I can understand everything from this article except one single word that causes me trouble in getting the complete meaning of the following sentence:
The dietary swaps appeared to cause significant changes to the cells lining the gut as well as the bacteria that live in the bowel - with the US volunteers faring better.
I understand that the new diet caused important changes to the cells, modifying their structure, but what exactly happened to the gut by lining it and also to the bacteria (were they lined too or what?) I quite can't get.
Checking The Free Dictionary for the verb to line, I couldn't find an appropriate sense for the context in question.
So, please let me know which one of the following dictionary definitions matches:
v. lined, lin·ing, lines
- To mark, incise, or cover with a line or lines.
- To represent with lines.
- To place in a series or row.
- To form a bordering line along: Small stalls lined the alley.