It's best not to add the lemon juice to your water. Adding any taste to your mouth, even toothpaste, can set in motion a partial startup of the digestive system which will take energy from other parts of the body which need it more. And the Sugar in the lemon even at trace amounts could slow things down also. The body doesn't wait for food to be sitting in the stomach to begin digestion. It starts in the mouth so it's important not to be sending it the wrong signals.