Our bodies have a complex and relatively delicate system to process carbohydrates and maintain the proper blood-glucose levels. These systems are used to process protein, fat and, primarily, complex carbohydrates. Man has the ability to withstand constant onslaughts by dangerous organisms, pollution, injuries, etc. if our protective mechanisms are up to the task. This depends on what we eat, the raw materials of life. In addition, nutrition is often looked upon as being much simpler than it truly is.
Many people discuss nutrition as analogous to gas for a car.
This is true only as food is our source of energy, admittedly the most important aspect of eating. But nutrition plays other roles as well and it is in these roles that the modern diet is so lacking. Instead of a car, let's think of our bodies more like a cruise ship and instead of gas, let's think of food as everything that is taken on-board ship, except the people. This would include the fuel, of course. But also, all the food for the passengers and crew, all the spare parts, all the cleaning supplies, the dishes and utensils, all the towels and room amenities, and on and on.