A new study involving obese children found significant results in lowering blood pressure and cholesterol in less than two weeks.

Scientists behind the study said it showed that sugar was "metabolically harmful not because of its calories" but because it places particular strain on the body.

It follows the release of a controversial official report last week which called for a tax of between 10 and 20 per cent on sugary foods and drinks.

British PM David Cameron has rejected calls for such taxes, but has indicated that other measures - such as a clampdown on advertising and marketing deals - could form part of a future Government strategy on childhood obesity.

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.

The syndrome can result in high blood pressure, high blood glucose levels, excess body fat around the waist and abnormal cholesterol levels.

The research found that those who were made to cut back on sugar had a drop in blood pressure, cholesterol, improved liver function, fasting blood glucose levels while insulin levels were cut by one third.

Some 43 children aged nine to 18 took part in the study at the University of California San Francisco Benioff Children's Hospital.

The children were all obese and had at least one other chronic disorder, such as high blood pressure.

Over nine days, the children followed a meal plan that included all snacks and drinks, but restricted sugar intake.

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.

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."

Overall, the total dietary sugar in the meal plan 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.

Diastolic blood pressure decreased by 5mm, levels of key fats in the blood known as triglycerides dropped by 33 points, LDL-cholesterol, known as the "bad" cholesterol, fell by 10 points, and liver function tests improved.

Fasting blood glucose went down by five points, and insulin levels were cut by one-third, the study found.

"All of the surrogate measures of metabolic health got better, just by substituting starch for sugar in their processed food — all without changing calories or weight or exercise," said Dr Lustig.

"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."

Dr Lustig said: "This study demonstrates that a calorie is not a calorie.

"Where those calories come from determines where in the body they go."

"Sugar calories are the worst, because they turn to fat in the liver, driving insulin resistance, and driving risk for diabetes, heart, and liver disease.

"This has enormous implications for the food industry, chronic disease, and health care costs."

Dr Aseem Malhotra, a cardiologist and advisor to Action on Sugar, said: "It's time to abandon the outdated notion that a calorie is a calorie theory that continues to damage public health. This study provides further evidence that all calories do not have the same metabolic effects on the body with sugar calories being particularly harmful."

Tracy Parker, heart health dietitian at the British Heart Foundation, said: "This study is interesting, but we need more research to confirm these findings. 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."

The Telegraph, London