What does it mean to be lactose-intolerant?

Being lactose-intolerant refers to a condition in which an individual has a deficiency of the enzyme lactase. This enzyme is produced in the small intestine and is crucial for breaking down lactose, which is the sugar found in milk and dairy products. When a person lacks sufficient levels of lactase, they cannot adequately digest lactose, leading to various gastrointestinal symptoms when they consume dairy. These symptoms can include bloating, gas, diarrhea, and abdominal pain.

The other options point to different conditions not related to lactose intolerance. For instance, inability to digest proteins and inability to absorb fats refer to issues with different types of macromolecules and enzymes. An allergic reaction to milk involves the immune system and typically pertains to proteins in milk (like casein), rather than the sugar lactose itself. Therefore, the definition centered on lactase deficiency is accurate and directly addresses the essence of lactose intolerance.