Complex sentence: Castle then showed that tropical sprue was caused by intestinal impermeability to this and other hematopoietic factors in food and In related studies he defined the need for iron for the bone marrow to make hemoglobin.
Split into simpler sentences: Castle then showed that tropical sprue was caused by intestinal impermeability to this and other hematopoietic factors in food. In closely related studies Castle defined the need for iron for the bone marrow to make hemoglobin.