Previous research has shown that the sweetener fructose combined with a diet too high in calories can cause obesity and fatty liver diseases, but a new study has shown that fructose can rapidly cause liver damage even without weight gain. "Is a calorie a calorie? Are they all created equal? Based on this study, we would say not," said Kylie Kavanagh, a medico at Wake Forest Baptist and lead author of the study.
In a previous study Kavanagh's research team studied monkeys who were allowed to eat as much as they wanted of low-fat food with added fructose for seven years, as compared to a control group fed a low-fructose, low-fat diet for the same time period. Not surprisingly, the animals allowed to eat as much as they wanted of the high-fructose diet gained 50 percent more weight and developed diabetes at three times the rate of the control group.
The big question for Kavanagh was what caused the liver damage? Was it because the animals got fat from eating too much, or was it something else?
To answer that question, this new study was designed to remove weight gain as a factor. Over six weeks, one group of animals was fed a calorie-controlled diet consisting of 24 percent fructose, while the control group was fed a calorie-controlled diet with only 0.5 percent of fructose.
Both diets had the same amount of fat, carbohydrate and protein, but the sources were different, Kavanagh said. The high-fructose group's diet was made from flour, butter, pork fat, eggs and fructose (the main ingredient in corn syrup), similar to what many people eat, while the control group's diet was made from healthy complex carbohydrates and soy protein.
At the end of the study, the researchers measured biomarkers of liver damage through blood samples and examined what types of bacteria were present in the intestine.
"What surprised us the most was how quickly the liver was affected and how extensive the damage was, especially without weight gain as a factor," Kavanagh said. "Six weeks in monkeys is roughly equivalent to three months in humans." The study, published in the American Journal of Clinical Nutrition, reports that liver damage more than doubled in the animals fed a high-fructose diet as compared to those in the control group.
In the high-fructose group, the researchers found that the type of intestinal bacteria hadn't changed, but that they were migrating to the liver more rapidly and causing damage there. "It appears that something about the high fructose levels was causing the intestines to be less protective than normal, and consequently allowing the bacteria to leak out at a 30 percent higher rate," Kavanagh said.
"We can't say conclusively that fructose caused the liver damage," he cautioned. "What we can say is that high added sugars caused bacteria to exit the intestines, go into the blood stream and damage the liver."
Most strikingly, the liver damage began even in the absence of weight gain. This could have clinical implications as most scientists believed that it was the fat in and around tissues in the body that cause health problems such as diabetes.
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Source: Wake Forest Baptist Medical Center