Yeast intolerance is a poorly defined and controversial entity that indicates abnormal response to "yeast". Usually, it is used to describe a non-allergic yeast sensitivity that does not involve the immune system (hence not an allergy). However, the intolerance is sometimes also used synonymously with yeast "allergy". Both are very different conditions and true yeast allergy is an extremely rare condition. All this confusion exists because the intolerance is not a currently accepted medical diagnosis and hence not defined in medical literatures.
It is "believed" to afflict a significant number of people around the world. However, it is still a matter of research whether the myriad of non-specific symptoms described by so many people is really intolerance. Moreover, the symptoms attributed to the intolerance are very vague and not necessarily denote intolerance. Conditions like dysthymia (mild depression) and chronic fatigue syndrome might also result in very similar symptoms.
Moreover so called "resolution" of symptoms after cessation of yeast products (bread products, beer, etc.) and there "return" with eating or drinking yeast products does not necessarily suggests intolerance. It is more likely to be due to an individual bias by already presuming that he has intolerance (akin to placebo effect).
True allergic reaction to yeast proteins can occur like allergy to any other food (e.g. peanuts). However, this is extremely rare and will present as a clear cut allergic reaction (shortness of breath, wheezing, hives, itching, cutaneous flushing, swelling on the face, abdominal pain, nausea and vomiting, diarrhea, anaphylactic shock etc.). This allergic reaction is entirely different from the commonly described "intolerance" or "syndrome".
Overgrowth of the fungus Candida (yeast) has been suggested as the cause of intolerance. Increased dietary intake of yeast has also been implicated in increasing the "total body load of yeast" and thus giving rise to intolerance. However, currently there is not much acceptable research in this regard. The intolerance is also referred by various other names like candiadiasis hypersensitivity syndrome, yeast syndrome and chronic candiadiasis.
The hallmark symptoms is "feeling bad" or "feeling slow" after consuming yeast containing products like bread and alcohol. However, the spectrum of symptoms attributed to the intolerance is very wide and it can present with entirely different symptoms in different individuals.
The symptoms of intolerance may be due to involvement of multiple organ system:
Anti-fungal medications should never be used for yeast intolerance. Many people have reported significant relief from avoiding yeast products in diet (like beer, wine, bread, cheese, etc.) but it is not known whether yeast free diet is truly effective or the relief is just a placebo effect. It would be best to seek physician's advice for any symptoms rather than self-diagnosing yourself with intolerance and starting a troublesome yeast free diet.