Malnutrition is a condition that results from eating a diet in which nutrients are either not enough or are too much such that the diet causes health problems. It may involve calories, protein, carbohydrates, vitamins or minerals. Not enough nutrients is called undernutrition or undernourishment while too much is called overnutrition. Malnutrition is often used to specifically refer to undernutrition where an individual is not getting enough calories, protein, or micronutrients. If undernutrition occurs during pregnancy, or before two years of age, it may result in permanent problems with physical and mental development. Extreme undernourishment, known as starvation, may have symptoms that include: a short height, thin body, very poor energy levels, and swollen legs and abdomen. People also often get infections and are frequently cold. The symptoms of micronutrient deficiencies depend on the micronutrient that is lacking.

Undernourishment is most often due to not enough high-quality food being available to eat. This is often related to high food prices and poverty. A lack of breastfeeding may contribute, as may a number of infectious diseases such as: gastroenteritis, pneumonia, malaria, and measles, which increase nutrient requirements. There are two main types of undernutrition: protein-energy malnutrition and dietary deficiencies. Protein-energy malnutrition has two severe forms: marasmus (a lack of protein and calories) and kwashiorkor (a lack of just protein). Common micronutrient deficiencies include: a lack of iron, iodine, and vitamin A. During pregnancy, due to the body's increased need, deficiencies may become more common. In some developing countries, overnutrition in the form of obesity is beginning to present within the same communities as undernutrition. Other causes of malnutrition include anorexia nervosa and bariatric surgery.

Efforts to improve nutrition are some of the most effective forms of development aid. Breastfeeding can reduce rates of malnutrition and death in children, and efforts to promote the practice increase the rates of breastfeeding. In young children, providing food (in addition to breastmilk) between six months and two years of age improves outcomes. There is also good evidence supporting the supplementation of a number of micronutrients to women during pregnancy and among young children in the developing world. To get food to people who need it most, both delivering food and providing money so people can buy food within local markets are effective. Simply feeding students at school is insufficient. Management of severe malnutrition within the person's home with ready-to-use therapeutic foods is possible much of the time. In those who have severe malnutrition complicated by other health problems, treatment in a hospital setting is recommended. This often involves managing low blood sugar and body temperature, addressing dehydration, and gradual feeding. Routine antibiotics are usually recommended due to the high risk of infection. Longer-term measures include: improving agricultural practices, reducing poverty, improving sanitation, and the empowerment of women.

There were 793 million undernourished people in the world in 2015 (13% of the total population). This is a reduction of 216 million people since 1990 when 23% were undernourished. In 2012 it was estimated that another billion people had a lack of vitamins and minerals. In 2015, protein-energy malnutrition was estimated to have resulted in 323,000 deaths—down from 510,000 deaths in 1990. Other nutritional deficiencies, which include iodine deficiency and iron deficiency anemia, result in another 83,000 deaths. In 2010, malnutrition was the cause of 1.4% of all disability adjusted life years. About a third of deaths in children are believed to be due to undernutrition, although the deaths are rarely labelled as such. In 2010, it was estimated to have contributed to about 1.5 million deaths in women and children, though some estimate the number may be greater than 3 million. An additional 165 million children were estimated to have stunted growth from malnutrition in 2013. Undernutrition is more common in developing countries. Certain groups have higher rates of undernutrition, including women—in particular while pregnant or breastfeeding—children under five years of age, and the elderly. In the elderly, undernutrition becomes more common due to physical, psychological, and social factors.

In those with malnutrition some of the signs of dehydration differ. Children; however, may still be interested in drinking, have decreased interactions with the world around them, have decreased urine output, and may be cool to touch.

Malnutrition in children – here understood as undernutrition – is common globally and results in both short and long term irreversible negative health outcomes including stunted growth which may also be linked to cognitive development deficits, underweight and wasting. The World Health Organization (WHO) estimates that malnutrition accounts for 54 percent of child mortality worldwide, about 1 million children. Another estimate also by WHO states that childhood underweight is the cause for about 35% of all deaths of children under the age of five years worldwide.

