Food intolerance is an adverse reaction to common foods, does not occur rapidly and so the link to food is not obvious. Food intolerances are typically related to a lack of a specific enzyme to break down the food in question. Symptoms of intolerance vary widely: one person may have diarrhoea or constipation and fatigue, another rheumatoid arthritis, palpitations, and headache. Food intolerances tend to generate fewer symptoms that are slower in onset, and can therefore be more difficult to diagnose.
Casein is the main protein in raw milk. Gluten is a protein found in wheat, barley and rye. Casein and gluten have a similar molecular structure. Some people are allergic or intolerant to one or both; others have an inherited condition called celiac disease which pre-disposes to food intolerance.