A 24-year-old African-American female presents with complaints of
intestinal bloating, gas, cramps, and diarrhea following a meal includ-
ing dairy products. A lactose-tolerance test confirms your suspicion
that she had a deficiency of lactase in her intestine. Which of the fol-
lowing dairy products could you recommend that would be least likely
to cause her difficulties in the future?
A. Condensed milk
B. Cottage cheese
C. Ice cream
D. Skim milk
A nsw ers
E. The patient has normal levels of blood insulin and exhibits a nor-
mal glucose tolerance test. This indicates that glucose absorption
from the intestine is normal as is clearance of glucose from the blood.
The presence of glucose in the urine is most likely a kidney problem.
Because the defect seems to involve only D-glucose and no other
sugar, this points to a transporter with high specificity. The kidney
has the GLUT 2, SGLT1, and SGLT2 transporters. GLUT 2 and
SGLT1 are present in other tissues, and a defect in these would be
expected to result in more serious sequelae. SGLT2 is a sodium-
dependent glucose transporter specific to the kidney that has a high
specificity for glucose. The glucose is present in the urine because of
a failure to reabsorb it as a consequence of a defect in SGLT2. This
leads to a loss of water also, because it is reabsorbed with glucose.
B. Because the patient’s liver enzymes are normal and her symptoms
seem to correlate with her intake of fruit juices, most likely her prob-
lem stems from an inability to absorb fructose. Since removal of
cow’s milk from her diet did not eliminate the problem, a lactase defi-
ciency can be ruled out. GLUT 5 is the primary transporter of fruc-
tose in the intestine and a deficiency in this transporter would lead to
an inability to absorb fructose in the gut, making it a substrate for
bacterial metabolism that produces various gases, including hydrogen,
as well as organic acids.
E. The microorganisms that convert milk to yogurt
Lactobacillus delbrueckii bulgaricus)
metabolize most of the lactose in the milk, thus removing the source
of this patient’s intestinal disquietude. Yogurt is also a good source of