The main causes are unsafe water, inadequate sanitation or insufficient hygiene, factors related to society and poverty, diseases, maternal factors, gender issues and – overall – poverty.

There are three commonly used measures for detecting malnutrition in children:

- stunting (extremely low height for age),

- underweight (extremely low weight for age), and

- wasting (extremely low weight for height).

These measures of malnutrition are interrelated, but studies for the World Bank found that only 9 percent of children exhibit stunting, underweight, and wasting.

Children with severe acute malnutrition are very thin, but they often also have swollen hands and feet, making the internal problems more evident to health workers.

Children with severe malnutrition are very susceptible to infection.

Marasmus is commonly represented by a shrunken, wasted appearance, loss of muscle mass and subcutaneous fat mass. Buttocks and upper limb muscle groups are usually more affected than others. Edema is not a sign of marasmus and is only present in kwashiorkor, and marasmic kwashiorkor. Other symptoms of marasmus include unusual body temperature (hypothermia, pyrexia), anemia, dehydration (as characterized with consistent thirst and shrunken eyes), hypovolemic shock (weak radial pulse, cold extremities, decreased consciousness), tachypnea (pneumonia, heart failure), abdominal manifestations (distension, decreased or metallic bowel sounds, large or small liver, blood or mucus in the stools), ocular manifestations (corneal lesions associated with vitamin A deficiency), dermal manifestations (evidence of infection, purpura, and ear, nose, and throat symptoms (otitis, rhinitis). Dry skin and brittle hair are also symptoms of marasmus. Marasmus can also make children short-tempered and irritable.

Marasmus is caused by a severe deficiency of nearly all nutrients, especially protein, carbohydrates, and lipids, usually due to poverty and scarcity of food. Viral, bacterial, and parasitic infections can cause children to absorb too few nutrients, even when consumption is adequate. Marasmus can develop in children who suffer from weakening conditions such as chronic diarrhea.

Stunted growth, also known as stunting and nutritional stunting, is a reduced growth rate in human development. It is a primary manifestation of malnutrition (or more precisely undernutrition) and recurrent infections, such as diarrhea and helminthiasis, in early childhood and even before birth, due to malnutrition during fetal development brought on by a malnourished mother. The definition of stunting according to the World Health Organisation (WHO) is for the "height for age" value to be less than two standard deviations of the WHO Child Growth Standards median.

As of 2012 an estimated 162 million children under 5 years of age, or 25%, were stunted in 2012. More than 90% of the world's stunted children live in Africa and Asia, where respectively 36% and 56% of children are affected. Once established, stunting and its effects typically become permanent. Stunted children may never regain the height lost as a result of stunting, and most children will never gain the corresponding body weight. Living in an environment where many people defecate in the open due to lack of sanitation, is an important cause of stunted growth in children, for example in India.

Vitamin poisoning is the condition of overly high storage levels of vitamins, which can lead to toxic symptoms. The medical names of the different conditions are derived from the vitamin involved: an excess of vitamin A, for example, is called "hypervitaminosis A".

Iron overload disorders are diseases caused by the overaccumulation of iron in the body. Organs commonly affected are the liver, heart and endocrine glands in the mouth.

Obesity is caused by eating too many calories compared to the amount of exercise the individual is performing, causing a distorted energy balance. It can lead to diseases such as cardiovascular disease and diabetes. Obesity is a condition in which the natural energy reserve, stored in the fatty tissue of humans and other mammals, is increased to a point where it is associated with certain health conditions or increased mortality.

The low-cost food that is generally affordable to the poor in affluent nations is low in nutritional value and high in fats, sugars and additives. In rich countries, therefore, obesity is often a sign of poverty and malnutrition while in poorer countries obesity is more associated with wealth and good nutrition. Other non-nutritional causes for obesity included: sleep deprivation, stress, lack of exercise, and heredity.

