Eighteen prisoners have been hospitalised, including 13 who are being tube fed, said spokesman for the detention centre Captain John Adams on Tuesday.
The other five are receiving intravenous fluids.
Thirty-nine prisoners have joined the strike since Friday, Adams said.
He said the number of hospitalised detainees had reached 22, but four were returned to their cells after their conditions improved.
All are being monitored by doctors at the camp in eastern Cuba, he added.
"Everyone is stable at this time," Adams said. "If their condition appears to weaken, they will be brought to the hospital and either fed intravenously or nose fed."
The prison at Guantanamo holds about 500 prisoners from 40 countries. More than 230 others have been released or transferred to the custody of their home governments.
The detainees are accused of ties to the al-Qaida network or Afghanistan's ousted Taliban government. Most have been held for more than three years without charge.
The military said the latest strike began on 8 August, saying 76 detainees were refusing meals. On Friday, Guantanamo officials said the number had risen to 89.
Some detainees have threatened
to starve to death
The military's accounts conflict with a 31 August report from the New York-based Centre for Constitutional Rights, which said 210 prisoners were refusing meals.
The centre, which has filed legal challenges on behalf of detainees, said some have threatened to starve to death unless they are put on trial or released.
Lawyers from the centre said detainees were angry because the military allegedly reneged on promises to bring the prison into compliance with the Geneva Conventions if detainees ended a hunger strike involving nearly 200 prisoners that started in late June.
The military has said that only 52 detainees participated in the June strike, which was first disclosed by two detainees who had been released and sent home to Afghanistan.
Sergeant Justin Behrens, another Guantanamo spokesman, denied that the military had reneged on any promises.
He said each cell block had chosen a prisoner to talk with military authorities about conditions at the camp.
The military has also denied the New York centre's claim that at least three detainees were abused by the military's Extreme Reaction Force.
Lawyers from the centre said an interrogator threw a mini-refrigerator and a chair at a detainee on 5 August. Military police then allegedly beat him up.