Sugar – Understanding the Difference a Bond Makes
The Food and Drug Administration (FDA) defines sugar as sucrose obtained from sugar cane or sugar beets. The FDA further specifies that the glucose and the fructose in sucrose are molecularly bonded (linked together).
The digestion of sucrose requires the action of a specialized enzyme – sucrase. The bond linking the glucose and fructose in sugar must first be broken by sucrase before the freed fructose and freed glucose can enter the human bloodstream for subsequent metabolism. Sucrase is not required when glucose or fructose is molecularly free (not bonded).
How sucrase influences metabolism is an intensely debated scientific topic. It is unknown if sucrase modifies how the glucose and fructose in sucrose enter the bloodstream. It is also unknown if the molecular bond in sugar causes variations in the metabolism of glucose and fructose.
Results of a recent study provide insight into understanding the biological influence of the molecular bond in sugar. Equivalent amounts of sucrose and a 50/50 mixture of glucose and fructose were fed to rats for 16 weeks. Sugar and a 50/50 mixture of glucose and fructose are each half glucose and half fructose. The only difference is the presence of the molecular bond – glucose and fructose are linked in sugar but are unlinked in the 50/50 mixture.
Sucrose and the 50/50 mixture represented 60 percent of the calories throughout the study. Feeding an excessive amount of a nutrient – sucrose or the 50/50 mixture in this case – is a common design used in animal studies to shorten the time where the expected metabolic results can be observed. This design strategy also permits earlier detection of metabolic differences when nutrients – sucrose and the 50/50 mixture in this case – are compared. Another justification for conducting initial feeding experiments with animals is the ability to know exactly what and how much each animal eats.
The international team of scientists compared the effects of sucrose and a 50/50 mixture of free glucose and free fructose on the production of uric acid, triglycerides and fat buildup in the liver. Uric acid was measured because it is a well-recognized indicator of hypertension and its associated maladies. Differences in triglyceride levels were determined since elevated triglycerides is a recognized predictor of increased risk of heart disease. Increased fat buildup in the liver is the leading cause of non-alcoholic liver disease.
The 50/50 mixture of free glucose and free fructose generated meaningfully greater levels of liver uric acid, triglycerides and fat accumulation. For example, liver triglycerides were 1.5-times higher with the 50/50 mixture midway (eight weeks) through the study than they were with sucrose. By the end (sixteen weeks) of the study, liver triglycerides were 170 percent higher with the 50/50 mixture of free glucose and free fructose.
Significance – This study is the first to provide evidence that the molecular bond between glucose and fructose makes sugar biologically distinct from a mixture containing the same relative amounts of unbonded glucose and unbonded fructose. These findings suggest the presence (sugar) or absence (50/50 mixture) of a molecular bond between glucose and fructose determines metabolic outcome. Molecularly bonded glucose and fructose differ from molecularly free glucose and fructose, even when the relative amounts of glucose and fructose are the same.
Citation: “Comparison of free fructose and glucose to sucrose in the ability to cause fatty liver.” LG Sanchez-Lozada, W Mu, C Roncal, YY Sautin, et al. European Journal of Nutrition, January 2010