@haidut I suspect that 5-HT is high in babies because constipation/impaction of the gut would be life threatening for a baby. 5-HT keeps things moving along in the intestine; it increases/intensifies peristalsis.
Serotonin Deficiency Is Associated With Delayed Gastric Emptying
I suspect that the issue of serotonin (5-HT) getting through the intestinal wall and into the blood stream is the primary concern; so long as the 5-HT remains inside the intestine (along with the endotoxin) things should be fine. If the integrity of the intestinal wall is compromised and endotoxin and 5-HT are able to seep through into the blood stream major problems happen (sepsis, high serotonin symptoms).
The health of the epithelial cells that line the intestine are important because if they fail, leaky gut happens. Thiamine is believed to be important for the epithelial cell function. See here: Dietary supplementation of thiamine enhances colonic integrity and modulates mucosal inflammation injury in goats challenged by lipopolysaccharide and low pH "The results show that dietary thiamine supplementation could improve the colon epithelial barrier function and alleviate mucosal inflammation injury in goats after lipopolysaccharide and low pH challenge."
Another important body part that requires excellent epithelial cell function is the blood/brain barrier. Thiamine deficiency compromises the blood/brain barrier too.
Thanks for all you do! I always appreciate your posts.