Acute overeating can also be a symptom of an eating disorder.

Goitrogenic foods can cause goitres by interfering with iodine uptake.

Protein–energy malnutrition (PEM) refers to a form of malnutrition which is defined as a range of pathological conditions arising from coincident lack of protein and/or energy in varying proportions.The condition vary in forms ranging from mild through moderate to severe degrees.

Types include:

- Kwashiorkor (protein malnutrition predominant)

- Marasmus (deficiency in calorie intake)

- Marasmic Kwashiorkor (marked protein deficiency and marked calorie insufficiency signs present, sometimes referred to as the most severe form of malnutrition)

PEM is fairly common worldwide in both children and adults and accounts for 6 million deaths annually. In the industrialized world, PEM is predominantly seen in hospitals, is associated with disease, or is often found in the elderly.

Note that PEM may be secondary to other conditions such as chronic renal disease or cancer cachexia in which protein energy wasting may occur.

Protein–energy malnutrition affects children the most because they have less protein intake. The few rare cases found in the developed world are almost entirely found in small children as a result of fad diets, or ignorance of the nutritional needs of children, particularly in cases of milk allergy.

The defining sign of kwashiorkor in a malnourished child is pitting edema (swelling of the ankles and feet). Other signs include a distended abdomen, an enlarged liver with fatty infiltrates, thinning hair, loss of teeth, skin depigmentation and dermatitis. Children with kwashiorkor often develop irritability and anorexia. Generally, the disease can be treated by adding protein to the diet; however, it can have a long-term impact on a child's physical and mental development, and in severe cases may lead to death.

In dry climates, marasmus is the more frequent disease associated with malnutrition. Another malnutrition syndrome includes cachexia, although it is often caused by underlying illnesses. These are important considerations in the treatment of the patients.

Growth stunting is identified by comparing measurements of children's heights to the World Health Organization 2006 growth reference population: children who fall below the fifth percentile of the reference population in height for age are defined as stunted, regardless of the reason. The lower than fifth percentile corresponds to less than two standard deviations of the WHO Child Growth Standards median.

As an indicator of nutritional status, comparisons of children's measurements with growth reference curves may be used differently for populations of children than for individual children. The fact that an individual child falls below the fifth percentile for height for age on a growth reference curve may reflect normal variation in growth within a population: the individual child may be short simply because both parents carried genes for shortness and not because of inadequate nutrition. However, if substantially more than 5% of an identified child population have height for age that is less than the fifth percentile on the reference curve, then the population is said to have a higher-than-expected prevalence of stunting, and malnutrition is generally the first cause considered.

Kwashiorkor is a form of severe protein–energy malnutrition characterized by edema, irritability, ulcerating dermatoses, and an enlarged liver with fatty infiltrates. Sufficient calorie intake, but with insufficient protein consumption, distinguishes it from marasmus. Kwashiorkor cases occur in areas of famine or poor food supply. Cases in the developed world are rare.

Jamaican pediatrician Cicely Williams introduced the name into the medical community in a 1935 "Lancet" article, two years after she published the disease's first formal description in the Western medical literature. The name is derived from the Ga language of coastal Ghana, translated as "the sickness the baby gets when the new baby comes" or "the disease of the deposed child", and reflecting the development of the condition in an older child who has been weaned from the breast when a younger sibling comes. Breast milk contains proteins and amino acids vital to a child's growth. In at-risk populations, kwashiorkor may develop after a mother weans her child from breast milk, replacing it with a diet high in carbohydrates, especially sugar.

