Any existing sheets will appear to have their data removed (even if they aren't then written with the data pulled from this add-on), which isn't user-friendly. One thing you can do to resolve this is to restore from a previous version of your sheet, then click Add-Ons -> Fit sync -> Refresh Data. This will allow you to pull your fit data into your sheet without deleting existing sheet data :D
Also it could definitely use some improvements. I'd love to see water as well as nutrition added. The great thing is this project appears to be on GitHub so maybe some open source improvements can happen!