Ageusia (pron.: /əˈɡjuːziə/ ə-GEW-zee-ə) is the loss of taste functions of the tongue, particularly the inability to detect sweetness, sourness, bitterness, saltiness, and umami (meaning "pleasant/savory taste"). It is sometimes confused with anosmia - a loss of the sense of smell. Because the tongue can only indicate texture and differentiate between sweet, sour, bitter, salty, and umami, most of what is perceived as the sense of taste is actually derived from smell. True ageusia is relatively rare compared to hypogeusia — a partial loss of taste — and dysgeusia — a distortion or alteration of taste.
To discover the extent of the ageusia, a scientist attempts to discern the minimum level of a chemical that a patient can detect by that the patient can differentiate. Various methods are used, including the "sip, spit, and rinse" test and direct application of chemicals to the tongue.
Neurological damage 
Tissue damage to the nerves that support the tongue can cause ageusia, especially damage to the lingual nerve and the glossopharyngeal nerve. The lingual nerve passes taste for the front two-thirds of the tongue and the glossopharyngeal nerve passes taste for the back third of the tongue. Neurological disorders such as Bell's palsy, Familial dysautonomia, and Multiple sclerosis cause similar problems to nerve damage, as do certain infectious conditions like primary amoeboid meningoencephalopathy. The lingual nerve (which is a branch of the trigeminal V3 nerve, but carries taste sensation back to the chorda tympani nerve to the geniculate ganglion of the facial nerve) can also be damaged during otologic surgery, causing a feeling of metal taste.
Problems with the endocrine system 
Deficiency of vitamin B3 (niacin) and zinc can cause problems with the endocrine system, which may cause taste loss or alteration. Disorders of the endocrine system, such as Cushing's syndrome, hypothyroidism and diabetes mellitus, can cause similar problems. Ageusia can also be caused by medicinal side-effects from antirheumatic drugs such as penicillamine, antiproliferative drugs such as cisplatin, ACE inhibitors, and other drugs including azelastine, clarithromycin and zopiclone.
Other causes 
Local damage and inflammation that interferes with the taste buds or local nervous system such as that stemming from radiation therapy, glossitis, tobacco use, and denture use also cause ageusia. Other known causes include loss of taste sensitivity from aging (causing a difficulty detecting salty or bitter taste), anxiety disorder, cancer, renal failure and liver failure.
In fiction 
The movie "Aile ou cuisse" (The Wing and The Thigh) with Louis de Funés, which was an early mention of the badness/evilness of fast food and canned food sold as fresh in fast food restaurants, the main character is depicted to acquire ageusia due to being force-fed tons of junk food. (The parts about fast food industry are still valid). IMDB: imdb.com/title/tt0074103.
- MedTerms Online Medical Dictionary. "Ageusia". Retrieved April 15, 2005.
- Family Practice Notebook. "Taste Sensation". Retrieved April 15, 2005.
- Massachusetts Eye and Ear Infirmary. "Taste Disorders". Retrieved May 26, 2010.