The role of dietary fructose in the development of obesity
and fatty liver diseases remains controversial, with previous studies
indicating that the problems resulted from fructose and a diet too high in
However, a new study conducted in an animal model at Wake
Forest Baptist Medical Center showed that fructose rapidly caused liver damage
even without weight gain. The researchers found that over the six-week study
period liver damage more than doubled in the animals fed a high-fructose diet
as compared to those in the control group.
The study is published in the American Journal of Clinical Nutrition.
Not all kilojoules
"Is a kilojoule a kilojoule? Are they all created
equal? Based on this study, we would say not," said Kylie Kavanagh,
D.V.M., assistant professor of pathology-comparative medicine at Wake Forest
Baptist and lead author of the study.
In a previous trial which is referenced in the current
journal article, Kavanagh's 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 than the control group.
They developed diabetes at three times the rate of the
control group and also developed hepatic steatosis, or non-alcoholic fatty
The big question for the researchers 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 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 six weeks, one group was fed a kJ-controlled diet
consisting of 24 % fructose, while the control group was fed a kJ-controlled
diet with only a negligible amount of fructose, approximately 0.5 %.
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.
Every week the research team 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, the researchers measured
biomarkers of liver damage through blood samples and examined what type of
bacteria was in the intestine through faecal 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," Kavanagh said. "Six weeks in monkeys is roughly
equivalent to three months in humans."
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% higher
rate, Kavanagh said.
One of the limitations of the study was that it only tested
for fructose and not dextrose. Fructose and dextrose are simple sugars found
naturally in plants.
"We studied fructose because it is the most commonly
added sugar in the American diet, but based on our study findings, we can't say
conclusively that fructose caused the liver damage," Kavanagh said.
"What we can say is that high added sugars caused bacteria to exit the
intestines, go into the blood stream and damage the liver.
"The liver damage began even in the absence of weight
gain. This could have clinical implications because most doctors and scientists
have thought that it was the fat in and around tissues in the body that caused
the health problems."
The Wake Forest Baptist team plans to begin a new study
using the same controls but testing for both fructose and dextrose over a
longer time frame.