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Anorexia (deriving from the Greek "α(ν)-" (a(n)-, a prefix that denotes absence) + "όρεξη" (orexe) = appetite) is the decreased sensation of appetite. While the term in non-scientific publications is often used interchangeably with anorexia nervosa, many possible causes exist for a decreased appetite, some of which may be harmless, while others indicate a serious clinical condition or pose a significant risk.
For example, anorexia of infection is part of the acute phase response (APR) to infection. The APR can be triggered by lipopolysaccharides and peptidoglycans from bacterial cell walls, bacterial DNA, double-stranded viral RNA, and viral glycoproteins, which can trigger production of a variety of proinflammatory cytokines. These can have an indirect effect on appetite by a number of means, including peripheral afferents from their sites of production in the body, by enhancing production of leptin from fat stores. Inflammatory cytokines can also signal to the central nervous system more directly by specialized transport mechanisms through the blood–brain barrier, via circumventricular organs (which are outside the barrier), or by triggering production of eicosanoids in the endothelial cells of the brain vasculature. Ultimately the control of appetite by this mechanism is thought to be mediated by the same factors normally controlling appetite, such as neurotransmitters (serotonin, dopamine, histamine), corticotropin releasing factor, neuropeptide Y, and α-melanocyte-stimulating hormone.
Clinically important causes 
- Acute radiation syndrome
- Acute viral hepatitis
- Addison's disease
- Atypical pneumonia (mycoplasma)
- Anorexia nervosa
- Anxiety disorder
- Chronic pain
- Chronic kidney disease
- Congestive heart failure, perhaps due to congestion of the liver with venous blood
- Crohn's disease
- Drug addiction
- Hypervitaminosis D
- Metabolic disorders, particularly urea cycle disorders
- Sickness behavior
- Superior mesenteric artery syndrome
- Ulcerative colitis
- Zinc deficiency
- Amphetamine (Adderall), dextroamphetamine (Dexedrine & Dextrostat), lisdexamfetamine (Vyvanse)
- Antidepressants can have anorexia as a side effect
- Byetta, a Type II Diabetes drug, will cause moderate nausea and loss of appetite
- Dextromethylphenidate (Focalin)
- Abrupt cessation of appetite-increasing drugs, such as cannabis and corticosteroids
- Methamphetamine (Desoxyn) (treatment of ADD & ADHD and narcolepsy)
- Methylphenidate (Ritalin & Concerta)
- Chemicals that are members of the phenethylamine group. (Individuals with anorexia nervosa may seek them to suppress appetite)
- Stimulants such as Caffeine, Nicotine, and Cocaine
- Topiramate (Topamax) (as a side effect)
- Other drugs may be used to intentionally cause anorexia in order to help a patient preoperative fasting prior to general anesthesia. It is important to avoid food before surgery to mitigate the risk of pulmonary aspiration, which can be fatal.
- Opiates act upon the digestive system and can reduce the physical sensation of hunger in the same way that they reduce physical sensations of pain. They also frequently cause delayed gastric emptying (gastroparesis) and can sometimes lead to changes in metabolism with long-term use.
- During the post-operative recovery period for a tonsillectomy or adenoidectomy, it is common for adult patients to experience a lack of appetite until their throat significantly heals (usually between 10–14 days).
- Altitude sickness
- Significant emotional pain caused by an event (rather than a mental disorder) can cause an individual to temporarily lose all interest in food
- Physical exercise
- Langhans W (October 2000). "Anorexia of infection: current prospects". Nutrition 16 (10): 996–1005. PMID 11054606.
- Exton, M. S. (1997). "Infection-Induced Anorexia: Active Host Defence Strategy". Appetite 29 (3): 369–383. doi:10.1006/appe.1997.0116. PMID 9468766.
- Murray, M. J.; Murray, A. B. (1979). "Anorexia of infection as a mechanism of host defense". The American journal of clinical nutrition 32 (3): 593–596. PMID 283688.
- Home Care After Tonsillectomy and Adenoidectomy