Mohamed Soltan, 26, has lost a third of his weight and is unable to stand unassisted after 100 days on hunger strike

A US citizen who has been on hunger strike in an Egyptian jail for 100 days has lost at least a third of his bodyweight and is unable to stand unassisted, according to an independent medical report facilitated by the US embassy and seen by the Guardian.

Mohamed Soltan, a 26-year-old graduate of Ohio State University who says he campaigned for Barack Obama in 2008, grew up in the US but moved to Egypt in 2013. He was shot in the arm during the forced clearance of a protest camp last August, arrested 10 days later and has been held in custody ever since on suspicion of helping to plan a series of attacks on the state. He is the son of a leader of the banned Muslim Brotherhood, but is not a member of the group himself and has previously said he was ideologically independent of his father.

Soltan now uses a wheelchair and his weight has fallen from 120kg to 78kg, according to a 19 April report by a private doctor commissioned by Soltan's mother and who was given credentials to access prison by US embassy officials. This week, Soltan's friends said his health had worsened further, that he had been transferred to a civilian hospital and that they feared he was dying.

A dual-national, Soltan is one of at least 16,000 Egyptians arrested – according to the interior ministry – for political reasons since the overthrow of the president Mohamed Morsi last year.

Abdel Fatah al-Sisi, the army leader who ousted Morsi and who is likely to be elected Egypt's next president, admitted this week that some violations may have been committed during the crackdown on dissent. But said on Thursday that his presidency would not herald the immediate rise of western-style democracy, and told Egyptian journalists to temper their criticisms of the country's political course in order to avoid worrying the wider population.

"We always turn to the image of stable democracies in states that preceded us and compare them with Egypt," Sisi said in a Facebook post that summarised his meeting with leading Egyptian editors. "Applying the models of western democracies in the case of Egypt does an injustice to Egyptians. Egyptian society still faces time before it enjoys true democracy as it should be."

In letters smuggled from prison, Soltan has criticised both Sisi and Washington's failure to censure him strongly. In a letter from last November, he wrote: "I often get asked sarcastically by judges, officers, and even inmates 'Where is this first-world country [the US] that takes such pride in defending human rights and freedoms? Where are they now to help you?' Of course, I am left speechless every time."

In a letter he wrote this week, Soltan wondered whether his treatment would have been better had he been born with "blond hair and blue eyes".

An Egyptian government spokesman did not respond to a request for comment on Soltan's treatment.

• This article was amended on 9 May 2014. It originally stated that Soltan is the only US citizen in detention in Egypt. This is not the case and the statement has been removed. The US embassy's position in regard to the private doctor who examined Soltan in prison has also been clarified.