Reducing sugar in diets even without cutting calories or losing weight has the power to dramatically improve health, experts say.
A involving obese children found impressive results in lowering blood pressure and cholesterol in as little as 10 days.
Scientists behind the study said it showed that sugar was "metabolically harmful not because of its calories" but because it is sugar.
"What this is saying is that sugar is toxic because it’s sugar; not because it’s calories," the study's lead author, Dr Robert Lustig told the .
"This proves conclusively, beyond a shadow of doubt, that a calorie is not a calorie."
The study, published in the journal Obesity, looked at the effect of restricting sugar on metabolic syndrome, a cluster of conditions that increase the risk of heart disease, stroke and Type 2 diabetes.
Metabolic syndrome can include high blood pressure, high blood glucose levels, excess body fat around the waist and abnormal cholesterol levels.
A total of 43 children aged nine to 18 took part in the study at the University of California San Francisco Benioff Children's Hospital.
All the children had a Latino or African-American background because of their higher risk for certain conditions associated with metabolic syndrome, such as high blood pressure and high blood glucose levels.
The children were all obese and had at least one other chronic disorder, such as high blood pressure.
'Cutting sugar improved health in nine days'
Over a period of nine days, the children followed a meal plan that included all snacks and drinks, but restricted sugar intake.
Added sugar was banned but fruit was allowed.
"I have never seen results as striking or significant in our human studies," said the study's senior author, Jean-Marc Schwarz.
"After only nine days of fructose restriction, the results are dramatic and consistent from subject to subject.
"These findings support the idea that it is essential for parents to evaluate sugar intake and to be mindful of the health effects of what their children are consuming."
The diet overall had the same fat, protein, carbohydrate, and calorie levels as their previous diets at home, with the carbohydrate from sugar replaced by foods such as bagels, cereals and pasta. Hot dogs, crisps and pizza from local supermarkets all featured in the diet.
Initial fasting blood levels, blood pressure, and glucose tolerance were assessed before the new meals were eaten.
During the study, if the children did lose weight, they were given more of the low sugar foods to keep weight stable.
Overall, the total dietary sugar in the meal plan was was reduced from 28 per cent to 10 per cent, and fructose from 12 per cent to 4 per cent of total calories.
The results showed that the new meal plan led to dramatic improvements in health in a short time, with a drop in blood pressure and cholesterol, and improved liver function.
Fasting blood glucose levels fell by five points while insulin levels were cut by a third, researchers from the University of California San Francisco and Touro University in California said.
Lead author, Dr Robert Lustig, said: "This study definitively shows that sugar is metabolically harmful not because of its calories or its effects on weight; rather sugar is metabolically harmful because it's sugar.
"This internally controlled intervention study is a solid indication that sugar contributes to metabolic syndrome, and is the strongest evidence to date that the negative effects of sugar are not because of calories or obesity."
Jean-Marc Schwarz, senior author of the paper, added: "I have never seen results as striking or significant in our human studies.
"After only nine days of fructose restriction, the results are dramatic and consistent from subject to subject.
"These findings support the idea that it is essential for parents to evaluate sugar intake and to be mindful of the health effects of what their children are consuming.
"When we took the sugar out, the kids started responding to their satiety cues.
"They told us it felt like so much more food, even though they were consuming the same number of calories as before, just with significantly less sugar.
"Some said we were overwhelming them with food."
'Study needs to be viewed with caution'
Authors of the study acknowledged its limitations, writing in the study that it was based on a small sample and relied on self-reported data.
“The results are not convincing to me – this is a very small study, and it has not been statistically well controlled," said Professor of Metabolic Medicine, Naveed Sattar from University of Glasgow.
"Also, when people are losing weight, even if modest, their metabolic changes can seem larger than they actually are – one needs to see results once folk return to their habitual state after they’ve finished losing weight. Overall, this study is of modest interest but is far from convincing.”
Dr Sonia Caprio, professor of pediatrics at Yale Medical School, said the study still had value despite its small sample size.
"It addressed the issue in an original way and tried to isolate the effect of sugar on metabolic syndrome and insulin resistance," she told the New York Times.
But Tom Sanders, professor emeritus of nutrition and dietetics at King’s College London, said the study's findings needed to be viewed with caution.
He said the absence of a control group and a reliance on the weight and health of children before the study lessens the strength of any causal conclusions that can be drawn.
"It is well known that obese children underestimate and under-report food intake, particularly of soft drinks and snack foods,” Sanders told the Guardian.
“This is a fundamental flaw in the study. It is likely that the changes in metabolic outcomes observed can be explained by the experimental diet providing fewer calories than the children’s usual intake.”
Dietitian Tracy Parker from British Heart Foundation said further research would be needed to confirm the study's findings.
"This study is interesting, but we need more research to confirm these findings," she said.
"Previous studies have suggested that eating too much added sugar increases a person’s risk of development of the various diseases, including cardiovascular disease, type 2 diabetes, hypertension and high cholesterol, because of the link with excess calorie intake leading to obesity."
