MOSCOW — Yulia Navalnaya, wife of jailed Russian opposition leader Alexei Navalny, went on "the best date of my life" Tuesday.

True, it was in a tiny room in a prison where she spoke to her husband on a telephone receiver while gazing at him through glass. He was still “the coolest guy ever,” she wrote in an Instagram post.

But she departed with fears about his frailty on his 13th day of a hunger strike to demand medical care: He was so weak that he had difficulty speaking and had to put the receiver down, stop talking and lie down several times during the meeting, she said.

And yet he was “just as cheerful and fun.” She posted a photograph of herself outside the prison in Pokrov, 64 miles east of Moscow.

She said his weight was down to 76 kilograms (168 pounds), compared with 93 kilograms (205 pounds) when he was jailed in February. Navalny, 44, is 6-foot-2.

She said he was skin and bones, his skin stretched tight on his skull, “but I know that he is not going to give up.” She said that prison authorities were denying him his basic right to see a doctor. Navalny has been suffering severe back pain and loss of feeling in his legs, according to social media posts from his team.

“But after the visit with Alexei, I worry about him even more,” Navalnaya said.

Earlier, a post on Navalny’s Instagram account said he was suing Russian prison authorities for their refusal to provide him with a copy of the Koran, which he wishes to study to better understand Islam. The posts are made by members of his team, as he does not have access to social media.

Navalny had mentioned his Christian belief during court hearings in February. He said Tuesday that he had read the Koran before but that as a Christian, he wished to “deeply study and understand the Koran.”

“But in my ‘friendly concentration camp’ they hate books! I came here a month ago and brought a bunch of books. And ordered a bunch of books. But so far I have not been given a single one. Because all of them ‘must be checked for extremism,’ ” the post said. “Now, on the 13th day of my hunger strike, I am in a very philosophical mood. Books are everything, and if I have to sue for the right to read, I will sue.”

Earlier in his political career, Navalny issued videos and statements that were offensive to immigrants and Muslims. In recent years his focus has been on the corruption of Russian officials and oligarchs who benefit from President Vladimir Putin’s rule.

Navalny was poisoned in August with a chemical nerve agent, an attack that he blames on Putin. The United States and European Union have issued sanctions over Russia’s use of a chemical nerve agent that they say was used to poison Navalny.

Navalny was arrested in January on his return from Germany, where he was treated for the poisoning. Russian authorities prosecuted him for his failure to report to police while in Germany, which a court ruled breached probation conditions related to a previous trial. In February, Navalny was ordered to serve 2½ years in prison. He has another charge hanging over him for fraud, meaning he could serve many years in prison. He says the cases are political, designed to silence him.

With Navalny jailed, police have been harassing members of Navalny’s regional teams, several of whom have been detained or arrested in recent days.

Navalnaya said her husband sent greetings to everyone but no longer had the strength to add his usual optimistic comment that “everything will be fine.”

“So I’ll add it,” she said. “He is simply the best. Everything will definitely be fine.”