Why You Used to Drink Milk and Now It Upsets You — Medical Explanation for Lactose Intolerance
“When you were a kid you drank milk without any problems… and now it doesn’t sit well?” This is one of the most common questions in the clinic, and it has a very specific medical explanation.
What Is Lactose Intolerance?
Lactose is the natural sugar found in milk and dairy products. To digest it properly, the body needs an enzyme called lactase, which is produced in the small intestine.
When there isn’t enough lactase, lactose isn’t digested well. It then moves into the gut where bacteria ferment it, causing:
Bloating
Gas
Abdominal pain
Diarrhea
Feeling heavy or uncomfortable after eating dairy
Why Didn’t It Happen Before, But It Happens Now?
As we get older, lactase production naturally decreases in many people — this is completely normal.
Lactase levels can also drop temporarily if the intestine becomes inflamed or damaged — conditions that are common in people with SIBO, gastritis, enteritis, long-term stress, very restrictive diets, or after taking antibiotics.
So even if you tolerated milk in the past, your digestive system might now struggle to break it down.
Not Everyone Reacts the Same Way
Your symptoms depend not just on lactose but on:
The amount of dairy you eat
Your personal level of lactose tolerance
The sensitivity of your gut
The health of your microbiome (gut bacteria)
This means some people can handle a small coffee with milk but not a full glass, or they tolerate certain dairy foods better than others.
Important: It Doesn’t Always Mean Avoiding All Dairy Forever
Having lactose intolerance doesn’t automatically mean you must eliminate all dairy forever. Many people:
Tolerate small amounts
Do well with fermented dairy like yogurt or aged cheese
Can use lactose-free products
Improve as the underlying gut condition (like SIBO) gets treated
Cutting out all dairy without guidance can lead to unnecessarily restrictive diets and nutrient deficiencies.
When to Get Checked
If your symptoms happen often, are strong, or affect your life, it’s a good idea to talk to a healthcare provider and evaluate what’s really going on — not all dairy sensitivity is lactose intolerance, and not all digestive discomfort is caused by the same thing.