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Macronutrients in Health and Disease: Carbohydrate Intolerance

Lactose intolerance. Intolerance of certain kinds of carbohydrate is common and may mimic medical disorders. Lactase, the enzyme in the jejunum that cleaves the milk sugar lactose, normally disappears sometime after the age of weaning. Its disappearance is gradual and extremely variable in age of onset. For some, the enzyme disappears in early childhood; in others, it wanes in late adulthood. The non–persistence of lactase was once regarded as an abnormal condition, referred to as lactose intolerance. It is now known to be the biological norm, occurring in the vast majority of individuals. For most Caucasians, however, lactase persists throughout life, a condition referred to as lactase persistence.

After lactase disappearance, the consumption of milk or other lactose–containing products can cause bloating, cramping, diarrhea, and flatulence, which may be mistaken for a number of gastrointestinal diseases.8 Some authorities advise such individuals to limit their consumption of lactose–containing products to small amounts consumed throughout the day, or to use lactase–treated dairy products, soymilk, rice milk, or other substitutes. However, after the age of weaning, there is no nutritional requirement for either milk or milk substitutes, and their inclusion in the diet is based on preference, rather than nutritional needs. 

Sucrase deficiency.  This condition is rare in the general public. It has been reported in adults with renal calculi,9 and one study found sucrase deficiency in 31% of a population of individuals with HIV.10

Fructose malabsorption. Fructose malabsorption can also cause significant gastrointestinal symptoms (eg, bloating, cramps, osmotic diarrhea) that may not respond to medications or surgical interventions.11 However, this condition is only likely to be problematic for individuals consuming concentrated amounts of this sugar (eg, beverages with high–fructose corn syrup), as opposed to those eating reasonable quantities of fruit.12 Malabsorption appears to be a problem mainly with fructose intakes over roughly 25 grams per meal, although in patients with functional bowel disease malabsorption may occur with amounts <15 g.13

Congenital carbohydrate intolerances. Congenital carbohydrate intolerances are rare but life threatening. These conditions include sucrase–maltase deficiency, glucose–galactose malabsorption, and alactasia (a total absence of lactase).14


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