Lactose intolerance

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Introduction0:00–0:32

I Lactose intolerance is a condition where the body has difficulty digesting lactose, which is the main sugar found in milk and dairy products.
This happens because of a deficiency in lactase, which is the enzyme responsible for breaking down lactose. Now when we drink milk or eat dairy products, lactose begins its journey through the digestive system and eventually reaches the small intestine.
But there's a problem. Lactose is a disaccharide.
It's too big and complex for the body to absorb as it is. So in order to absorb it into the bloodstream, first we need to cut it into smaller, simpler sugars called monosaccharides.

Physiology0:32–1:30

That's where lactase comes in. Lactase is a brush border enzyme that sits right on the surface of the intestinal cells called enterocytes on their tiny finger-like projections called microvilli.
When lactose reaches enterocytes, lactase acts as a pair of scissors, snipping lactose into glucose and galactose. The body then absorbs these two monosaccharides into the bloodstream.
How much lactase a person produces depends on their genes. The instructions for making lactase come from the LCT gene on chromosome 2.
In infancy, this gene is highly active because milk is the main source of nutrition. After weaning, most individuals are genetically programmed to gradually reduce lactase production, as milk is no longer a major part of their diet.

Lactose metabolism1:30–2:26

Over time, this leads to declining lactase levels. At this point, you might be thinking, if lactase production drops after childhood, how can adults still digest dairy products at all?
The answer is genetics. In certain populations, particularly those with a long history of dairy consumption, genetic variants allow lactase persistence into adulthood.
In lactose intolerance, there isn't enough lactase in the intestines, so a person can't break it down into simple monosaccharides.
In other words, the underlying cause of lactose intolerance is lactase deficiency. Based on the underlying cause, lactase deficiency is subdivided into four main types.

Lactase persistence phenomena2:26–3:10

The most common type is primary lactase deficiency, also known as lactase nonpersistence. From an evolutionary standpoint, this is actually the normal state.
After weaning, when milk is no longer a major part of the human diet, humans are genetically programmed to gradually reduce lactose production.
Over time, the enzyme levels drop, and the ability to digest lactose decreases. Secondary lactase deficiency occurs when something causes damage to the lining of the small intestine, destroying the enterocytes, along with their microvilli and brush border enzymes.

Pathology3:10–4:12

Common causes include gastrointestinal infections, celiac disease, Crohn's disease, and small intestinal bacterial overgrowth.
Next, there's congenital lactase deficiency, a rare autosomal recessive condition caused by mutations in the LCT gene itself.
Because of this mutation, infants are born with little to no functional lactase, impairing lactose digestion from their very first feeds.
Finally, there's developmental lactase deficiency, which is seen in premature infants born before 34 weeks of gestation.
Because lactase production ramps up late in fetal development, preterm babies may have temporarily insufficient lactase.
Now, moving on to clinical manifestations, imagine someone enjoying their favorite vanilla ice cream. Everything feels perfectly fine, so at first no symptoms at all.
But about 30 minutes to 2 hours later things start going downhill. That's because lactose doesn't cause symptoms right away.

Causes4:12–8:51

It takes time. Regardless of the type of lactase deficiency, the outcome is the same.
Without enough lactase, lactose never gets broken down and remains undigested in the small intestine. And because lactose is osmotically active, it draws water into the intestinal lumen, resulting in watery stools.
And the story doesn't end there. The undigested lactose then passes into the colon where billions of bacteria live.
These bacteria break lactose down by fermentation into short chain fatty acids. As a byproduct, the bacteria produces gasses like hydrogen, carbon dioxide, and methane.
As gas builds up, symptoms appear, including bloating, excessive flatulence, and abdominal cramping. However, keep in mind that the severity of symptoms is not always the same.
Severeity depends on two things. First, how much lactose a person consumes, and second, how much lactase their intestine still produces.
That's why many individuals with lactose intolerance can tolerate small amounts of lactose without significant symptoms.
Once the lactose moves through the digestive system, the symptoms fade. Next up is diagnosis, which primarily relies on history.
If symptoms consistently occur after consuming dairy products and resolve with lactose elimination, that's a strong clue.
No lactose coming in means no osmotic pool of water, so symptoms improve. That's why a two-week trial of a lactose-free diet can help confirm the suspicion.
If uncertainty remains, the hydrogen breath test can help confirm the diagnosis. In this test, after fasting, a person drinks a solution containing lactose.
Then every 30 minutes for up to 4 hours, they breathe into a collection device. In lactase deficiency, gasses produced from bacterial fermentation don't just stay in the intestines, they diffuse into the bloodstream, travel into the lungs, and get exhaled.
A significant rise in hydrogen or methane in the breath confirms lactose intolerance. This test is extremely difficult to perform in infants, so instead we examine the stool.
High levels of short chain fatty acids make the stool acidic, which is why infants with lactase deficiency often have low stool pH.
Also, acidic stool can irritate the perianal area, resulting in skin irritation. Finally, the key question becomes why lactase is low.
If the symptoms persist despite dietary changes, think of secondary lactase deficiency. In those cases, an intestinal biopsy can help clarify the underlying cause.
In primary lactase deficiency, intestinal histology is normal because the problem is reduced enzyme activity, not structural damage.
In contrast, secondary lactase deficiency often shows mucosal damage, reflecting injury to the brush border where lactase is normally located.
Treatment for lactose intolerance usually starts with cutting back on foods that contain lactose. People often feel better once they reduce or avoid these foods.
Some still want to enjoy dairy, and that's where lactose supplements come in. For example, they can take a tablet before eating or drinking something with lactose, which helps their body break it down.
All right, as a quick recap, lactose intolerance happens. When there isn't enough lactase to properly digest lactose.
The most common cause is primary lactase deficiency. A normal drop in lactase production after weaning.
When lactose isn't broken down, it pulls water into the intestine, leading to watery diarrhea. On top of that, undigested lactose then moves into the colon.
Where bacteria ferment it, producing short chain fatty acids and gasses. As gas builds up, it causes bloating, excessive flatulence, and abdominal cramping.