High intake of sugar-sweetened beverages is often highlighted as a main cause of obesity and type 2 diabetes. To find conclusive evidence, researchers therefore gave 66 overweight men without diabetes large quantities of fructose drinks each day for 12 weeks. However, this did not immediately affect their sugar metabolism or the ability of insulin to regulate this.
Sugary drinks, sweets and junk food are among the sinners in the obesity and diabetes epidemic that is ravaging the world. Many studies have indicated that the type of sugar called high-fructose corn syrup is one of the main causes because of its high calorie count and its lack of nutritional value. Nevertheless, researchers have not yet been able to establish whether these lifestyle diseases are caused by the excess calories or inherently by the direct adverse effects of sugar.
A Finnish-led international research collaboration examined this. Sixty-six overweight men were given extra sugar alongside their regular diet. In the study, the participants consumed 1 litre of a lemon-flavoured drink containing 75 grams of fructose each day.
“The results were striking, since the increase in fructose intake apparently did not affect the participants’ ability to regulate blood glucose during the 12-week study. This implies that insulin resistance is not the first metabolic change resulting from high sugar intake in the short term,” explains Marja Riitta Taskinen, Professor of Internal Medicine at the University of Helsinki in Finland.
Weight gain but no insulin resistance
To study whether added fructose has effects that cause diabetes, the researchers measured the participants’ blood glucose and the fluctuation in the concentrations of such hormones as insulin, which regulates blood glucose, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotrophic peptide (GIP), which are gut-derived hormones stimulating insulin production. In addition, they measured the amount of fat in the participants’ liver using magnetic resonance spectroscopy.
The researchers measured the participants’ glucose and hormone concentrations before the 12-week study started and could therefore monitor how each person’s glycaemic control and fat metabolism changed during the trial. As expected, the researchers observed adverse effects on their weight and the fat content of the liver, both of which increased slightly. Further, plasma triglyceride concentrations increased during fructose intake.
“Nevertheless, the increase in fructose intake did not influence the ability to regulate blood glucose. Both the concentrations and effects of insulin, GLP-1 and GIP seemed to be unaffected. This result suggests that high fructose intake does not seem to affect blood glucose regulation – at least not over 3 months.”
A very surprising result
The study further implies that insulin resistance is not the first metabolic change resulting from high sugar intake in the short term. Instead, the researchers believe that the high fructose intake mainly influences the ability of the body to metabolize triglycerides, which are deposited as liver fat and also secreted into the blood.
"Although the subjects gained weight and got more fat in the liver, it is nevertheless surprising that we did not see any signs of the onset of insulin resistance. Even though the subjects only drank those sugary drinks for just under 3 months, we had nevertheless expected to see an initial effect. So the findings give a definite opportunity to re-evaluate our view of the mechanisms of obesity and type 2 diabetes," explains co-author Jens Juul Holst, Professor, Novo Nordisk Foundation for Basic Metabolic Research, University of Copenhagen.
Insulin resistance and thus type 2 diabetes are frequent among overweight people, and there is still a long way to go before this mechanism is fully understood. Nevertheless, this study emphasizes once again how the high sugar content in modern diets strongly influences the obesity epidemic.
“Fructose intervention for 12 weeks does not impair glycemic control or incretin hormone responses during oral glucose or mixed meal tests in obese men” has been published in Nutrition, Metabolism and Cardiovascular Diseases. In 2013, the Novo Nordisk Foundation awarded a grant to Marja-Riitta Taskinen for the project "Is Inflamed Adipose Tissue a Missing Link between Insulin Resistance and Cardiometabolic Risk Factors Induced by Fructose Intake in Obese Men?" Researchers from The Novo Nordisk Foundation Center for Basic Metabolic Research participated in the research.