By Adjust, you mean using the Adjust button on the Photo itself?
If you just open Adjust, it will not change anything.
If you make changes, then you are changing the base data and the picture can and will change its sharpness and sometimes the focus can go off as well. If you are using a JPG that is not the highest quality the Camera can offer, then it is already compressed and has lost some data in that compression. For true editing, you need those pictures in the highest JPG format the camera offers, or, turn on RAW, if supported, and edit the RAW images. RAW does not compress.
iPhoto is not a professional editor, it is a basic editor and does some nice stuff, but you need a more sophisticated app like Aperture or Lightroom
to be able to adjust the data so it is as true as possible to the original.
You can always revert the setting to its original data set and then make changes again. But radical changes, like adding Exposure or reducing Contrast will affect the sharpness. You have to play with the balance of those setting to achieve the picture you want without losing anything.
I do this as well and some of mine are not the sharpest anymore either, but I like the softer tone of Photorealistic pictures for some of my shots. For serious, gotta be perfect editing, I use Aperture from Apple on my RAW pictures so I have the most room to work with in data.
Hope this helps,