The Scot, 40, hopes to publicise the plight of UK resident Shaker Aamer, held without charge at the US detention camp since 2002.
Around two-thirds of the way through his week, he tweeted: "Hello Day 5. Stomach flattening but moobs putting up spirited resistance."
Boyle began fasting on Thursday, joining Aamer's lawyer Clive Stafford Smith, who had already fasted for seven days.
Stafford Smith had previously tweeted that the comic was prepared to "take over from me when I fall" in the protest, which aims to raise awareness of Guantanamo prisoners on hunger strike, some of whom have reportedly been force-fed.
Stafford Smith says British actress Julie Christie has agreed to join the hunger strike for a week.
Aamer is the last British resident held in Guantanamo. It is estimated 96 of the 166 prisoners at Guantanamo are still classified as hunger strikers by the US, down from a high of 106.