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Prime Minster David Cameron revealed this week in the House of Commons that he has trouble stopping his three children from over-indulging on sugary, fizzy drinks.

His comments came after Labour MP Keith Vaz, who has Type 2 diabetes, reminded him that a third of all primary school leavers are obese or overweight, yet they consume cans of cola that contain up to eight teaspoons of sugar.

So how important is sugar in the fight against obesity?

investigated the link between sugar consumption and body weight by looking at the results of previous studies. It found that getting people to reduce sugar intake in their diet was associated with a reduction in their weight of about 2.2lb (1kg) in adults.

The findings also suggested that sugar increases body weight by promoting overconsumption of energy. In other words, the taste of sugar could lead us to want to eat more of it.

The idea that sugar is bad for our health is not new.

Forty years ago, a book written by British physiologist John Yudkin claimed that high sugar consumption was linked to heart disease.

We know that sugar also causes tooth decay and that eating too much sugar-laden food can lead to a poor diet lacking in nutrients.

But some experts say that sugar has actually helped to fuel the obesity epidemic.

Robert Lustig, professor of paediatrics at the University of California, is well-known for his research into the effects of dietary sugar. He believes that sugar is addictive.

In a recent interview he said: "There are five tastes on your tongue: sweet, salty, sour, bitter and umami.

"Sugar covers up the other four, so you can't taste the negative aspects of foods. You can make dog poop taste good with enough sugar."

Lustig goes on to say that table sugar known as sucrose, which is a made of two sugars (glucose and fructose) chemically bound to each other, is identical to high fructose corn syrup - which he describes as a "chronic toxin".

Dr Alex Richardson, senior research fellow at the University of Oxford and founder director of the UK charity Food and Behaviour Research, agrees with Lustig and says that there is far too much sugar and empty carbohydrates in children's diets.

"We find that highly processed foods are making up massively more of children's diets. Things like cakes, biscuits, snacks and crisps.

"Fruit and vegetables are so vital for children. They provide essential vitamins and minerals, but so often a third of a plate of child's food is sugary rubbish and a small amount is veg or fruit."

She warns that a diet high in sugar could lead, in the long term, to Type 2 diabetes.

Sugar comes in all shapes and sizes. It can occur naturally in fruit and milk, which is not a concern, but when sugar is added to foods such as cereals, desserts, confectionery, processed meals and soft drinks it can become a problem in large quantities.

Sugar can be listed under the names sucrose, glucose, fructose and maltose in the ingredients.

'Added sugars', as they are known, are a good source of energy but provide no other nutrients.

Sugary drinks are thought to represent more of a health issue because they do not fill us up as quickly as the sugars in solid food do.

The British Dietetic Association's advice on sugar says: "Some research suggests that sugary drinks make it harder for us to regulate the overall amount of calories eaten and a regular intake may be a factor contributing to obesity in children."

But the BDA maintains that sugar does not makes us fat.

"Sugary foods and drinks can only make us gain weight if overall we eat more calories than we use for energy."

Sugar Nutrition UK, a research body which is funded by the sugar manufacturers, refutes the suggestion that sugar is toxic or addictive.

"Sweet treats are not toxic. Major expert committees have considered the evidence in regards to sugars and all of the diseases addressed by Lustig, and all have concluded that there is no evidence of any harm attributed to current sugar consumption levels."

They also said that sugar intake was not increasing in line with obesity rates.

"Sugar consumption in the UK has declined whilst obesity and diabetes rates have increased. Data from the government's national dietary surveys show that intakes of sugars have declined over the last decade, whilst rates of obesity and diabetes have been increasing."

The Labour party has called on the government to set legal limits on sugar, fat and salt in some foods to tackle the rise in obesity.

The Health Secretary Jeremy Hunt responded by saying that through voluntary agreements with food manufacturers progress was being made to encourage the production of healthier foods - and tackle obesity.

At present, the World Health Organization recommends that added sugar intake should be limited to 10% of total energy intake but the American Heart Association suggests a lower limit of 5%.

Improving the quality of carbohydrates and reducing intakes of refined grain products and potatoes is also recommended in a bid to lower sugar intake in the general population.