from an article by Tara Parker-Pope on nytimes.com:
In humans, triglycerides, which are a type of fat in the blood, are mostly formed in the liver. Dr. Parks said the liver acts like “a traffic cop” who coordinates how the body uses dietary sugars. When the liver encounters glucose, it decides whether the body needs to store it, burn it for energy or turn it into triglycerides.
But when fructose enters the body, it bypasses the process and ends up being quickly converted to body fat.
“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 people were given three different drinks. In one test, the breakfast drink was 100 percent glucose. 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 drinks were given at random, and neither the study subjects nor the evaluators were aware who was drinking what. 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 the study subjects drank the drinks with fructose. When fructose was given at breakfast, the body was more likely to store the fats eaten at lunch.
Dr. Parks noted that the study likely underestimates the fat-building effect of fructose because the study subjects were lean and healthy. In overweight people, the effect may be amplified.
So this doesn’t mean you should stay away from fruit though, as fruit has nutrients and fiber. But fruit juice will pack on the pounds.