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The causes of malnutrition are diverse, because both a deficit and a surplus of nutrients can cause malnutrition. Undernutrition is the condition most people associate with malnutrition, which is usually caused by inadequate food supplies. However, overnutrition, particularly obesity, is one of the fastest-growing health concerns in the United States.

One of the most common causes of undernutrition is malabsorption, which is the inability of the body to absorb nutrients. It can be caused by numerous factors, some of which are hereditary and some of which relate to existing health problems. For example, chronic diseases such as cystic fibrosis, liver disease, kidney disease, cancer (especially of the gastrointestinal system), AIDS and inflammatory bowel disease (IBD) can all cause malabsorption of certain nutrients into the bloodstream. Malabsorption also often occurs following gastric bypass surgery. In addition, some medications used to treat conditions such as epilepsy, high blood pressure and acid indigestion can cause malabsorption.

Some cases of malabsorption are temporary and the body is able to effectively absorb nutrients after the source of malabsorption is treated. Other cases of malabsorption are permanent. People suffering from pernicious anemia (a deficiency in vitamin B12 which causes anemia) cannot absorb B12 and have to undergo regular supplementation for life.

Another cause of undernutrition is loss of appetite. Although this may seem trivial, the body relies on appetite to instruct it when and how much to eat. The disruption of this natural mechanism can cause undernutrition. There are many reasons why people lose their appetite, including:

• Mental health conditions. Conditions such as depression or dementia may cause people to be apathetic about food or to simply forget to eat. Also, eating disorders such as anorexia nervosa and bulimia can cause severe malnutrition.
  
  
• Dietary restrictions. Special diets, such as low-salt or low-fat diets, can cause everyday meals to become bland and unappealing, therefore contributing to under-eating.

Other causes of undernutrition include:

• Hospitalization. Most people suffer from some degree of malnutrition after major surgery. This is especially the case if nutrients are being supplied through an intravenous (I.V.) drip, as patients are sometimes released before the body has recovered from the loss of nutrients.
  
  
• Diarrhea. During prolonged periods of diarrhea, the body is unable to retain all the nutrients it ingests, often resulting in malnutrition.
  
  
• Sensory or dental problems. As the body ages, it becomes less sensitive to taste and smell, which can lead to a lack of interest in eating. In addition, conditions such as dysgeusia (taste disorder) can make some foods taste unpleasant. Dental problems such as pain while chewing or swallowing can make some people reluctant to eat.
  
  
• Substance abuse. People who abuse alcohol may eventually develop malabsorption. Alcohol impedes the absorption of folate, thiamine and other nutrients. It can also cause tissue injury and liver and kidney disease, which can have implications for absorbing nutrients. Heavy drug users may find that the need to eat is suppressed by the substances they are taking. Also, some drug users lose interest in self-preservation, often resulting in malnutrition.

Overnutrition can be caused by the following:

• Overeating. The most common cause of obesity and overnutrition is an imbalanced, overindulgent diet. This type of diet can leave the body deficient in many vitamins and minerals. For example, a diet that lacks fresh fruit and vegetables can be deficient in vitamin C, folate and other vitamins. In addition, eating disorders such as binge-eating can cause serious overnutrition problems.
  
  
• Excess intake of micronutrients or macronutrients. The over-consumption of any one micronutrient or macronutrient can cause an imbalance in nutrients, placing undue stress on the body. This is often the result of excessive vitamin supplementation. For some vitamins – especially the fat-soluble vitamins A, D, E, and K – excess levels are stored in the body for longer and toxicity can easily occur if these levels are then added to by supplements.
  
  
• Overuse of diets. Diets that concentrate on one food type, as many fad diets do, may lead to malnutrition because other nutrients are ignored. Some people, particularly those who are recovering from surgery or serious illness, are prescribed high-calorie or high-protein diets in order to facilitate faster recovery. Although these diets may be effective for short periods of time, staying on a high-calorie or high-protein diet for longer than necessary can cause severe overnutrition and eventually obesity.

In addition to the above causes, there are a number of risk factors for malnutrition. The most important of these are age and socioeconomic status.

Age is a significant risk factor for malnutrition because of the constantly changing nutritional needs of people throughout their life. The following life stages are most at risk for malnutrition:

• Infancy. In infancy, the body requires large quantities of protein, calories and other nutrients in order to grow at a steady and healthy rate. If infants do not receive adequate amounts of nutrients from breast milk or formula they are at serious risk for protein-energy malnutrition and other life-threatening conditions.
  
  
• Childhood and adolescence. As children grow and become more active, they require larger amounts of energy. However, many children in these stages begin to adopt unhealthy lifestyle habits that can cause overnutrition and obesity. It is essential that children and teenagers are encouraged to maintain an active lifestyle that efficiently burns the calories that they consume. Anorexia nervosa, a form of starvation, may affect adolescent girls.
  
  
• Pregnancy and lactation. Pregnant and breastfeeding women have extra nutritional needs. During pregnancy, nausea and vomiting may make proper nutrition difficult to maintain. Breastfeeding women must nourish themselves enough to continue to produce breast milk for their infants.
  
  
• Later life. As the body ages, its nutritional requirements change. The body’s metabolism, which regulates the rate at which nutrients are broken down for energy, slows down. This often results in a decline in appetite and a reduction of muscle, bone and organ mass. A diminished sense in taste and smell, loneliness, physical and mental handicaps, immobility, and chronic illness can result in inadequate intake and malnutrition in the elderly.  However, older people still require certain levels of nutrients, especially micronutrients. In some cases, requirements for nutrients increases with age. In addition to metabolism changes, many of the diseases that accompany old age and the medications to treat them cause malabsorption, which is one of the major causes of malnutrition.
  
  
• Socioeconomic status may also be considered a risk factor for malnutrition. One of the biggest non-medical causes of malnutrition is poverty. It is estimated that 4.4 million American households experience hunger, according to the U.S. Department of Agriculture (USDA). In addition, obesity rates are higher in poor areas, possibly because there is less access to nutritious foods and easier availability of less healthy foods.

Researchers continue to study the causes of malnutrition. For example, one recent study found that infants and children who are placed on elimination diets are at heightened risk of malnutrition. These diets are used to pinpoint the source of food allergies.