Coeliac disease is a condition where the lining of the small intestine (the gut) is damaged. The small intestine is the site for digestion of food and absorption of nutrients.
Digestion is the process of breaking down food into its constituent components.
Absorption occurs when the nutrients are carried across the lining of the intestine and into the blood stream.
If the small intestine is damaged then the body may not absorb sufficient nutrients. This can result in weight loss and vitamin deficiencies. Similarly, fluid that is not absorbed passes through the intestine and causes diarrhoea.