This study demonstrates that TDCA supplementation to the diet of mice results in protection from injury-induced intestinal apoptosis, diminished crypt cell proliferation, and decreased villus height. This protection of the intestinal mucosa correlates with a marked increase in animal survival when lethal doses of LPS are given. This is the first study to date that shows bile acid supplementation to be beneficial in critical illness in vivo. Importantly, we see that the survival benefit due to TDCA supplementation is associated with intestinal protection and not protection from systemic insult.
Our data support the growing evidence that suggests that intestinal injury and impaired intestinal barrier is the “motor” of the septic state (21
). This theory stems from that fact that during critical illness the small intestinal absorptive and barrier functions are damaged. This leads to bacterial translocation which is the first step towards proinflammation in the “gut-liver-lung axis” (23
). Research suggests that in order to maintain the gut barrier function, the focus should be on maintaining the integrity of the gastrointestinal mucosa (24
Previous studies on cirrhotic rats have shown enteral bile acid administration to be associated with decreased bacterial overgrowth and decreased bacterial translocation (19
). Our study focused on previously healthy mice that underwent a septic insult. We were able to show that enteral bile acid administration resulted in maintained integrity of the intestinal mucosa which translated into increased survival. More studies are needed to determine whether this is associated with decreased bacterial translocation or some other mechanism.
Although this study shows a correlation between TDCA supplementation and increased protection after intestinal injury, there are a few limitations. First, the mice were given the TDCA supplementation in a liquid diet. Although we estimated that each animal received 50mg/kg/day, this was not measured precisely; mice may have received more or less of the daily dose depending on how much food they consumed. Another possible limitation is that the mice were treated with the TDCA supplementation before the time of injury. In clinical practice, however, patients usually present at the time of injury. However, supplemental bile salts may be beneficial to prevent or minimize intestinal injury in high risk disease processes or in conditions were bile salt concentrations are low. Separate studies were done on non-injured mice (data not shown) given diets with or without TDCA supplementation. In these studies, there was a slight increase in proliferative cells but no difference seen in apoptotic cells or villus height. This shows that the effect we see in the current study is secondary to the protective effect of bile salts during intestinal injury and not simply the effect of bile salts on normal intestine. More studies are needed to further our knowledge regarding timing of the bile salt-induced protection.
Despite these limitations, this study shows that TDCA supplementation is protective from intestinal injury and leads to increased survival when used in the treatment of systemic shock. Importantly, we demonstrate that TDCA protection is limited to the intestine and not other organ systems. This study gives further credence to the theory that protection of the intestine is essential in critical illness.
Enteral supplemention of bile salts resulted in clearly beneficial effects where there was normal bile salt homeostasis. There are many conditions where bile salt production, secretion, and absorption are abnormal such as cholestasis, liver failure, short gut syndrome and malabsorbtion from a variety of causes. Bile salts may be critical for intestinal mucosal integrity in these conditions.
Results from our study could also be expanded to other diseases of intestinal injury, like necrotizing enterocolitis or inflammatory bowel disease. In premature infants, duodenal bile salt concentrations are significantly decreased, and the concentrations increase as postnatal age increases (25
). Further, in premature low birth weight infants there is low hepatic bile acid clearance (26
). There is also a physiologic cholestasis in the newborn (27
). Therefore, the intestinal mucosa of the premature infant, at high risk for NEC, is exposed to less bile salt than the full term neonate. Further studies are necessary to determine if simple measures such as enteral supplementation of bile salts in premature infants could diminish the incidence of NEC, or prove beneficial for patients with intestinal injury as part of their spectrum of disease.