Authorities at the military base say most of the prisoners arrived slightly underweight but have since gained an average of nine kilograms.
Most are now "normal to mildly overweight or mildly obese", said US navy Commander Robert Durand.
The military base spokesmen mentioned one detainee's weight has almost doubled to 186kg.
Human rights groups attribute the weight gain to lack of mobility in the detainees' small cells. They also cited accounts of released detainees who said they were at times allowed to exercise fewer than three times a week.
Hefty meals
The detainees' meals total a whopping 4,200 calories per day. US government dietary guidelines for weight maintenance recommend 2,000 to 3,000 calories per day.
Commander Durand said detainees are served a wide variety of food and expected to choose what appeals to them.
About 460 detainees are currently at Guantanamo, with some, including Australian man David Hicks, held for more than four years on suspicion of links to al-Qaeda and the Taliban.
The meals include meats prepared according to Islamic guidelines, fresh bread and yogurt.
With nearly all detainees fasting in the daytime during Ramadan, authorities have arranged for two separate meals -- a post-sunset meal and a midnight meal -- to be delivered after dark. Traditional desserts and honey are served during the Ramadan observances.
Guantanamo's heaviest detainee, at 186kg, arrived in 2002 weighing 98kg, Commander Durand says. His identity was not disclosed.
Limited exercise
The Guantanamo Bay spokesman says all prisoners, including those held at maximum-security Camp 5, are allowed at least two hours of daily recreation -- the minimum called for by the International Committee of the Red Cross.
However, reporters who visited Camp 5 last month were told that the exercise time had been reduced to one hour after the suicides of three detainees in June.
During the August visit, a "high-value" detainee could be seen walking in circles around a three by 5.5 metre fenced-in "recreation yard".
The exercise time has since been increased to 90 minutes, the commander of the camp guards said, and there were plans to restore the two-hour exercise periods.
The most compliant detainees receive up to 12 hours of exercise time and have access to treadmills, stationary bikes and other exercise equipment, Commander Durand said.
The conflicting accounts of prisoners' exercise time highlight a need for neutral monitors to examine conditions at the prison and report their findings, said Curt Goering, of Amnesty International
USA.
"The army says one thing, camp commanders say one thing, and then there's this other information available," he said.
"It's really important that someone independent makes that assessment."
A delegation from the International Committee of the Red Cross arrived at the prison last week for interviews with detainees.
The humanitarian organisation, which does not publicly release some findings as a condition of its access to prisoners, was hoping to get a first glimpse of 14 top alleged al-Qaeda figures and other terrorist suspects previously locked up in CIA secret prisons.