Air carrier Captain reported a loss of aircraft control during climb when encountering an area of severe turbulence. Flight crew; ATC and Dispatch all discussed options to avoid the forecast area of turbulence in Denver area prior to departure.

Narrative

Taxiing out from DEN ; when we checked in with GND (Ground); they told us to check the new ATIS and determine if we would like to continue. We did so and saw that there was severe turbulence reported near our route but well below the altitude that we were planning to be at. We had just flown in from that direction and had only experienced light to moderate turbulence. We asked ground if they could suggest a better route and they offered a different departure that they thought might be better but when we contacted dispatch; his information was that the pireps for severe turbulence were not on our route and our route only had light to moderate turbulence reports. Since we had not had problems on the arrival only about an hour prior; we accepted what dispatch said. Also; based on what the ATIS said; it seemed there could be severe turbulence in the direction of the other route that ground had suggested. We went ahead; having already briefed the crew and the passengers to expect turbulence and remain seated. Reaching about FL180; we encountered moderate turbulence that intensified and developed into severe around FL240 to FL250. The airplane struggled to maintain attitude; altitude; and airspeed; losing 20 to 30 knots at a time and losing climb capability. Once we climbed out of it; I contacted the flight attendants. They said there were items strewn about the cabin but everyone was okay. Cause: Extreme winds at DEN. Suggestions: We were aware of severe turbulence reports. It seemed like they would not affect our route; but ground had suggested an alternate route that could've been slightly better. Even though dispatch thought we would be fine; we could have taken the alternate route. Maybe ground knew something dispatch didn't. We also could've considered cancelling the flight or delaying.