Protein malnutrition is detrimental at any point in life, but protein malnutrition prenatally has been shown to have significant lifelong effects. During pregnancy, one should aim for a diet that consists of at least 20% protein for the health of the fetus. Diets that consist of less than 6% protein "in utero" have been linked with many deficits, including decreased brain weight, increased obesity, and impaired communication within the brain in some animals. Even diets of mild protein malnutrition (7.2%) have been shown to have lasting and significant effects in rats. The following are some studies in which prenatal protein deficiency has been shown to have unfavorable consequences.

- Decreased brain size: Protein deficiency has been shown to affect the size and composition of brains in rhesus monkeys. Monkeys whose mother had eaten a diet with an adequate amount of protein were shown to have no deficit in brain size or composition, even when their body weight amounted to less than one-half of that of the controls, whereas monkeys whose mothers had eaten low-protein diets were shown to have smaller brains regardless of the diet given after birth.

- Impaired neocortical long-term potentiation: Mild protein deficiency (in which 7.2% of the diet consists of protein) in rats has been shown to impair entorhinal cortex plasticity (visuospatial memory), noradrenergic function in the neocortex, and neocortical long-term potentiation.

- Altered fat distribution: Protein undernutrition can have varying effects depending on the period of fetal life during which the malnutrition occurred. Although there were not significant differences in the food intake, there were increased amounts of perirenal fat in rats that were protein-deprived during early (gestation days 0–7) and mid (gestation days 8–14) pregnancy, and throughout pregnancy, whereas rats that were protein-deprived only late in gestation (gestation days 15–22) were shown to have increased gonadal fat.

- Increased obesity: Mice exposed to a low-protein diet prenatally weighed 40% less than the control group at birth (intrauterine growth retardation). When fed a high-fat diet after birth, the prenatally undernourished mice were shown to have increased body weight and adiposity (body fat), while those who were adequately nourished prenatally did not show an increase in body weight or adiposity when fed the same high-fat diet after birth.

- Decreased birth weight, and gestation duration: Supplementation of protein and energy can lead to increased duration of gestation and higher birth weight. When fed a supplement containing protein, energy, and micronutrients, pregnant women showed more successful results during birth, including high birth weights, longer gestations, and fewer pre-term births, than women who had consumed a supplement with micronutrients and low energy but no protein (although this finding may be due to the increase of energy in the supplements, not the increase of protein).

- Increased stress sensitivity: Male offspring of pregnant rats fed low-protein diets have been shown to exhibit blood pressure that is hyperresponsive to stress and salt.

- Decreased sperm quality: A low-protein diet during gestation in rats has been shown to affect the sperm quality of the male offspring in adulthood. The protein deficiency appeared to reduce sertoli cell number, sperm motility, and sperm count.

- Altered cardiac energy metabolism: Prenatal nutrition, specifically protein nutrition, may affect the regulation of cardiac energy metabolism through changes in specific genes.

- Increased passive stiffness: Intrauterine undernutrition was shown to increase passive stiffness in skeletal muscles in rats.

From these studies it is possible to conclude that prenatal protein nutrition is vital to the development of the fetus, especially the brain, the susceptibility to diseases in adulthood, and even gene expression. When pregnant females of various species were given low-protein diets, the offspring were shown to have many deficits. These findings highlight the great significance of adequate protein in the prenatal diet.

Disease-related malnutrition can be considered in four categories:

Weight loss issues related to specific diseases include:

- As chronic obstructive pulmonary disease (COPD) advances, about 35% of patients experience severe weight loss called pulmonary cachexia, including diminished muscle mass. Around 25% experience moderate to severe weight loss, and most others have some weight loss. Greater weight loss is associated with poorer prognosis. Theories about contributing factors include appetite loss related to reduced activity, additional energy required for breathing, and the difficulty of eating with dyspnea (labored breathing).

- Cancer, a very common and sometimes fatal cause of unexplained (idiopathic) weight loss. About one-third of unintentional weight loss cases are secondary to malignancy. Cancers to suspect in patients with unexplained weight loss include gastrointestinal, prostate, hepatobilary (hepatocellular carcinoma, pancreatic cancer), ovarian, hematologic or lung malignancies.

