Archaea and the human gut: New beginning of an old story

Table 2 Possible association between several human diseases and methanogens

Pathology	Pros for a role or an association	Cons
Colorectal cancer	80% of colorectal cancer patients are methane producers[127]	No significant relation with the amount of breath methane[48,71,129-131]
	Methane production increases with the severity of colorectal cancer[128]	Native Africans with high level of methanogenic archaea are less susceptible to sporadic colorectal cancer[48]
Obesity	Higher levels of methanogens in obese[132]	Decreased proportion of M. smithii in obese[72,73,133]
Anorexia	Higher levels of M. smithii in anorexic people (adaption to diet restriction?)[134]
IBD	Lower proportion of methane-producers/methanogen carriers in patients with IBD[104,135]
IBS	Correlation with higher production of methane (also compared to IBD patients)[136]	No significant difference in methane production with controls[137]
Diverticulosis	Subjects with diverticulosis often with high level of methanogens[45]
Constipation	Association with chronic constipation/transit time[138,139]	No significant difference between constipated children and controls[144]
	Level of methane significantly associated with the severity of constipation[136,140]
	Association with IBS constipation/increase of transit time[141-143]
Periodontitis	Possible pathogenic role and increased proportions of hydrogenotrophs and methanogens in severe cases[24,145,146]
	After therapy, decrease in the prevalence of archaea[147]

IBD: Inflammatory bowel disease; IBS: Irritable bowel disease.