Complex sentence: He contends it can impair digestion and nutrient absorption, cause sugar intolerance, provide a haven for pathogens and parasites, cause diarrhea, and promote the development of bowel cancer, allergies and skin conditions.
Split into simpler sentences: He contends it can impair digestion and nutrient absorption, cause sugar intolerance, provide a haven for pathogens and parasites and cause diarrhea. In addition the condition is alleged to promote the development of bowel cancer, allergies and skin conditions.