Complex sentence: Gluten, a mixture of proteins present in wheat, rye, barley and all their species and hybrids, is toxic for a small, but growing proportion of susceptible people, in whom it may trigger coeliac disease.
Split into simpler sentences: Gluten, a mixture of proteins present in wheat, rye, barley and all their species and hybrids. It is toxic for a small proportion of people, in whom it may trigger coeliac disease.