food (for example, in people with ulcerative colitis or severe pancreatitis). Food given intravenously (parenteral nutrition) can supply part of a person's nutritional requirements (partial parenteral nutrition) or all of them (total parenteral nutrition). Because total parenteral nutrition requires a large intravenous tube (catheter), it is inserted into a large vein, such as the subclavian vein, located under the collarbone.

Intravenous feeding can also cause problems: Infection: Infection is a constant risk because the catheter is usually left in place for a long time and the solutions that pass through it contain a lot of glucose—a sugar—which promotes the growth of bacteria. People receiving total parenteral nutrition are closely monitored for signs of infection. Too much water (volume overload): Giving too much water can cause fluid to collect in the lungs, making breathing difficult. Thus, doctors monitor the person's weight and the amount of urine excreted regularly. They can sometimes reduce the risk by calculating the amount of water required before starting feedings. Nutritional imbalances and deficiencies: Rarely, deficiencies of certain vitamins and minerals occur. Doctors measure the blood levels of dissolved minerals (electrolytes), sugar (glucose), and urea (a measure of kidney function) to identify certain nutritional imbalances. They can then adjust the solution accordingly. They periodically monitor the levels. Decreased bone density: In some people, total parenteral nutrition causes bone density to decrease. The reason is unknown, and the best treatment is to temporarily or permanently stop this type of feeding. Liver problems: Total parenteral nutrition can cause liver malfunction, most commonly in premature infants. Blood tests are done to monitor liver function. Gallbladder problems: Gallstones may develop. Treatment involves adjusting the solution, stopping feedings for a few hours a day, and, if possible, providing food by mouth or feeding tube.

Drugs: People who are very undernourished are sometimes given drugs to increase appetite, such as dronabinol or megestrol, or drugs to increase muscle mass, such as growth hormone or an anabolic steroid (for example, nandrolone or testosterone).

Last full review/revision August 2007 by David R. Thomas, MD

Source: The Merck Manual Home Edition