A new study into the health effects of sugar by the University of Otago aims to disentangle what has been a confused and controversial issue, its author says.
The study, commissioned by the World Health Organisation and published today in the influential British Medical Journal (BMJ), found cutting down on sugar had a small but significant effect on body weight.
It has already received a ringing endorsement from United States nutrition experts, who have backed its findings in an editorial also published today in the BMJ.
Study co-author Professor Jim Mann, who has been researching the health effects of sugar since the 1970s, said the study's methodology helped it to disentangle conflicting information about the role of sugar in human health.
"The story about sugar and teeth and dental care is fairly clear cut, but with regard to other health issues, there has been a certain amount of controversy, to some extent fuelled by the industry,'' he said.
Dr Mann and his co-author Dr Lisa Te Morenga searched through almost 8000 trials and 10,000 studies before aggregating the findings of 68 studies that looked directly at the effects of free sugars on body weight.
He said such an analysis had the advantage of giving researchers an overall picture.
The study found reducing sugar intake had a small but significant effect on adult body weight - an average reduction of 0.80kg - while increasing sugar intake led to a corresponding 0.75kg increase in weight.
Dr Mann said that might not sound like very much - but the effect was "incredibly important'' because it was cumulative.
"The longer you continue to eat less sugar, the more the effect. And equally, if you start eating more sugar, the greater the effect.''
The studies in which people increased their sugar intake showed they gained weight "in an incredibly short period of weeks or months''.
"So what are you doing to yourself if you are having a high intake of sugar over the years?''
Dr Mann said the study was one of the first to acknowledge sugar was not the cause of the obesity epidemic but a significant contributor.
The evidence was less consistent in children, mainly due to poor compliance with dietary advice, but it showed an increased risk of obesity among children who consumed more sugar-sweetened beverages.
Dr Mann said the results were heartening because they were completely compatible with his own research into the effects of sugar, first published in The Lancet, another respected medical journal, in the 1970s.
"It is encouraging to see that probably we were right.''
The BMJ editorial backing the study argues for action at many levels - including education programmes, better food and drink options at schools and worksites, and policy changes like taxes on sugar-laden drinks.
But Dr Mann said such health messages could take a long time to get through to the public, citing smoking as an example.
"I think the sugar story has been very confused by people who have exaggerated the badness of sugar. Sugar is not inherently a bad substance - if you like a bit of sugar on your cereal, that's fine. What you don't want is an excessive intake of sugar for the amount of calories that you need,'' he said.
"I think the problem over the years is that you've had two camps. You've had people who've said sugar is a disaster, it's the cause of all ill-health - which it clearly isn't - and then you've had people, particularly the sugar industry, that has understandably rubbished that.''