Cucumbers and lettuce can be similarly compared in regards to nutritional value: there’s not much they contain apart from water. They do, however, have some good vitamins that can be of benefit for humans and dogs.

In general, yes, you are able to feed your dog lettuce but you need to be aware of the lettuce type and the portion size you’re allowing your dog to eat. Some lettuce such as iceberg, romaine, and arugula are made up of 90% water.

Dark leafy greens such as kale and spinach should be avoided. They contain elements harmful to pups. 

Just like asparagus, it is better to cut the lettuce you’re giving your dog into smaller pieces. Lettuce can contain high amounts of fiber, making it quite hard for your dog to digest. It’s also crucial for you to thoroughly wash your lettuce before feeding it to your dog (or yourself).You can put you and your pet at risk for various diseases such as listeria and E. coli if your lettuce is contaminated and not washed properly.