CARBOHYDRATE MALABSORPTION,
INCLUDING LACTOSE
MALABSORPTION
Carbohydrate malabsorption is a frequent clinical condition caused by fermentation of unabsorbed carbohydrates by colonic flora and giving rise to symptoms. Although lactose is the most commonly malabsorbed sugar, other carbohydrates, including oligosaccharides, disaccharides, and monosaccharides such as fructose, can cause symptoms related to malabsorption.
Lactose
intolerance refers to the development of abdominal pain,
flatulence, nausea, bloating, and diarrhea after ingestion of lactose, whereas malabsorption
refers to the inefficient digestion of lactose, a disorder that might not
cause symptoms. Lactose is the main source of sugar from milk and milk
products from all mammals except the sea lion. It is hydrolyzed by lactase, an
intestinal brush-border enzyme that cleaves lactose into glucose and galactose.
Intestinal
lactase activity is maximal at birth but starts to decline after 2 years of
age, a process that might help weaning. Most people in the world (70%) have low
lactase activity after childhood (lactase nonpersistence). The
proportion is higher in Native Americans and some populations of Southeast Asia
and Africa, where it reaches 90% to 95%. Low activity should be distinguished
from congenital lactase deficiency, a rare autosomal recessive disease
that affects infants from birth, and acquired lactose intolerance, which
is a consequence of loss of intestinal brush-border enzyme activity associated
with infectious enteritis or celiac disease. In approximately 30% of the population,
the lactase activity level does not decline and continues at the maximal
neonatal level well into adulthood (lactase persistence). This occurs
mainly in people of Northern European descent and may indicate a natural
selection in those populations who relied on mammalian milk in times of poor
harvest. Individuals with lactase nonpersistence are able to tolerate low doses
of lactose. Studies indicate that symptoms are likely to be negligible if
lactose intake is limited to 12.5 g, equivalent to 240 mL of milk per day.
The
average adult with a western diet consumes approximately 15 g of lactose per
day.
Lactose
that is not absorbed by the small intestine rapidly reaches the colon and is
fermented to short-chain fatty acids and hydrogen gas, which are responsible
for the ensuing symptoms. Symptoms, however, are variable and may depend on the
fat content of the food, the intestinal transit time, and the composition of
the colonic flora. Furthermore, short-chain fatty acids can be used up as an
energy source by colonocytes, and this is one mechanism whereby
lactose-deficient individuals adapt to ingestion of lactose.
The
presence of lactose intolerance is usually suggested by the patient’s history;
however, testing may be necessary to confirm the diagnosis. Lactose tolerance
testing and breath testing after ingestion of a standard dose of lactose are
easy, noninvasive tests of lactose malabsorption.
Complete
restriction of foods containing lactose may be recommended for a short period
of time to alleviate symptoms and confirm the diagnosis. After this period, however, strict elimination may not be
necessary, and small quantities may slowly be reintroduced. Preparations
containing bacterial or yeast β-galactosidases are
commercially available and can be added to foods containing lactose or ingested
with meals to prevent symptoms. Products containing
predigested milk or other dairy substances or nondairy milk products such as
almond milk are possible alternatives. Live culture yogurt,
which contains endogenous β-galactosidase,
is well tolerated and can be a good source of calcium.
Avoidance of dairy products has been associated with low calcium and vitamin D
levels; therefore, these levels should be monitored and properly supplemented.
Fructose
Malabsorption
One fourth of the general population is estimated to suffer from varying degrees of fructose malabsorption. This should not be confused with hereditary fructose intolerance, in which infants develop hypoglycemia owing to a deficiency of the enzyme fructose-1-phosphate aldolase. Fructose is naturally present in fruits such as apples but can also be produced from corn and is a major ingredient in high-fructose corn syrup. Fructose is not efficiently absorbed, as glucose or galactose is, and the capacity of the gut to absorb fructose can be easily over-whelmed when large quantities are ingested. Unabsorbed fructose can reach the colon, where fermentation by colonic bacteria leads to abdominal symptoms. Fructose malabsorption seems more common in patients with functional bowel disease and can be present in up to 80% of cases. Avoidance of fructose from the diet or even reduction of intake eliminates symptoms.
