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Medical Author:

Siamak T. Nabili, MD, MPH

Siamak T. Nabili, MD, MPH

Dr. Nabili received his undergraduate degree from the University of California, San Diego (UCSD), majoring in chemistry and biochemistry. He then completed his graduate degree at the University of California, Los Angeles (UCLA). His graduate training included a specialized fellowship in public health where his research focused on environmental health and health-care delivery and management.

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Medical Editor:

Bhupinder Anand, MD

It is important to recognize that deficiency in the lactase enzyme does not necessarily translate into lactose intolerance. Many people with mild degrees of lactase deficiency have no symptoms and are able tolerate lactose ingestion. On the other hand, people with severe lactase deficiency may have symptoms even with minimal amount of lactose ingestion.

The amount of lactose in the diet and the difference in the make up of bacteria in the colon are other factors that determine the variability and severity of symptoms in some individuals.

Symptoms of lactose intolerance include:

• bloating
• abdominal pain
• diarrhea
• flatulence (passing gas)
• nausea

The production of gas (flatus) is the result of the activity of bacteria in the large intestine (colon). As the large lactose molecule passes unchanged through the small intestines, it is metabolized by the bacteria that are normally present in the colon. As a result, certain gases, such as hydrogen, are produced and are released from the rectum.

Additionally, a portion of the lactose reaching the colon does not get metabolized by the bacteria. Because these larger molecules are accompanied with an increase in secretion of water through osmosis, this results in the passage of loose stools and diarrhea.

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