A decrease in appetite is quite common during illness, such as when suffering from a bout of influenza or other relatively short-term illnesses. Nausea is also associated with decreased appetite.

Appetite can also decrease in times of emotional upset, depression, boredom or anxiety.

Generally, when the illness or emotional problem is resolved, the appetite returns to normal. If a decreased appetite is persistent, is accompanied by other symptoms such as those listed below, or there is unexplained weight loss, medical attention should be sought.

Infections That Can Cause Decreased Appetite

Infection can lead to a decrease in appetite. Infections that are often associated with a loss of appetite include:

Respiratory infections that affect the lungs
Pneumonia
Influenza
Kidney infection, such as pyelonephritis
Hepatitis and conditions that cause liver inflammation
HIV/AIDS

Other Causes

Factors that can affect appetite are many and can vary greatly. Other causes can include:

Psychological issues such as stress, anxiety, depression, grief or eating disorders like anorexia
Stimulants, weight loss medications, and medications for attention deficit and hyperactivity disorder (ADHD) like amphetamines (e.g., Adderall)
Illicit drugs like hallucinogenics, inhalants, and LSD
Migraine headaches
Hypercalcemia, a condition in which there is too much calcium in the blood
Hypokalemia, a condition where there is too little potassium in the blood
High alcohol intake
Congestive heart failure
Cancer and treatments like chemotherapy
Acid reflux
Food allergies
Food poisoning
Acute pain
Intestinal blockage
Gastrointestinal ulcers
Appendicitis
Digestive disorders such as irritable bowel syndrome (IBS), Crohn's disease, celiac disease and colitis

When to Seek Help

If a loss of appetite is accompanied by chest pains, shortness of breath, sweating, blurred vision or confusion—the warning signs of a heart attack or stroke—seek medical attention immediately.

Also, seek medical attention if any of the following signs and symptoms accompany decreased appetite:

Unable to eat or drink for more than 12 hours
A high fever above 103 degrees Fahrenheit (40 degrees Celsius)
A severe headache
Extreme abdominal pain
Stiffness in the neck
Dehydration with symptoms including dizziness, increased thirst, and low urine output

Weight Loss From Decreased Appetite

During periods of decreased appetite, weight loss can obviously occur, but there are occasions when weight loss is unexplained and may be unrelated to decreased appetite.

When drastic weight loss reaches more than 10% of the starting weight over a short period of time (for example, 3 months), a search for medical causes should be undertaken, and medical attention is required.

Typically, dramatic weight loss is due to an inability to digest and absorb food properly, or the intake of calories is insufficient and outstripped by the energy needs of the body.

Weight Loss From Malabsorption

Malabsorption is the term used to describe the inability of the body to absorb enough nutrients from the consumption of food and drink. This can lead to malnutrition and unexplained weight loss.

The most common nutrients that are not sufficiently absorbed are fats (lipids); however, malabsorption can apply to nutrients like carbohydrates, proteins, minerals like iron and calcium, vitamins, and electrolytes like potassium and sodium.

Malabsorption can be caused by several factors, including:

Digestive enzyme problems
Gallbladder problems
Small intestine problems
Parasitic infections
Some medications causing long-term diarrhea
Hyperthyroidism creating an excessive energy demand
Fever
Cancer
Drug abuse