Sorbitol, also known as glucitol, is a sugar alcohol. It possesses a sweet taste and is naturally present in a variety of fruits, including apples, pears, peaches, and prunes. Beyond its natural occurrence, sorbitol is frequently manufactured from potato or corn starch and used as a common artificial sweetener. Its presence is widespread in sugar-free products like chewing gum, candies, and diet foods, where it serves as a humectant, sweetener, and texture modifier.
How Sorbitol is Processed
When ingested, sorbitol is processed in the human body. Unlike simple sugars such as glucose, sorbitol is absorbed more slowly and less completely in the small intestine. This slower absorption means a portion of the consumed sorbitol reaches the large intestine, where it can be fermented by bacteria.
Absorbed sorbitol is primarily metabolized in the liver. Here, an enzyme called sorbitol dehydrogenase (SDH) plays a central role in converting sorbitol into fructose.
Once converted to fructose, this sugar can then be further metabolized. Fructose can be processed into glucose or converted into glycogen, a stored form of energy, for various bodily functions. This pathway allows the body to derive some caloric energy from dietary sorbitol, although its overall caloric contribution is lower than that of sucrose.
The Polyol Pathway and Health Conditions
A distinct metabolic route, known as the polyol pathway, involves the conversion of glucose into sorbitol within the body. This pathway begins with the enzyme aldose reductase (AR), which converts glucose into sorbitol. Under typical physiological conditions, this pathway is a minor contributor to glucose metabolism.
However, in states of elevated glucose levels, such as uncontrolled diabetes, the polyol pathway becomes significantly more active. Aldose reductase has a low affinity for glucose, meaning high concentrations of glucose are required to activate this enzyme and increase its activity. The subsequent enzyme in this pathway is sorbitol dehydrogenase (SDH), which converts sorbitol into fructose.
A significant issue arises in tissues that are permeable to glucose but have low or absent levels of sorbitol dehydrogenase, such as the lens of the eye, retina, kidneys, and nerve cells. In these cells, sorbitol can accumulate because it is produced from glucose but cannot be efficiently converted to fructose or readily exit the cell. This intracellular accumulation of sorbitol leads to osmotic stress, drawing water into the cells and causing swelling.
The overactivation of the polyol pathway also depletes NADPH, a co-factor important for the body’s antioxidant defenses. This depletion impairs the cell’s ability to counteract oxidative stress, contributing to cellular damage. Such damage and osmotic effects are implicated in the development of long-term diabetic complications, including diabetic retinopathy (damage to the retina), nephropathy (kidney damage), and neuropathy (nerve damage).
Sorbitol’s Impact on Digestion and Blood Sugar
Sorbitol’s metabolic properties contribute to its effects on the human body. It has a lower caloric value compared to table sugar, providing approximately two-thirds of the calories. This reduced caloric content, combined with its minimal impact on blood sugar levels, makes it a suitable sweetener for individuals managing diabetes. Sorbitol metabolism does not directly rely on insulin, making it a favorable alternative for those monitoring glucose intake.
The incomplete absorption of sorbitol in the small intestine leads to its slow transit through the digestive system. When consumed in larger quantities, the unabsorbed sorbitol reaches the large intestine, where it can exert an osmotic effect, drawing water into the colon. This increased water content in the bowel contributes to sorbitol’s mild laxative effect, which can manifest as bloating, gas, and diarrhea.
Individual tolerance levels for sorbitol vary considerably; some individuals may experience gastrointestinal discomfort with as little as 10 grams, while others can tolerate higher amounts without symptoms. The Food and Drug Administration (FDA) requires a warning label for foods that could lead to consumption exceeding 50 grams daily, indicating that “excess consumption may have a laxative effect”. Sorbitol should be used as directed to avoid severe digestive side effects and potential electrolyte imbalances.