"My understanding is that he had been struggling (with cardiovascular problems) since last August but it worsened after repeated visits to Mount Paektu," Daily NK reported.
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Daily NK is a website based in South Korea that gathers information from informants inside the secretive and isolated country north of its border.
Bloomberg said the health of the North Korean leader is one of the state's most closely guarded secrets, typically only known by a handful of people in the inner circle of leadership.
New Zealand-born journalist Anna Fifield, author of a book on Jong Un, said on Twitter his biggest risk factor was his obvious poor health, in particular the risk of cardiac problems.
"Kim Jong Un is five feet, seven inches tall, and weighs about three hundred pounds = BMI of 45, or 'extremely obese'," she wrote.
Three hundred pounds is the equivalent of about 136 kilograms.
"There have been a number of recent rumours about Kim's health (smoking, heart, and brain). If Kim is hospitalised, it would explain why he wasn't present on the important April 15th celebrations," former CIA deputy division chief for North Korea Bruce Klingner told CNN.
"But, over the years, there have been a number of false health rumours about Kim Jong Un or his father. We'll have to wait and see."
Kim Jong Un is one of the world's most mysterious men, with very little known about his private life and childhood — apart from a string of bizarre myths, such as his weather-controlling powers.
However, in her book, The Great Successor, Fifield managed to dig up some fascinating details of the leader's personal life.
In an interview with Vox last year, she described him as a pampered child raised to believe his own greatness.
"He was a spoiled brat. He was brought up to believe that he was a demigod from the age of 3or 4, probably as long as he can remember. He was given a specially modified car so he could drive it himself at the age of 7," she told the publication.
"Then there was the time the family's personal sushi chef, who had basically been hired to be Kim's friend, went fishing for sea bass with Kim and his brother."
