Title: Alternative Sweeteners

Crystalline Fructose 377
A. Absorption
Fructose in the diet occurs either bound covalently to dextrose in the disaccharide,
sucrose, or as the free monosaccharide. Free monosaccharide fructose can originate
from dietary fruits and berries or from sweeteners like honey, high fructose
corn syrup, and crystalline or liquid fructose. Ingested sucrose is hydrolyzed to
fructose and glucose by sucrase enzymes associated with the brush border of the
intestinal epithelium. The resulting monosaccharides are immediately transported
through the brush border membrane by the disaccharidase-related transport system,
without being released to the lumen (19). Free dietary glucose is transported
through the brush border membrane by one or more specific carrier systems. It
is generally considered that fructose is absorbed across the intestinal mucosa of
humans by facilitated diffusion.
Riby et al (20) reported that the capacity for fructose absorption is small
compared with sucrose and glucose. They found that the simultaneous ingestion
of glucose could prevent fructose malabsorption by increasing intestinal absorptive
capacity for fructose. This suggests that the pair of monosaccharides might
be absorbed by the disaccharidase-related transport system as if they were products
of the enzymatic hydrolysis of sucrose. The human intestinal capacity for
fructose absorption in the absence of glucose appears to vary significantly from
one individual to another. It is important to note U. S. dietary consumption data,
which indicate that typical glucose/fructose ratios are more than adequate to support
fructose absorption in the general population (21). Those individuals who
experience symptoms of malabsorption are advised to avoid consumption of
products in which fructose is the sole carbohydrate (20).
B. Metabolism
All dietary fructose is absorbed and transported by the intestinal epithelium into
the hepatic portal vein. The active hepatic enzyme system for metabolizing fructose
efficiently extracts this sugar into the liver, leaving relatively low fructose
concentrations in systemic blood vessels (22). Significantly, its entry into liver
cells and subsequent phosphorylation by fructokinase is insulin independent (23).
After cleavage by liver aldolase, the resulting trioses can be used for gluconeogenesis
and glycogenesis or the synthesis of triglycerides, or they can enter the
glycolytic pathway. The ultimate fate of these trioses depends on the metabolic
state of the individual. Figure 3 illustrates the pathways involved in fructose
metabolism.
C. Fructose and Diabetes
Studies by Crapo, Kolterman, and Olefsky in the early 1980s showed that acute
administration of fructose results in lower glycemic and insulin responses in nor-