Henry VIII became the tyrannical monster remembered by history because of a personality change following a serious jousting accident, according to a new historical documentary.

After the accident – just before he became estranged from the second of his six wives, Anne Boleyn – the king, once sporty and generous, became cruel, vicious and paranoid, his subjects began talking about him in a new way, and the turnover of his wives speeded up.

The accident occurred at a tournament at Greenwich Palace on 24 January 1536 when 44-year-old Henry, in full armour, was thrown from his horse, itself armoured, which then fell on top of him. He was unconscious for two hours and was thought at first to have been fatally injured.

But, although he recovered, the incident, which ended his jousting career, aggravated serious leg problems which plagued him for the rest of his life, and may well have caused an undetected brain injury which profoundly affected his personality, according to the History Channel documentary Inside the Body of Henry VIII. The programme focuses on the king's medical problems which grew worse in his later years, especially his ulcerated legs and his obesity: measurements of his armour show that, between his 20s and his 50s, the 6ft 1in monarch's waist grew from 32in to 52in, his chest expanded from 39in to 53in, and, by the time of his death in 1547 at the age of 56, he is likely to have weighed 28 stone – at which time his doctors recorded that he had badly ulcerated legs, was unable to walk, his eyesight was fading, and he was plagued by paranoia and melancholy.

However, Henry had excellent health as a young man, being universally admired for his manly physique. An ambassador at the Tudor court reported: "His Majesty is the most handsomest potentate I have ever set eyes on. Above the usual height with an extremely fine calf to his leg and a round face so very beautiful it would become a pretty woman."

He may have had a bout of smallpox at the age of 23, but the experts speculate that his real medical problems began at the age of 30 when he appears to have contracted malaria, which is thought to have returned throughout his life. They were intensified by two factors: open sores on his legs and sporting injuries.

The sores – varicose ulcers, which began on his left leg when he was 36, and later affected his right – may have been caused by the restrictive garters he wore to show off his calves. They never healed, and increasingly restricted his mobility. He also suffered various injuries because of his well-known love of sports – he excelled at archery, wrestling and 'real' tennis, and, playing the latter he seriously injured his foot.

But it was jousting – two armoured horsemen charging at each other with wooden lances – which proved the most dangerous. His first serious accident occurred in 1524 when he failed to lower the visor on his helmet and was hit by his opponent's lance just above the right eye, after which he suffered from migraines.

Jousting nearly killed him 12 years later. The fall at Greenwich left him "speechless" for two hours, and Anne Boleyn, the woman for whom he had divorced his original queen, Katherine of Aragon, was told that he would die – the shock of which news, she said, caused her to miscarry the child she was expecting. The miscarried baby was male, and it was immediately after this that Henry told Anne they would clearly never have male children together, and turned against her. Less than six months later, Anne had been executed and Henry had married the third of his six wives, Jane Seymour.

But the jousting accident may have affected his whole personality, the new documentary suggests. "We posit that his jousting accident of 1536 provides the explanation for his personality change from sporty, promising, generous young prince, to cruel, paranoid and vicious tyrant," says historian Lucy Worsley. "From that date, the turnover of the wives really speeds up, and people begin to talk about him in quite a new and negative way. After the accident, he was unconscious for two hours; even five minutes of unconsciousness is considered to be a major trauma today." Henry may have suffered a brain injury, according to Dr Worsley. "Damage to the frontal lobe of the brain can perfectly well result in personality change."

The end of his jousting combined with his leg ulcers to restrict his movement, and Henry – who had a large appetite anyway – began to put on weight rapidly. The programme reconstructs his diet, suggesting he may have eaten up to 13 dishes a day, the majority comprising meat such as lamb, chicken, beef, game, rabbit, and a variety of birds including peacock and swan. And he may have drunk 10 pints of ale a day as well as wine, as water was unsafe.

Henry, the programme says, "became a comfort-eating paranoid recluse – a 28-stone
man-mountain".