"The really interesting finding is that increasing and decreasing sugar had virtually identical results [on weight], in the opposite direction of course," says researcher Jim Mann, DM, PhD, professor of human nutrition and medicine at the University of Otago.
Mann and his team analyzed the results of 30 clinical trials and 38 other studies. Their goal was to summarize the evidence on the link between dietary sugars and body weight in adults and children.
However, those studies have produced conflicting findings.
The new review is published in BMJ.
Sugar & Weight: The Studies
Since 2003, the World Health Organization (WHO) has recommended that intake of "free" sugars be limited to 10% of calories daily. The WHO is in the process of updating that recommendation, and it commissioned the review of this research to help that process.
Free sugars are defined as sugars added to foods by the manufacturer, cook, or consumer, or those naturally present in fruit juice, syrups, and honey.
In some of the studies evaluated, adults were told to lower their sugar intake.
Those who did lowered their weight slightly, Mann says. Over the course of the studies, which lasted for up to eight months, the average weight loss was about 1.7 pounds.
Adults who increased their sugar intake over the same average time period increased their weight nearly the same amount.
Over time, Mann says, that decrease (and increase) could be more. "There is also evidence that the longer the study, the more striking the effect," he says.
The link between sugar intake and weight was less consistent in studies of children, Mann found. He says that is mainly due to the kids not following the diet advice.
When they looked only at sugary beverages, however, the link between a sugary beverage habit and being overweight in children was strong.
Those who had the highest intake after a year were about 1.5 times as likely to be overweight or obese as those with the lowest intake, Mann found.
Sugar & Weight: Industry Responds
The Sugar Association declined to comment on the study, but says the news release "is a classic example of misrepresented science."
But the American Beverage Association issued this statement: "This study confirms that it's calories that count when it comes to weight loss, not uniquely calories from sugar. As the authors noted, when calories from sugar were replaced with calories from carbohydrates, there was no change in weight. This would not have been the case if sugars had a unique effect on body weight."
Sugar & Weight: Perspective
The results suggest sugar increases body weight mainly by encouraging overeating, according to Walter Willett, MD, PhD, MPH, chair of nutrition at Harvard School of Public Health. He co-wrote an editorial to accompany the study.
However, he writes, many questions remain about sugar and weight.
The 10% limit set by the WHO ''could be viewed as a realistic practical goal," he says.
Keeping sugar to 10% of calories, if on a 2,000-calorie-a-day diet, would mean eating and drinking just 200 calories a day of sugars.
One way to cut down is to eat whole fruit and not drink sweetened juices, says Ping Zhou, MD, a pediatric endocrinologist at The Children's Hospital of Montefiore Medical Center in the Bronx.
Sugars from whole fruits have not been linked with weight gain, according to Willett.