Conversion of fructose to lipid occurred quickly, usually within four hours after ingestion, Elizabeth Parks, Ph.D., of the University of Texas Southwestern Medical Center, and colleagues reported in the June issue of the Journal of Nutrition.
Moreover, consumption of a high-fructose drink for breakfast increased liver-mediated fat storage after lunch, the researchers said.
"Our study shows for the first time the surprising speed with which humans make body fat from fructose," said Dr. Park. "Once you start the process of fat synthesis from fructose, it's hard to slow it down."
The findings provide strong support for clinical guidelines that recommend limiting processed carbohydrates, which often contain high-fructose corn syrup, she added.
Studies involving controlled feeding have shown that fructose could increase serum triacylglycerol levels and maintain the increase throughout the day in healthy individuals and in patients with diabetes. Chronic elevation of triacylglycerol levels could lead to accumulation of atherogenic lipoprotein remnants, the authors said.
Replacement of glucose with fructose in a fat-containing breakfast drink has been shown to increase the four-hour appearance of the meal's fatty acids in VLDL, suggesting increased reesterification of breakfast fat in the liver, they continued.
So the authors hypothesized that a fructose-induced rise in lipogenesis in the morning would further increase triacylglycerol concentrations following lunch. They also sought to determine the lipogenic effects of two different doses of fructose in healthy, relatively lean individuals.
Four men and two women volunteered for the study. Their mean age was 28 and they had a mean body mass index of 24.3 and mean serum triacylglycerol level of 1.03 mmol/L.
On separate days, the volunteers consumed breakfast energy drinks sweetened with 100% glucose, a 50-50 mix of glucose and fructose, and a 25-75 mix of glucose and fructose. The volunteers ate a standardized lunch four hours after consuming the drink.
Lipogenesis was assessed by serial testing for four hours after breakfast, and postprandial lipemia was measured following the lunch meal.
The drink containing only glucose led to a peak fractional lipogenesis of 7.8%. In contrast, the 50-50 mix and the 25-75 mix more than doubled peak fractional lipogenesis (15.98% and 16.9%, respectively, P<0.02).
Fructose consumption at breakfast induced a dramatic rise in postprandial lipemia after the lunch meal. Consumption of the fructose-containing drinks was associated with an increase in postprandial serum triacylglyerols of 11% to 29% compared with the glucose-only drink.
Concentrations of triacylglycerol-rich lipoproteins increased by 76% to 200% with the 50-50 and 25-75 mix of glucose and fructose.
"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 . . . 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."
The message should not be misconstrued by people who are trying to lose weight, she continued. Specifically, they should not eliminate dietary fruits, which have high fructose concentrations.
Overeating and excess caloric consumption remain the principal drivers of weight gain and obesity, she concluded.
The biggest limitation of the study, the researchers acknowledged, is its small sample size. However, they said, the repeated measures design supports the notion that the differences were real and would be reproducible.
Another limitation is the fact that the drinks were consumed first thing in the morning when participants were in the fasting state. That could lead to an underestimation of lipogenesis.
|The study was supported by the National Institutes of Health, the Cargill Higher Education Fund, and the Sugar Association.
Zalman S. Agus, MD Emeritus Professor
University of Pennsylvania School of Medicine
Journal of NutritionSource Reference: Parks EJ, et al "Dietary sugars stimulate fatty acid synthesis in adults" J Nutr 2008; 138: 1039-1046.