Researchers found that people who consumed extra fructose baked into breads or sprinkled into drinks didn't gain any extra weight compared to those who had other types of carbohydrates instead -- when they ate the same number of total calories.
On the other hand, when study participants supplemented a standard diet with extra calories in the form of straight fructose, they did gain weight.
"Fructose probably isn't any different than other sources of carbohydrates," said lead author Dr. John Sievenpiper, a research fellow at St. Michael's Hospital in Toronto.
The finding, he told Reuters Health, "represents pretty reasonable evidence that fructose in and of itself doesn't contribute to weight gain. But when it contributes extra energy, that's when you do see weight gain."
Researchers have wondered whether there's something about fructose -- typically found in fruits as well as baked goods and sugar-sweetened beverages -- that makes people store fat and gain weight faster than other carbohydrates.
That's especially a concern because high-fructose corn syrup is a main ingredient in many common foods and drinks, including soda.
To see where the evidence stands, Sievenpiper and his colleagues looked back at studies that compared weight gain in people assigned to eat diets high in fructose or another carbohydrate instead, most commonly starch or glucose.
In 31 studies including 637 people, participants on both diets ate an equal number of calories, but those in the fructose groups got about 17 percent of their calories from fructose, on average.
The studies included participants who were normal weight, overweight or obese, depending on the trial. Some of the study diets were designed to promote weight loss, while others aimed for maintenance or weight gain.
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In the other 10 trials, with 119 participants, people assigned to the high-fructose groups ate the extra sugar on top of the normal calories fed to all participants -- and they took in more than twice as much sugar as people in the equal-calorie studies.
In those trials, over an average of one and a half weeks on the diets, participants eating and drinking the extra sugar gained 1.2 pounds more than those in the comparison groups.
The results suggest it's not the fructose itself that causes weight gain, according to the researchers.
"It's not any one source of calories -- it's calories in general," Sievenpiper said.
One theory has been that because of the way fructose is processed in the liver, people who eat a lot of it may be more likely to become insulin-resistant than those who choose other carbs. The research team didn't look at individuals' insulin levels, so the new analysis doesn't say anything about the effects of fructose on the blood sugar-regulating hormone, according to Sievenpiper.
It also doesn't show how weight was distributed in people who ate extra fructose. Dr. Frank Hu, a nutrition researcher at the Harvard School of Public Health in Cambridge, Massachusetts, said that fructose may increase fat around the belly and organs more than glucose, for example.
That type of fat has been linked to heart disease and diabetes.
"In terms of body weight, it looks like the same amount of fructose or glucose would have the same effects," Hu, who wasn't involved in the study, told Reuters Health.
"But the other metabolic effects can be different," he said.
"We have to look beyond just body weight when we talk about the effects of different sugars."
Because most of the studies they analyzed were small and didn't follow dieters for very long, the researchers said that people shouldn't base their own nutrition decisions on the new findings.
There's also a need for larger studies to compare the effects of natural fructose, like the kind in fruits and vegetables, with the kind of sugar added to other food and drinks in amounts most people get in a typical day, Sievenpiper said.
The researchers report receiving grants from Coca-Cola, but said the company was otherwise not involved in the current study. The primary funding source for the report was the Canadian Institutes of Health Research.
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