Glycerol, also known as glycerin or glycerine, is a simple sugar alcohol. It is a colorless, odorless, sweet-tasting, and viscous liquid. As a polyol compound, it is highly miscible with water and attracts moisture. Glycerol intolerance is a negative reaction to the substance, distinct from an allergy, as it relates to a metabolic difficulty in processing the compound rather than an immune response. Glycerol is used extensively as a food additive (E422), in pharmaceuticals, and in cosmetics, making exposure widespread and identifying the source of an intolerance challenging.
Glycerol’s Function in the Body and Diet
Glycerol occurs naturally in the body as the backbone of triglycerides, the main components of fats and oils. When stored fat is broken down for energy, glycerol and fatty acids are released into the bloodstream. The liver converts this glycerol into glucose through gluconeogenesis, or it can be used directly for energy via glycolysis.
In the food industry, glycerol is widely used and labeled as the food additive E422. It functions as a humectant, solvent, bulking agent, and sweetener. Common food sources include processed foods, low-fat products, confectionery, baked goods, and sugar substitutes. Beyond food, it is a common ingredient in:
- Cough syrups
- Elixirs
- Eye drops
- Toothpaste
- Various skin care products
Recognizing the Signs of Intolerance
Gastrointestinal distress is a primary symptom of glycerol intolerance, particularly with excessive consumption. Symptoms include flatulence, abdominal pain, and diarrhea. This digestive discomfort is attributed to the osmotic effects of glycerol, which draws water into the intestines.
More serious reactions, often seen in cases of metabolic disorders or high-dose exposure, involve systemic and neurological symptoms. These manifest as confusion, lethargy, sweating, and irritability. In rare and severe instances, especially in young children consuming large amounts, the intolerance can trigger hypoglycemia (low blood sugar), potentially leading to seizures or loss of consciousness. Symptom severity correlates with the amount of glycerol ingested, ranging from mild digestive upset to metabolic crisis.
Underlying Causes and Diagnostic Procedures
Inborn Errors of Metabolism
The definitive causes of glycerol intolerance relate to inborn errors of metabolism, which impair the body’s ability to break down the compound. The primary metabolic pathway involves the enzyme Glycerol Kinase (GK), which converts glycerol into glycerol-3-phosphate. A deficiency in this enzyme, known as Glycerol Kinase Deficiency (GKD), is an X-linked recessive disorder. This deficiency results in an accumulation of glycerol in the blood (hyperglycerolemia) and urine (glyceroluria).
GKD presents in several forms, ranging from a benign adult form with no symptoms to a severe infantile form. The severe form can be part of a complex genetic syndrome involving conditions like Duchenne muscular dystrophy. Another related condition is Glycerol Intolerance Syndrome (GIS), sometimes associated with a deficiency in the enzyme fructose-1,6-bisphosphatase (FDPase). GIS symptoms are precipitated by glycerol ingestion and include hypoglycemia and metabolic acidosis.
Diagnosis
Diagnosing a glycerol metabolism disorder begins with reviewing the patient’s history, noting episodic symptoms such as vomiting, lethargy, or acidemia following glycerol exposure. Laboratory testing detects elevated glycerol levels in the blood and urine. Confirmation of GKD is achieved through enzyme assays to measure GK activity in cells like leukocytes or fibroblasts, or through genetic testing to identify a mutation or deletion in the GK gene on the X chromosome. For less clear-cut cases, a physician may recommend a controlled elimination diet to see if symptoms resolve when glycerol sources are removed.
Treatment and Dietary Management Strategies
The primary strategy for managing glycerol intolerance, especially in metabolic disorders like GKD or GIS, is the strict avoidance of glycerol in the diet and medications. This requires careful attention to ingredient labels, as glycerol is often listed under various names. Consumers must look for “glycerin,” “glycerine,” or the food additive code “E422” on packaged foods and beverages.
Patients must also be cautious of pharmaceutical and personal care products, where glycerol is used as a solvent, sweetener, and humectant in items like cough syrups, lozenges, and topical creams. Working with a registered dietitian is highly beneficial to identify hidden sources of glycerol and ensure the restricted diet remains nutritionally balanced.
For those with GKD, dietary management focuses on avoiding prolonged fasting, which exacerbates hypoglycemic episodes. Consuming frequent, carbohydrate-rich meals, particularly those with a low glycemic index, helps stabilize blood sugar levels. During an acute metabolic crisis, immediate medical treatment involves the intravenous infusion of glucose to correct hypoglycemia and supportive care for associated symptoms.