Answer:
Some water filters do de-mineralize the water and others do not. The most effective form of home water filtration, a reverse osmosis system, will filter out minerals as well as virtually all contaminants. To add the minerals back in, you can add a post-calcite filter which is easy to add on to any reverse osmosis system.
Other, non-reverse osmosis type water filters, such as pitcher filters, standard countertop or undercounter, do not filter out the minerals.