The news that Boris Johnson was admitted to hospital for tests on Sunday night with persistent coronavirus symptoms has shone a spotlight on the prime minister’s health.

While Johnson is not known to have any underlying health conditions, he has been open about his struggle with obesity – a known risk factor for coronavirus.

According to a report published last week, more than 70% of patients placed in intensive care after being infected with Covid-19 were overweight, obese or severely obese on the body mass index scale.

In December 2018, when Johnson was foreign secretary, he said in the Spectator that he weighed 16 and a half stone, which at 5ft 9in would have put him in the obese category. Johnson has lost weight since then, but it has fluctuated significantly throughout his political career.

Sonia Purnell, author of Just Boris: A Tale of Blond Ambition, said that the prime minister’s usual wellbeing was striking. “By and large, he is remarkably fit considering he is actually a bit overweight,” she said. “He’s not really that active, his runs aren’t very long or very energetic and he is very fond of cheese. In fact, he likes eating a lot – nothing wrong with that – but he does have a very robust constitution. In the time I’ve known him since the 90s, illness has never really come up.”

Johnson became a devoted runner in the early 2000s. In his book Boris: The Rise of Boris Johnson, Andrew Gimson notes the prime minister also took up cycling in London in 1999 and quickly became “the most famous cyclist in Britain”. Gimson added that cycling had several attractions for Boris: “It was energetic, dangerous, anarchic.” While he was mayor of London, Johnson was involved in several near-miss accidents while cycling, including a 2009 incident in east London.

In 2013, Johnson went on a major health kick before taking part in the first Prudential RideLondon-Surrey 100, a now annual closed-roads cycle ride out of the capital and into the countryside. At the start of his training regime, which he styled Operation Chiselled Whippet, Johnson, who was then 48, conceded that he was “pushing 17 stone, and since my normal cycling speed is so slow that my wife says it is a miracle I stay upright, I have decided to get in shape”.

When Theresa May made him foreign secretary in 2016, Johnson lamented the impact the job had on his lifestyle. While he was back on his bike after returning to the backbench, his cycling days were limited, as he would soon be entering No 10.

As well as jogging and cycling, Johnsonis a keen tennis player. At an auction for the Conservative party’s annual winter ball, a game of tennis with the prime minister was auctioned for £45,000.

Purnell traces Johnson’s appetite for sport to his childhood, when he was encouraged to run, jump, and play “whiff-whaff”, better known as table tennis. At Eton, he played rugby and cricket, and took part in the Wall Game, a sport unique to the private school.

More recently, Johnson is reported to have given up his running due to issues with his knees. The prime minister instead participates in online classes that include yoga and pilates.

Purnell says the hospital admission will be difficult for Johnson who, like his father, rarely admits to illness or notices it in others. “When I worked with him as his deputy [in the Daily Telegraph’s Brussels bureau], he was certainly very, very intolerant of illness in others, and considered it as sort of a weakness.”

Additional reporting by Peter Walker