A new study determined if fructose from sweetened drinks increased the risk of type 2 diabetes compared to other fructose-containing foods.
Type 2 diabetes is one of the leading causes of mortality worldwide. It is a chronic condition in which the body either resists the effects of insulin, a hormone that regulates glucose levels, or the body does not produce enough insulin to maintain a normal glucose level.
Diet plays an important role in the risk of developing type 2 diabetes and other metabolic diseases. Fructose, a monosaccharide, in its pure form or from high fructose corn syrup is added to many foods such as sweetened drinks, pastries, ice creams and yogurt, that are included in our diets today.
Many studies have linked the consumption of fructose with a higher risk of type 2 diabetes. Some other studies failed to show the adverse effects of fructose and yet some suggested that isocaloric substitution of some carbohydrates with fructose may even be beneficial.
The significance of consumption patterns and levels of fructose
Fructose is almost always present in combination with similar amounts of glucose in various foods. Sucrose, or table sugar, is also a combination of fructose and glucose. The high fructose corn syrup is another example of the fructose and glucose combination. Natural foods such as fruits and vegetables and less refined foods such as maple, agave, and honey are all a combination of fructose and glucose in varying proportions.
The metabolism of fructose is influenced by the proportion of fructose and glucose eaten together. Some studies suggest that the risk of type 2 diabetes is dependent on the source of fructose. For example, is it from a “nutrient-rich” source such as fruits and honey or a “nutrient poor” source such as sugar-sweetened beverages?
Effect of the source of fructose on blood sugar levels
Most dietary guidelines recommend lowering intake of free sugar, especially the added fructose in sweetened beverages. However, it is not clear if the effect of added or free sugars and sugar-sweetened drinks on the risk of type 2 diabetes is true for all fructose-containing food sources. The scientists are working to understand the significance of food sources of fructose in the prevention and management of diabetes.
A new systematic review and meta-analysis published in the British Medical Journal compared the results of 155 studies including 5086 participants to assess the effect of food sources of fructose at different calorie levels on blood glucose levels in people with and without diabetes. The participants were monitored for at least 12 weeks and their levels of glycated hemoglobin (HbA1C), fasting blood glucose, and fasting insulin were measured as the main outcome of the study.
The researchers in St Michael’s and the University of Toronto analyzed the results based on four study designs:
- Substitution studies that compared fructose-containing sugars with non-fructose-containing carbohydrates
- Addition studies in which energy from sugars was added to background diets
- Subtraction studies in which energy from sugars was subtracted from background diets
- Ad libitum studies that compared fructose-containing sugars with non-fructose-containing carbohydrates without any control of study foods or background diets.
The beneficial effect of fructose sugars
The results of the substitution study showed a beneficial effect of fructose on HbA1c- a measure of the amount of glucose attached to red blood cells- while there was no effect on fasting blood glucose or insulin. The researchers observed no harmful effect of foods containing fructose on blood glucose levels when no excess calories were consumed. The addition studies, however, showed a harmful effect on fasting blood insulin without having an effect on HbA1c or fasting blood glucose.
Significance of food sources of fructose
The researchers found that fruits and fruit juices that did not provide excess calories had a beneficial effect on blood glucose and insulin control in participants with diabetes. On the other hand, “nutrient poor” foods that added excess calories to the diet, such as sugar-sweetened beverages, had harmful effects and posed a higher risk of type 2 diabetes.
The reason why fructose may be beneficial
Fructose has a low glycemic index compared with other carbohydrates. A low glycemic index allows a slow release of sugars thus keeping the blood sugar level in control. There is evidence that suggests that foods with high glycemic index cause an increase in triglycerides and a decrease in HDL which are the markers of insulin resistance. Since sugar from fruits has a low glycemic index, this study showed significant improvement in HbA1c levels when fruits were the main source of fructose compared with sugar-sweetened beverages or sweets that have a high glycemic index.
Reducing added sugars may help prevent type 2 diabetes
The findings of the study suggest that fructose present in common food sources has no harmful effects if calorie intake is not excessive. However, foods with added sugars and excess energy such as sugar-sweetened beverages may increase the risk of type 2 diabetes. Therefore, the harmful effects of fructose sugars on blood glucose are a result of energy intake and source of the sugars.
More high-quality studies are needed to provide a high level of evidence
The main strength of this meta-analysis is the in-depth process of search and selection of trials and a detailed assessment of evidence of quality. The main limitations include small sample sizes, short follow-up periods, and a limited variety of food sources in the trials of the meta-analysis. The researchers noted that although the analysis had no risk of bias, but the quality of evidence is low. Therefore, more high-quality studies are needed to provide evidence to support these results.
The consumption of sugar is becoming increasingly debatable. Foods such as sweetened juices and pastries that contain high fructose corn syrup should be avoided. The researchers concluded that policymakers should consider the role of energy and source of fructose-containing foods while making dietary recommendations to reduce the consumption of sugars to prevent type 2 diabetes.
Written by Preeti Paul, MS Biochemistry
Reference: Vivian L Choo et al., Food sources of fructose-containing sugars and glycaemic control: systematic review and meta-analysis of controlled intervention studies. BMJ2018;363:k4644|doi:10.1136/bmj.k4644
