Yes lettuce is one of the vegetables that is hard for our bodies to digest fully and correctly in a short time.
However your body can still digest the lettuce eventually if you have no other stomach problems.
When eating lettuce be sure to chew it up really good to make it easier for your stomach to digest and to avoid an upset stomach.
However if you're having digestive issues or stomach problems when eating lettuce then you may want to avoid the lettuce even though it's healthy for you.