There are several reasons why dairy does not agree with some people; 1) Lactose intolerance, 2) Dairy allergy (IgE), and 3) Dairy sensitivity (IgG).
Lactose, also known as “milk sugar,” is the primary carbohydrate in milk products. During the digestion process, lactose is broken down into glucose and galactose for proper absorption. This step occurs in the small intestine with the assistance of an enzyme known as Lactase. Many people have or develop a shortage of lactase, and therefore are unable to properly digest some or all of the lactose they consume. The unabsorbed lactose passes into the colon where it causes water to rush into the intestine (e.g. loose stools). Also, the undigested lactose gets fed on by the gut bacteria causing gas, bloating, and gastrointestinal discomfort. This lactase deficiency and any resulting gastrointestinal symptoms, are what is typically referred to as lactose intolerance.
Dairy Allergy & Dairy Sensitivity
A food allergy and food sensitivity is identified as an abnormal and heightened response of the immune system (IgE or IgG) to certain components (most notably proteins) within a food. In milk, the two leading allergy/sensitivity offenders are the milk proteins known as casein and whey. Casein is the curd that forms when milk is left to sour. Whey is the watery part that is left after the curd is removed.