Perm, Russia - "Kroshik" (translation: "crumb") the cat weighed a whopping 17 kilograms (about 37 lbs) before his passing at the age of 12.

The sad news was shared on Instagram by the Matroskin cat rehabilitation center in Perm, Russia.

Kroshik – aka "the world's fattest cat" – most likely became obese because he lived in the basement of a hospital for a long time and was fed unhealthy food such as cookies and soup by various staff and patients.

At some point, the overweight stray cat ended up at the animal shelter where staff recognized the problem and found the animal a place in a rehabilitation clinic.

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There, the animal keepers and vets put together a strict diet plan and prescribed lots of exercise and massage sessions.

Initially, they found success! Kroshik lost three kilos (about 7 lbs) after just a few weeks and weighed in at just under 14 kilos (about 31 lbs).

So how did the kitty's health take such a tragic turn?

Over the weekend, the cat's health suddenly deteriorated.

Kroshik was admitted to the animal hospital with breathing difficulties and was even placed in an oxygen chamber.

All of his internal organs (such as his liver, gall bladder, kidneys, and bladder) were affected, the rehabilitation center wrote in a post.

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Only a short time later, the fat cat – who gained huge notoriety on Instagram due to his weight – died.

"According to doctors, Kroshik had several growths in his spleen and metastases in other internal organs," the rehabilitation clinic said.

These growths were only discovered shortly before his death and a precise ultrasound examination was not possible beforehand due to his obesity.

Fans who had followed Kroshik's journey are now mourning the loss of their internet idol.