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One
of the reasons people on low-carbohydrate diets may lose weight is that
they reduce their intake of fructose, a type of sugar that can be made
into body fat quickly, according to a researcher at UT Southwestern
Medical Center.
Current Health and Weight Loss Guidelines
Current health
guidelines suggest that limiting processed carbohydrates, many of which
contain high-fructose corn syrup, may help prevent weight gain, and the
new data on fructose clearly support this recommendation.
Fructose Weight Loss Study
Dr. Elizabeth Parks, associate professor of clinical nutrition and
lead author of a study appearing in a current issue of the Journal of
Nutrition, said her team’s findings suggest that the right type of
carbohydrates a person eats may be just as important in weight control
as the number of calories a person eats.
“Our
study shows for the first time the surprising speed with which humans
make body fat from fructose,” Dr. Parks said. Fructose, glucose and
sucrose, which is a mixture of fructose and glucose, are all forms of
sugar but are metabolized differently.
“All three can be made
into triglycerides, a form of body fat; however, once you start the
process of fat synthesis from fructose, it’s hard to slow it down,” she
said.
In humans, triglycerides are predominantly formed in the
liver, which acts like a traffic cop to coordinate the use of dietary
sugars. It is the liver’s job, when it encounters glucose, to decide
whether the body needs to store the glucose as glycogen, burn it for
energy or turn the glucose into triglycerides. When there’s a lot of
glucose to process, it is put aside to process later.
Fructose,
on the other hand, enters this metabolic pathway downstream, bypassing
the traffic cop and flooding the metabolic pathway.
“It’s
basically sneaking into the rock concert through the fence,” Dr. Parks
said. “It’s a less-controlled movement of fructose through these
pathways that causes it to contribute to greater triglyceride
synthesis. The bottom line of this study is that fructose very quickly
gets made into fat in the body.”
For the study, six healthy individuals performed
three different tests in which they had to consume a fruit drink
formulation. In one test, the breakfast drink was 100 percent glucose,
similar to the liquid doctors give patients to test for diabetes – the
oral glucose tolerance test. In the second test, they drank half
glucose and half fructose, and in the third, they drank 25 percent
glucose and 75 percent fructose. The tests were random and blinded, and
the subjects ate a regular lunch about four hours later.
The
researchers found that lipogenesis, the process by which sugars are
turned into body fat, increased significantly when as little as half
the glucose was replaced with fructose. Fructose given at breakfast
also changed the way the body handled the food eaten at lunch. After
fructose consumption, the liver increased the storage of lunch fats
that might have been used for other purposes.
“The message from
this study is powerful because body fat synthesis was measured
immediately after the sweet drinks were consumed,” Dr. Parks said. “The
carbohydrates came into the body as sugars, the liver took the
molecules apart like tinker toys, and put them back together to build
fats. All this happened within four hours after the fructose drink. As
a result, when the next meal was eaten, the lunch fat was more likely
to be stored than burned.
“This is an underestimate of the
effect of fructose because these individuals consumed the drinks while
fasting and because the subjects were healthy, lean and could
presumably process the fructose pretty quickly. Fat synthesis from
sugars may be worse in people who are overweight or obese because this
process may be already revved up.”
Dr. Parks said that people
trying to lose weight shouldn’t eliminate fruit from their diets but
that limiting processed foods containing the sugar may help.
“There
are lots of people out there who want to demonize fructose as the cause
of the obesity epidemic,” she said. “I think it may be a contributor,
but it’s not the only problem. Americans are eating too many calories
for their activity level. We’re overeating fat, we’re overeating
protein; and we’re overeating all sugars.”
About Fructose
Though fructose, a
monosaccharide, or simple sugar, is naturally found in high levels in
fruit, it is also added to many processed foods. Fructose is perhaps
best known for its presence in the sweetener called high-fructose corn
syrup or HFCS, which is typically 55 percent fructose and 45 percent
glucose, similar to the mix that can be found in fruits. It has become
the preferred sweetener for many food manufacturers because it is
generally cheaper, sweeter and easier to blend into beverages than
table sugar.
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Nutrition 
