Almost 25% of Brits are obese. Treating obesity and the knock on effects on our health is costing the NHS £5.1 billion every year. Sugar is thought to be a source of those excess pounds, says a report published earlier this year by the Scientific Advisory Committee on Nutrition. It advises we should be halving our consumption but how we do this is proving controversial. Professor Ian Young, Queen's University, Belfast was one of the authors of that very report and Chris Smith asked him the question that's on everyone's lips: Why do we need to slash our consumption...

Ian - Sugars are small molecules, which fall into the group of monosaccharides or disaccharides and, in particular, in the SACN report we have used the term 'free sugars,' which means the sugars which are added to foods or drinks by the manufacturer, plus the sugars which naturally are present in honey, and syrups and unsweetened fruit juices. And we don't count the sugars in the cellular structure of foods, so whole fruits and vegetables.

Chris - Because, for instance, I could argue that a potato is full of carbohydrates, but that's lots of sugar molecules stuck together to make another bigger molecule called starch, which is a very different beast than the glucose, sugar molecules, that make it up, isn't it?

Ian - It is, so the SACN report deals with carbohydrates in total, which include the complex carbohydrates like starch, the sugars as I've described, and also fibre. We didn't find any harmful effects of the amount of carbohydrate which was in the diet. So the recommendation is that it should remain at around 50% but we did find some harmful effects around free sugars...

Chris - When you say 50%, do you mean 50% of the calories that a person takes in should come from some sort of carbohydrate source?

Ian - Yes around 50% of calories should come from carbohydrate source.

Chris - How much are people eating at the moment?

Ian - The amount of carbohydrate is appropriate, it's around between 50 and 60% but, in terms of sugars, it's running between 12% and 15% of energy from free sugars, which we think is far too much.

Chris - And so what are you recommending instead?

Ian - We are recommending that free sugars should be reduced to less than 5% of energy intake.

Chris - And why are you suggesting that?

Ian - Well the harmful effects we found with free sugars were firstly, an increased risk of tooth decay in large prospective studies, secondly, we found that in adults when people eat a diet with a high sugar content that they tend to consume more energy overall, and that's likely to increase the risk of obesity and, for sugar sweetened drinks in particular, increased intakes are associated with increased risk of type 2 diabetes in adults.

Chris - Now about 20 years ago, the proportion of the population of the average western country that was obese was about 10%, it's now about 30%. Can we account for why there's been this dramatic difference in the last 20 years?

Ian - So the dominant theory is that it's due to an imbalance between energy intake and energy utilisation. So people have been eating more calories, and certainly the increase in sugar intake is likely to play a part in that....

Chris - But has that really changed in 20 years? Are people dramatically eating three times different amounts of things now compared with what they did 20 years ago?

Ian - No, they're eating somewhat more but, in addition to that, there's evidence of reduced energy expenditure and it's the balance between the two, the energy intake and the energy utilisation, which probably leads to the increased risk of obesity.

Chris - So why do you think that sugar, and reduction of sugar is the answer then. Because if it's down to lifestyle factors and activity, why will just cutting out some sugar - halving it from 10-15% down to around 5% of calorie intake, why will that make a difference?

Ian - So, I think it will make a difference. I don't think anybody would say that it's the answer. If you eat less free sugars, then you are better able to regulate your energy intake and we believe if sugar intake reduced from 10% of energy to 5%, that on average people would eat 100 calories per day less if you were an adult, and that would certainly make a contribution to reducing the incidence of obesity.

Chris - Ian, is there any evidence that people get 'hooked' on sugar, or is that just a figure of speech?

Ian - Certainly people like sugar but the idea that sugar is addictive in the way that, for instance, some drugs of abuse are addictive, I don't think there's any significant evidence to support that.