People on low-carbohydrate diets may lose weight because they have reduced their intake of a type of sugar the body turns quickly into fat, new research suggests.

A researcher at the University of Texas Southwestern Medical Center has found that fructose, commonly found in processed carbohydrates, turns into fat much faster than glucose and sucrose. All three are forms of sugar but the body metabolizes them differently.

The study found that fructose bypasses sugar's usual metabolic pathway through the liver. The liver decides whether sugar that passes through needs to be stored as glycogen, burned for energy, or converted to the form of body fat known as triglycerides.

"It's basically sneaking into the rock concert through the fence," study author Dr. Elizabeth Parks said in a statement. "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."

The findings are published in the Journal of Nutrition.

Parks, an associate professor of clinical nutrition, said the study indicates that the type of carbohydrate a person eats may be just as important for weight loss as factors such as counting calories.

High levels of fructose are found naturally in fruit. However it is also added as a sweetener to many processed foods, particularly in the form of high-fructose corn syrup.

For the study, researchers asked six healthy volunteers to drink three different fruit beverage formulas for breakfast. One drink was pure glucose, one drink was half glucose and half fructose and the third drink was 25 per cent glucose and 75 per cent fructose.

The participants ate a regular lunch about four hours after consuming each drink.

The researchers found that the process through which the body turns sugar into fat increased significantly among participants when they drank the higher fructose beverages.

As well, the subjects' bodies stored more fats from lunch when they consumed the high-fructose drinks.

"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," Parks said. "Fat synthesis from sugars may be worse in people who are overweight or obese because this process may be already revved up."

Parks said that the consumption of sugar is not the only contributor to a growing North American obesity epidemic. However, restricting processed carbohydrates is still a good tactic for weight control.

Abstract:

Dietary Sugars Stimulate Fatty Acid Synthesis in Adults

Elizabeth J. Parks, Lauren E. Skokan, Maureen T. Timlin, and Carlus S. Dingfelder

The goal of this study was to determine the magnitude by which acute consumption of fructose in a morning bolus would stimulate lipogenesis (measured by infusion of 13C1-acetate and analysis by GC-MS) immediately and after a subsequent meal. Six healthy subjects [4 men and 2 women; aged (mean 6 SD) 28 6 8 y; BMI, 24.3 6 2.8 kg/m2; and serum triacylglycerols (TG), 1.03 6 0.32 mmol/L] consumed carbohydrate boluses of sugars (85 g each) in a random and blinded order, followed by a standardized lunch 4 h later. Subjects completed a control test of glucose (100:0) and a mixture of 50:50 glucose:fructose and one of 25:75 (wt:wt). Following the morning boluses, serum glucose and insulin after 100:0 were greater than both other treatments (P , 0.05) and this pattern occurred again after lunch. In the morning, fractional lipogenesis was stimulated when subjects ingested fructose and peaked at 15.9 6 5.4% after the 50:50 treatment and at 16.9 6 5.2% after the 25:75 treatment, values that were greater than after the 100:0 treatment (7.8 6 5.7%; P , 0.02). When fructose was consumed, absolute lipogenesis was 2-fold greater than when it was absent (100:0). Postlunch, serum TG were 11-29% greater than 100:0 and TG-rich lipoprotein-TG concentrations were 76-200% greater after 50:50 and 25:75 were consumed (P,0.05). The data demonstrate that an early stimulation of lipogenesis after fructose, consumed in a mixture of sugars, augments subsequent postprandial lipemia. The postlunch blood TG elevation was only partially due to carry-over from the morning. Acute intake of fructose stimulates lipogenesis and may create a metabolic milieu that enhances subsequent esterification of fatty acids flowing to the liver to elevate TG synthesis postprandially.