- People with HIV often experience weight loss, and it is associated with poorer outcomes. Wasting syndrome is an AIDS-defining condition.

- Gastrointestinal disorders are another common cause of unexplained weight loss – in fact they are the most common non-cancerous cause of idiopathic weight loss. Possible gastrointestinal etiologies of unexplained weight loss include: celiac disease, peptic ulcer disease, inflammatory bowel disease (crohn's disease and ulcerative colitis), pancreatitis, gastritis, diarrhea and many other GI conditions.

- Infection. Some infectious diseases can cause weight loss. Fungal illnesses, endocarditis, many parasitic diseases, AIDS, and some other subacute or occult infections may cause weight loss.

- Renal disease. Patients who have uremia often have poor or absent appetite, vomiting and nausea. This can cause weight loss.

- Cardiac disease. Cardiovascular disease, especially congestive heart failure, may cause unexplained weight loss.

- Connective tissue disease

- Neurologic disease, including dementia

- Oral, taste or dental problems (including infections) can reduce nutrient intake leading to weight loss.

Malnutrition is an important health concern in Tibet. According to a study conducted in 1994/1995 in eleven districts of Tibet, malnutrition affected more than half of the children from 1–7 years old. The major cause was poverty.

Overnutrition or hyperalimentation is a form of malnutrition in which the intake of nutrients is oversupplied. The amount of nutrients exceeds the amount required for normal growth, development, and metabolism.

The term can also refer to:

- Obesity, which "usually" occurs by overeating, as well as:

- Oversupplying a "specific" nutrient, such as dietary minerals or vitamin poisoning. This is due to an excessive intake or a nutritional imbalance caused by fad diets.

For mineral excess, see:

- Iron poisoning, and

- Low sodium diet (a response to excess sodium).

Overnutrition may also refers to greater food consumption than appropriate, as well as other feeding procedures such as parenteral nutrition.

Unintentional weight loss may result from loss of body fats, loss of body fluids, muscle atrophy, or even a combination of these. It is generally regarded as a medical problem when at least 10% of a person's body weight has been lost in six months or 5% in the last month. Another criterion used for assessing weight that is too low is the body mass index (BMI). However, even lesser amounts of weight loss can be a cause for serious concern in a frail elderly person.

Unintentional weight loss can occur because of an inadequately nutritious diet relative to a person's energy needs (generally called malnutrition). Disease processes, changes in metabolism, hormonal changes, medications or other treatments, disease- or treatment-related dietary changes, or reduced appetite associated with a disease or treatment can also cause unintentional weight loss. Poor nutrient utilization can lead to weight loss, and can be caused by fistulae in the gastrointestinal tract, diarrhea, drug-nutrient interaction, enzyme depletion and muscle atrophy.

Continuing weight loss may deteriorate into wasting, a vaguely defined condition called cachexia. Cachexia differs from starvation in part because it involves a systemic inflammatory response. It is associated with poorer outcomes. In the advanced stages of progressive disease, metabolism can change so that they lose weight even when they are getting what is normally regarded as adequate nutrition and the body cannot compensate. This leads to a condition called anorexia cachexia syndrome (ACS) and additional nutrition or supplementation is unlikely to help. Symptoms of weight loss from ACS include severe weight loss from muscle rather than body fat, loss of appetite and feeling full after eating small amounts, nausea, anemia, weakness and fatigue.

Serious weight loss may reduce quality of life, impair treatment effectiveness or recovery, worsen disease processes and be a risk factor for high mortality rates. Malnutrition can affect every function of the human body, from the cells to the most complex body functions, including:

- immune response;

- wound healing;

- muscle strength (including respiratory muscles);

- renal capacity and depletion leading to water and electrolyte disturbances;

- thermoregulation; and

- menstruation.

