Almost 70 per cent of women and more than 50 per cent of men are obese - with 80 per cent of the adult population overweight, according to the Lancet.
Up to 40 per cent of the population is thought to have type 2 diabetes and life expectancy has dropped like a stone in recent years, from the mid-70s to just 64.
It's a country in which size and social standing are virtually interchangeable - Tongan King Tupou IV, who died in 2006, holds the world record for being the fattest-ever monarch, weighing an impressive 33 stone (210kg).
And the problem is now so severe entire families get diabetes.
One 14-year-old girl had a wound on her leg that refused to heal for four years because of the condition, reports the BBC.
In the end her limb had to be amputated.
A big reason behind the epidemic is diet - which is so poor some health food restaurants even serve fish and chips as a low-fat meal.
Tonga's national dish is mutton flaps - a cheap fatty off cut imported from New Zeland, and it estimated Tongans can wolf down 1kg of the unhealthy meat in a single sitting.
The islanders' traditional diet used to consist of fish, vegetables and coconuts.
But cheap meats from western civilisations, including turkey tails from the US, began to be seen as aspirational dishes.
Papiloa Bloomfield Foliaki, 82, said: "People think something imported is superior.
"And you have a situation where fisherman spear their fish - sell it - and go and buy mutton flaps.
"People don't have the education to know what is bad for their health."
In 1973, just seven per cent of the population had diabetes, but now there is a whole generation brought up on the mutton snack and Tongans are also said to have an unhealthy desire for fizzy drinks.
On top of that, lardy canned meat is popular - available in huge 2.7kg tins.
And the Tongan tradition of feasting looks, to the untrained eye, like an eating competition.
But some experts believe other factors may be behind the Tongans' worrying size.
In the past, Pacific islanders had to survive long periods without food so their bodies may have been programmed to hold on to fat.
There is also a social aspect, where being fat has long been associated with wealth.
Drew Havea, chair of the civil Society Forum of Tonga, said: "The bigger you are, that's beauty.
"We need to learn that if you are skinny you are not hungry."
Church leaders also have a role to play in a very religious nation - ministers are authority figures and 85 per cent of them obese.
In a bid to curb the nation's ever-increasing waistline, a tax was even introduced on mutton flaps.
But Lepaola Vaea, deputy Chief Executive at the Ministry of Revenue and Customs, said: "There was a large public outcry.
"People are addicted."
Mirror
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