Sugar intolerance, medically known as carbohydrate malabsorption, can develop later in life. This condition is a digestive issue, not a true food allergy involving an immune system reaction. Intolerance occurs when the body lacks the specific enzymes or transport proteins needed to break down certain sugars in the small intestine. Since the immune system is not involved, symptoms are confined to the digestive tract and are not life-threatening. The late onset of symptoms often relates to genetic changes or accumulated damage to the intestinal lining.
Common Forms of Sugar Intolerance
Lactose intolerance is one of the most common sugar intolerances affecting adults. This involves the sugar found in milk, which requires the enzyme lactase to be broken down into absorbable glucose and galactose. Fructose malabsorption is another common issue, where the small intestine has a reduced ability to absorb fructose. This usually results from a deficiency in the specialized transport protein, GLUT5, which moves fructose across the intestinal wall.
The sucrase-isomaltase enzyme complex handles the digestion of table sugar (sucrose) and starches. A deficiency in this complex causes sucrase-isomaltase deficiency, leading to digestive distress. Although often present from birth, symptoms may not be recognized until later in life. In all forms of intolerance, undigested sugar passes into the colon, where bacterial fermentation causes characteristic symptoms.
Mechanisms of Late-Onset Intolerance
The development of sugar intolerance in adulthood is driven by two distinct physiological mechanisms.
Primary Adult-Type Hypolactasia
The most frequent cause is Primary Adult-Type Hypolactasia, a genetically programmed decrease in lactase enzyme production. This is the ancestral state for most of the world’s population, where lactase production naturally slows or stops after childhood weaning. The down-regulation of lactase is controlled by specific genetic variants near the LCT gene. For individuals with lactase non-persistence, enzyme activity declines after age five, and symptoms become noticeable in adolescence or adulthood as levels drop significantly. This gradual, genetically determined decline explains why many adults suddenly cannot tolerate dairy products.
Secondary Intolerances
A second mechanism involves Secondary Intolerances, which result from damage to the small intestinal lining where digestive enzymes reside. Any condition causing inflammation or injury to the small intestine can temporarily or permanently reduce the production of lactase, sucrase, or other transport proteins. Examples include acute gastroenteritis, or chronic conditions like Celiac disease and Crohn’s disease. Small Intestinal Bacterial Overgrowth (SIBO) can also cause secondary sugar intolerance, as excess bacteria can damage the lining or inactivate digestive enzymes.
Recognizing Symptoms and Diagnostic Testing
Symptoms of sugar intolerance are primarily gastrointestinal and begin shortly after consuming the offending sugar. These include abdominal bloating, excessive gas, and cramping, as unabsorbed sugar draws water into the intestine. Diarrhea is also common, resulting from the osmotic effect of the sugar and the byproducts of bacterial fermentation. Symptom severity is usually proportional to the amount of the specific sugar consumed.
The gold standard for diagnosis is the non-invasive Hydrogen/Methane Breath Test. The patient ingests a specific sugar substrate, such as lactose or fructose. If the sugar is not digested, gut bacteria ferment it in the colon, producing hydrogen and methane gases. These gases are absorbed into the bloodstream and exhaled through the lungs. Measurements are taken over two to three hours, and a significant rise in gas levels indicates malabsorption.
Navigating Life with a Sugar Intolerance
The primary strategy for managing sugar intolerance is reducing or avoiding the specific carbohydrate that triggers symptoms. This requires careful attention to food labels, as sugars like lactose and high-fructose corn syrup are often hidden in processed foods, seasonings, and medications. Many individuals can tolerate a small threshold amount of the sugar, making complete avoidance unnecessary.
Over-the-counter enzyme supplements help manage accidental ingestion or permit occasional consumption. For lactose intolerance, lactase enzyme pills are taken immediately before eating dairy. Enzyme blends containing invertase are available for those with sucrose malabsorption.
When managing lactose intolerance, ensure adequate intake of calcium and Vitamin D, nutrients primarily sourced from dairy. Choosing fortified non-dairy milks, juices, and cereals helps maintain bone health. Hard cheeses and yogurt with live active cultures are often better tolerated because fermentation naturally reduces their lactose content.