Such access is worrying, the EU privacy watchdog said.
"The possible further combination and accumulation of sensitive personal data regarding people in Europe by a major tech company could entail a high level of risk to privacy and data protection," it said.
It urged the companies to assess their data privacy requirements and privacy implications in a transparent way and mitigate possible privacy and data protection risks before seeking EU antitrust approval for the deal.
Google said it would never sell personal information to anyone and that Fitbit health and wellness data would not be used for its ads while Fitbit users would have the option to review or delete their data.
"Protecting peoples' information is core to what we do, and we will continue to work constructively with regulators to answer their questions," the company said in a statement.
European Competition Commissioner Margrethe Vestager, who will vet the deal, in November voiced her concerns about big companies targeting data-heavy rivals.
The European Commission on Thursday in an emailed comment said it had yet to be formally notified.
"It is always up to the companies to notify transactions with an EU dimension to the European Commission," it said.
