oranges / lemons are a kind of food & eating oranges / lemons can prevent scurvy ->  eating a kind of food can prevent scurvy  eating a kind of food can prevent scurvy & scurvy is a kind of disease ->  eating a kind of food can prevent a disease  eating a kind of food can prevent a disease & eating / digestion is when an organism takes in nutrients from food into itself by eating ->  taking in nutrients by eating a kind of food can prevent a disease  taking in nutrients by eating a kind of food can prevent a disease & if something can prevent a something else then that something else is probably caused by a deficiency of that something -> if a disease can be prevented by eating a certain kind of food then that disease is probably caused by a nutritional deficiency