Egypt's most high-profile dissidenter was previously consuming one piece of fiber per week, an apple or a cucumber, as well as just 100 calories of liquid per day, to stay alive, his sister Sanaa Seif told Reuters after visiting him on Tuesday.
"During the visit he was leaning on the glass partition, he was struggling but is trying to keep it together," she said.
In June, his mother Laila Soueif expressed concerns her son's health could deteriorate rapidly.
Abd el-Fattah began his hunger strike on April 2 to protest his detention and prison conditions. He was jailed in December for five years on charges of spreading fake news, for sharing a social media post about the death of a prisoner. He acknowledges retweeting the post but maintains his sentence is unjust.
Egyptian officials have not responded to Reuters' phone calls for comment on Abd el-Fattah's case, but have said he was receiving meals and was moved to a prison with better conditions earlier this year.
The Egyptian government has defended judicial decisions against foreign criticism, including over Abd el-Fattah's conviction.
Neither Abd el-Fattah or his family expect Egypt, a close ally of the United States, Britain and other Western states, to release him anytime soon, his sister said.
Abd el-Fattah has been behind bars for much of the decade since the 2011 revolt that toppled Mubarak and which initially gave hope to a generation of activists in Egypt and beyond.
"We know for a fact that Alaa has become a well-known case to other governments so it's extremely frustrating that we did our part but we aren't achieving results, and that means that these governments are not doing their part," said Seif.
She said she pressed Abd el-Fattah's plight with numerous people, including members of the British parliament and the UK's Foreign Office, only to be told Britain was "pushing to the highest level for consular access," she said.
A Foreign, Commonwealth and Development Office spokesperson said: "We are working hard to secure Mr Abdel Fattah's release and are urging the Egyptian authorities to ensure his welfare needs are met."
A UK government official said Prime Minister Boris Johnson had raised the case directly with Egyptian President Abdel Fattah al-Sisi on a recent call.
Some critics also say the United States, a major source of arms and military aid, should suspend that aid due to Egypt's rights record. The U.S. State Department declined comment.
Sisi denies there are political prisoners in Egypt. He says stability and security are paramount and authorities are promoting rights by trying to provide basic needs such as jobs and housing.