Lactose intolerance

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Lactose intolerance


Peritoneum and peritoneal cavity disorders




Lactose intolerance


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USMLE® Step 1 questions

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High Yield Notes

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Lactose intolerance

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USMLE® Step 1 style questions USMLE

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A 27-year-old woman comes to her primary care physician for evaluation of bloating and flatulence. The patient’s symptoms have been ongoing for several months and tend to occur after she consumes ice cream or milk. The patient has not lost weight or noticed blood in her stool since symptom onset. Family history is notable for Crohn disease in the patient’s mother. Temperature is 37.0°C (98.6°F), blood pressure is 112/72 mmHg, pulse is 71/min, and respiratory rate is 14/min. Physical examination is unremarkable. Fecal occult blood test is negative. The patient is scheduled for a hydrogen breath test, which returns positive. Which of the following best describes the underlying cause of this patient’s symptoms?  

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lactase deficiency p. 79

lactose intolerance p. 390

Lactase deficiency p. 79

Lactose intolerance p. 390


Content Reviewers

The three most common forms of sugar are glucose, fructose, and galactose, and these are all types of monosaccharides, meaning they’re made of just one sugar molecule, molecules like this are called carbohydrates, because they’re made up of carbon, hydrogen, and oxygen, usually with a hydrogen-oxygen ratio of 2:1.

If you link two of these guys together, you get a disaccharide because “di” means two, and this is also a carbohydrate.

Now our body uses these sugar molecules for energy, right?

For us humans, glucose is our gasoline, our energy source, we’ll take galactose and fructose...but ultimately we need to use glucose, so almost all the fructose and galactose we ingest is converted to glucose, and then we use that glucose for energy.

Alright, but usually carbohydrates aren’t in monosaccharide form when we ingest them, and a lot of what we take in are in the disaccharide form, and one notorious disaccharide that tends to cause serious gastrointestinal distress for a lot of people, is lactose.

Lactose is a disaccharide that’s made up of a glucose molecule and a galactose molecule.

For us to use it as energy, though, we have to first break it down to those two monosaccharides.

In the milk of most mammals, lactose is generally the major carbohydrate, so when you have a glass of milk, and it gets through your stomach to the small intestine, that lactose gets chopped into glucose and galactose by an enzyme that’s fittingly called lactase.

The gene responsible for production of the lactase enzyme is expressed exclusively in the enterocytes lining the small intestine, which are cells that help absorb nutrients from stuff that we eat.

Once produced, the enzyme makes it’s way to the cell’s surface along the cell’s microvilli, these little tentacles that help increase surface area and absorb nutrients.

K, once lactose gets chopped by lactase, we’re good to go, and we absorb the glucose and galactose and all is well.

Now, as mammals, we’re wired to be able to ingest milk after birth, right?

So it makes sense that when we’re young we have a whole bunch of lactase enzyme, since that’s pretty much all we drink.

After weaning, in most mammal species, expression of the gene responsible for lactase is way down-regulated, and so production of lactase also goes way down.

The majority of humans actually follow this protocol as well, and down-regulate lactase production around 3-5 years of age.

Interestingly, though, the majority of caucasians, mainly those from northern european background, continue to have elevated lactase activity all the way into adulthood, and so they exhibit “lactase persistence”.


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  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
  4. "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
  5. "Milk Intolerance and Microbe-Containing Dairy Foods" Journal of Dairy Science (1987)
  6. "Genetics of Lactase Persistence and Lactose Intolerance" Annual Review of Genetics (2003)
  7. "Lactose Intolerance" Asia Pacific Journal of Clinical Nutrition (2015)

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