Lawyers for the prisoners said they were striking over harsh conditions at a new maximum-security prison.
Rights groups called on the US to halt the "brutal" tactic, which includes strapping detainees to a chair.
This latest hunger strike began in 2005, reaching a peak of 131 detainees.
Of the 13 current strikers, two had been refusing food since August 2005, while most of the others had joined the protest in January or February, Cdr Durand said.
He dismissed the protest as a stunt to attract the attention of the many journalists who were at the US naval base in Cuba to cover the March trial of Australian detainee David Hicks.
Cdr Durand said hunger strikes were a media tactic taught in the al-Qaeda training manual.
"As soon as the media left, the number of hunger strikers has been steadily dropping," he said.
Lawyers for some of the protesters said the strike was prompted by the conditions at the new maximum-security Camp 6 complex.
The 160 detainees at Camp 6 are confined in solid-wall cells most of the day, with recreation limited to a maximum of two hours a day. Contact with other detainees is severely restricted and they have little exposure to natural light.
Cdr Durant said the protesters were "in good health", defending the force-feeding as a common medical procedure used in many US civilian hospitals.
US-based campaign group Physicians for Human Rights has called on the US to halt the "brutal and inhumane force-feeding tactics".