In his first public comment since his arrest, Mr Kuznetsov writes - by hand - that "the Italians have been cynically ignoring my dietary habits for the past two months" - a reference to his vegetarian diet.
"They think that these restrictions can affect my position, and make me confess my guilt. But such efforts are futile."
A relative of Mr Kuznetsov's told me they had tried to deliver special food to the prison, but had been refused - and they were worried that he seemed "exhausted".
The Nord Stream pipelines were destroyed deep beneath the Baltic Sea at the beginning of Russia's full-scale invasion of Ukraine.
It was a high profile and controversial attack.
Many initially suspected Russian involvement and an attempt by Moscow to blackmail Europe by restricting its energy supplies.
But this summer, German prosecutors activated arrest warrants for two of the men they suspect of carrying out the attack: Serhiy Kuznetsov and a second Ukrainian citizen, Volodymyr Zhuravlyov, who was detained in Poland.
German prosecutors have cited charges of "anti-constitutional sabotage" as a basis for the extradition requests.
A judge in Warsaw ruled against Mr Zhuravlyov's extradition, saying that, if Ukrainians were involved, such an attack would count as legitimate self-defence in a "bloody, genocidal war".
In Italy, by contrast, a court in Bologna last month approved Serhiy Kuznetsov's extradition in a closed hearing. He is currently appealing against that ruling for a second time.
"He told me he had lost 9kg when I said that his clothes were hanging loosely," the Ukrainian's lawyer, Nicola Canestrini, said, after visiting his client on hunger strike.
The lawyer said Kuznetsov was refusing all solid food but was "doing fine and is clear of mind".