In addition, malnutrition can lead to vitamin and other deficiencies and to inactivity, which in turn may pre-dispose to other problems, such as pressure sores.

Unintentional weight loss can be the characteristic leading to diagnosis of diseases such as cancer and type 1 diabetes.

In the UK, up to 5% of the general population is underweight, but more than 10% of those with lung or gastrointestinal diseases and who have recently had surgery. According to data in the UK using the Malnutrition Universal Screening Tool ('MUST'), which incorporates unintentional weight loss, more than 10% of the population over the age of 65 is at risk of malnutrition. A high proportion (10-60%) of hospital patients are also at risk, along with a similar proportion in care homes.

Diseases of poverty is a term sometimes used to collectively describe diseases, disabilities, and health conditions that are more prevalent among the poor than among wealthier people. In many cases poverty is considered the leading risk factor or determinant for such diseases, and in some cases the diseases themselves are identified as barriers to economic development that would end poverty. Diseases of poverty are often co-morbid and ubiquitous with malnutrition.

These diseases triggered in part by poverty are in contrast to so-called "diseases of affluence", which are diseases thought to be a result of increasing wealth in a society.

Cutoff points may vary, but <80% (close to −2 Z-score) is often used.

- Adults:

- Body Mass Index (BMI) is the quotient between weight and height squared (kg/m). An individual with a BMI < 18.5 is regarded as a case of wasting.

- Percent of body weight lost (At Tufts, an unintentional loss of 6% or more in 6 months is regarded as wasting)

Wasting can be caused by an extremely low energy intake (e.g., caused by famine), nutrient losses due to infection, or a combination of low intake and high loss. Infections and conditions associated with wasting include tuberculosis, chronic diarrhea, AIDS, and superior mesenteric artery syndrome. The mechanism may involve cachectin – also called tumor necrosis factor, a macrophage-secreted cytokine. Caretakers and health providers can sometimes contribute to wasting if the patient is placed on an improper diet. Voluntary weight loss and eating disorders are excluded as causes of wasting.

Tea and toast syndrome is a form of malnutrition experienced by elderly people who are unable to prepare meals and tend to themselves. Their diets often dwindle to tea and toast resulting in a deficiency of vitamins and other nutrients. The syndrome often manifests itself as hyponatremia, a deficit of sodium, due to the lack of salt in the diet.

The syndrome often occurs once children have moved away, and a partner has died. An elderly person with nobody left to cook for, or without the skills to cook will revert to a diet of simple foods such as bread, cheese and crackers, and tinned foods.

Factors that lead to the syndrome include social isolation, psychological issues such as depression, illness and physical limitations.

One of the reasons a cat may stop eating is separation anxiety and the emotional stress that results. Moving, gaining or losing housemates or pets, going on vacation, or prolonged boarding are all common situations that pet owners report just prior to the onset of the disease, but it may develop without these conditions existing. Obesity is known to increase the risk of hepatic lipidosis; however, there is no known "official" cause of the disease. Severe anorexia usually precedes the onset of the disease. When the cat has no energy from eating, the liver must metabolize fat deposits in the body into usable energy to sustain life. The cat liver, however, is poor at metabolizing fat, causing a buildup of fat in the cells of the liver, leading to fatty liver. At this point the disease can be diagnosed; however, it will often not be diagnosed, and many animals are euthanized due to improper or no diagnosis.

EE is rarely symptomatic and is considered a subclinical condition. However, adults may have mild symptoms or malabsorption such as altered stool consistency, increased stool frequency and weight loss.

Hypolipoproteinemia, hypolipidemia, or hypolipidaemia (British English) is a form of dyslipidemia that is defined by abnormally lowered levels of any or all lipids and/or lipoproteins in the blood. It occurs through genetic disease (namely, Hypoalphalipoproteinemia and Hypobetalipoproteinemia), malnutrition, malabsorption, wasting disease, cancer, hyperthyroidism, and liver disease.