OK, so I'm not an expert, but I definitely want to be in a position to answer this Q too. However I believe that there are many different causes, and its going to take a lot more research to define them all. I focus only on "IBS" type causes- I think my problem fits in here. Broadly I think we can classify them into the following groups:
faecal incontience. by this I mean incontinence of solid of semisolid fecal matter. Causes could range from anthing that causes D, anatomical problems whereby there is poor sphincteric seal (traumatic chilbirth, surgical damage, nerve damage) or muscular dysfunction. Management would therefore be to eliminate the D, correct anatomical problems, perhaps surgically ("solesta injectons" reported to cure leaky gas by one user on another forum
www.ibsgroup.org/forums/topic/147145-leaky-gasfecal-body-odor-cured/ although this product is indicated for fecal incontince, and I also doubt it is permanent, since when used as a dermal filler, e.g. in the lips, it lasts only 6 months. I aslo suspect it may turn a "leaky" problem into a painful bloating problem). Biofeedback is supposed to be the standard medical management, I have not looked into this treatmetn in detail yet.
Leaky gas by this i mean incontinence of flatus, usually as a near constant leak of gas bubbles, and without the patient being aware. Docs would say this isn't a real condition, because flatus is normal, and it is also normal to pass flatus without being aware. Again some people seem to report leaky gas in relation to a "gaping" problem. Again this could be due to muscular dysfunction or nerve damage.
But if there is an increased production of gas, say because of bacterial overgrowth (all gas is produced by bacterial fermentation) and also undigested food in the GI tract, caused by food malabsorption and food intolerances, this would cause this gas to smell particularly foul. Remember that 90% of the weight of formed stool is bacterial.
If there is constipation, this stool will be stuck in the colon giving off gas, also explaining why FBO is worse before BM's. I belive this is why the low -FODMAP diet helps...because it reduces the amount of gas and shifts the bacterial flora to a less offensive one. The diet is composed of food that is easily absorbed by the body, leaving little for bacteria to break down, and hence their numbers reduce.
Then there are the problems related to C, like fecal impaction (leading to paradoxical diarrhoea, i.e. C punctuated by occasional bouts of D, where more liquid stool escapes past the blockage). Megacolon would lead to an increased storage capacity perhaps leading to decreased urge to defocate, meaning that stool lingers for longer, causing leaky gas. Another problem with C is passage of mucus (produced naturally to coat stools and lubricate their passage out - however if their is C, then the mucus continues to be produced). The mucus would feasible contain the same odor as the fecal matter itself.
The symptom of tenesmus (sensation of incomplete evacuation) is also I think important. If stools are left behind, again they may give off gas and causes FBO through leaky gas or through the "leaky gut" concept. The most likely probable cause for tenesmus is C. Other causes could be internal haemorroids, or intususseption, solitary rectal ulcer syndrome (I have not looked into in great detail yet)
Then there is the concept of "leaky gut" (different to leaky gas) where the GI tract lining is supposed to be more permeable (as a result of previous food poisioning) and allows toxins that should remain in the stool to be reabsorbed by the body and circulate, being given off in the sweat. It is easy to see how chronic constipation leaky gut could contribute together to cause FBO
Then, there are enzyme problems. Enzymes break down volatile organic compounds to their odourless state. (the "fish odour syndrome" is misnomer because the smell can also present as FBO). I also think there are enzyme conditions similar to TMAU that have not been discovered yet, which would cause FBO. These volatile organic compounds I understand are largely the result of bacterial fermentation, so if thenumber of bacteria increases, as in SIBO, or the diet contains an abundance of fermentable food for bacteria, then production of these compounds will exceed the ability of the level of enzymes to cope. Alternatively, enzyme deficiencies may be inherited as a genetic condition
Treatment of SIBO is well described in Dr Pimmentel's new book, however use of antibiotics may just lead to resistant strains and it may be more sensible to live long term on a strict low FODMAP diet.
Infection, especially anaerobic infection will cause odour Infected haemorrhoids, fissures, anal abscess, etc, etc. Some people have reported ENT infections to be the cause of their FBO.
Haemorroids, especially prolapsing haemorrhoids may still cause FBO even without being infected if they affect the seal causing leaky gas and/or fecal incontinence. Similarly, even without being infected, any abnormality in the anatomy that makes it harder to clean after a bowel movement could also cause FBO (i.e. haemorrhoids, skin tags, fistulas, fissures)
Others believe that a systemic/perianal/ENT candidiasis is the cause of their odor. Fungus smells mousy/ musty rather than fecal, and i think this is an important diagnostic clue early on. Management of candidiasis would be with antifungals, anticandida diet.
General treatments to reduce odor are chlorophyl, activated charcoal, bismuth compounds. Although TMAU management protocols tend to discourage exercise, this doesn't make much sense to me. Sweating is just another way for the body to excrete toxins, and can only help to reduce odor. Obviously a shower afterwards
Daily, vigorous exercise would also combat depression. IF their is anxiety (usually yes), then hypnotherapy is a very good tool. Specialized IBS hypnotherapy tapes available. Keeping well hydrated can only help too, if their is D to replace the lost fluid, and to try and reduce C.
Finally an important point to consider is the "referred olfactory syndrome". Some people are wrongly diagnosed with this when they have a real problem. I also think that the brain can create symptoms, perhaps a bit of a controversial thing to say. My thoughts on this topic on another forum:
www.ibsgroup.org/forums/topic/147159-fbo-cause-is-the-mind/I would add that I believe that most cases of FBO are related to leaky gas or other sphincter problems, and all other causes are much rarer. As you can tell my thoughts are all a bit jumbled atm...if you want a much better list try this site:
www.bodyodor777.com/causes_internal.html