A new study reported in the American Journal of Clinical Nutrition has revealed that dietary fructose (fruit sugar) rapidly causes liver damage in an animal model.

“Is a calorie a calorie? Are they all created equal? Based on this study, we would say not,” said lead author Dr Kylie Kavanagh from Wake Forest School of Medicine.

Dr Kavanagh with colleagues found that over the 6-week study period liver damage more than doubled in the animals fed a high-fructose diet as compared to those in the control group.

In a previous study, the team studied monkeys who were allowed to eat as much as they wanted of low-fat food with added fructose for 7 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 than the control group. They developed diabetes at three times the rate of the control group and also developed non-alcoholic fatty liver disease.

The big question for the researchers was what caused the liver damage. To answer that question, the current study was designed to prevent weight gain.

Ten middle-aged, normal weight monkeys who had never eaten fructose were divided into two groups based on comparable body shapes and waist circumference. Over 6 weeks, one group was fed a calorie-controlled diet consisting of 24 percent fructose, while the control group was fed a calorie-controlled diet with only a negligible amount of fructose, about 0.5 percent.

“Both diets had the same amount of fat, carbohydrate and protein, but the sources were different,” Dr 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.”

Every week the scientists weighed both groups and measured their waist circumference, then adjusted the amount of food provided to prevent weight gain.

At the end of the study, they measured biomarkers of liver damage through blood samples and examined what type of bacteria was in the intestine through fecal samples and intestinal biopsies.

“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. Six weeks in monkeys is roughly equivalent to three months in humans,” Dr Kavanagh said.

“In the high-fructose group, the team 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.”

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Bibliographic information: Kylie Kavanagh et al. Dietary fructose induces endotoxemia and hepatic injury in calorically controlled primates. Am J Clin Nutr, first published June 19, 2013; doi: 10.3945/ajcn.112.057331