Fructose, the most deadly form of sugar, is far more damaging to health than glucose - and now scientists can explain why.
And it is triggering an early form of fatty liver disease in youngsters, a condition normally associated with alcoholics which can lead to cancer, strokes and heart problems.
The illness is a growing issue among British children with one in four now clinically obese by the time they are 15.
First author Dr Samir Softic, of Joslin Diabetes Centre, Boston, said: “The disease is much more worrisome in a child of thirteen who goes from normal liver to fatty liver to liver inflammation over the span of several years than in somebody who has been overweight for 30 years.
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“Kids also eat more sugar than adults, so fructose may be even more of a risk factor in children, which would add to their years of poor health.”
Natural corn syrup
High-fructose corn syrup is used in sweetened beverages and many other processed foods.
It is found naturally in fruit but manufacturers remove the fibre and nutrients to give an instant sugar rush.
Now experiments have found mice fed fructose suffered much worse metabolic effects than those given similar calories of glucose, the other component in table sugar.
In the study published in the Journal of Clinical Investigation the animals were given either regular or high fat diets, and drank either plain water or water sweetened with fructose or glucose.
Over 10 weeks, none of the animals on a regular diet developed insulin resistance, a key factor in metabolic diseases such as diabetes, although those consuming either form of sugar gained substantially more weight.
But among mice on a high-fat diet, significant differences emerged between those drinking fructose and glucose.
Dr Softic, a paediatric gastroenterologist at Boston Children’s Hospital, said: “Fructose was associated with worse metabolic outcomes.”
Mice on the high-fat diet become much more obese and more insulin-resistant compared to their peers on the glucose diet.
And while both groups of animals added fat to their livers, its composition was quite different.
Co-author Professor Ronald Kahn explained that comparing the six diets “gave us a much more precise way of saying ‘what is the role of fructose versus glucose in the diet, and how bad is it when it’s added to a normal diet versus a diet high in fat.’”
The researchers also discovered production of an enzyme called Khk, required for the first step of fructose metabolism, was increased in the livers of mice who drank fructose.
When the scientists examined liver samples from obese human teenagers with fatty liver disease, they also found higher levels of Khk.
The Khk enzyme is specifically important in fructose, but not glucose, metabolism.
Prof Kahn said: “Although fructose and glucose are both sugars, cells handle them very differently.”
That may offer a target to clamp down on fructose metabolism.
Further experiments on mice found reducing the protein lowered liver weight and improved glucose tolerance, most strikingly among those on the high-fat and fructose diet.
Prof Kahn said: “This disease is almost always associated with obesity.
“Once your fat cells get really full of fat and they can’t hold any more, fat winds up going in other tissues, and the liver is the next best place.”
Almost all obese people with diabetes add some fat to their livers.
Dr Softic said: “These people are more at risk of developing fatty liver disease, just as those with fatty liver disease are more at risk of developing diabetes, since obesity is being a predisposing factor for both conditions.”
As obesity spreads worldwide, so will the burden of fatty liver disease and associated liver failure, which is predicted to become the most common factor driving the need for liver transplants, he added.
A healthy liver should contain little or no fat but it is estimated one in every three Britons has early stages of fatty liver disease where there are small amounts of fat in their liver.
It is also feared up to one in ten children now have liver disease.
There are 3.5 million people diagnosed with diabetes in the UK and an estimated 500,000 who are living undiagnosed with the condition.
A recent study suggested fructose is the deadliest sugar you can eat because it boost harmful cholesterol raising the risk of diabetes, heart attacks and stroke.