Your body is a car. The food is petrol (gas) and the fluids under the hood are the vitamins/minerals.
If you put nothing but gas in the car, the car will run. It could be a long time, in fact, before you run into any problems. Some cars last for 15,000 miles with no problems on pure gas, some do not make it past 5,000. If you don't take it into the shop for inspections (going to the doctor for physicals), and you never change or top off any fluids under the hood (eat properly), you will first start noticing small problems such as annoying noises, performance loss, and delayed ignition. Gradually, the brakes begin to wear out, your engine is tearing itself apart, and your radiator cannot keep up it's functions. After long enough, something major will give out and you'll be stuck with a dead car.
Also, if you DO put all those fluids in, get it taken care of, and properly upkeep it....that doesn't mean it won't have problems. If you let it sit in the garage for weeks on end and never run it, the fluids goo up and the gas turns to turpentine.
Even then, small things can happen to the best managed cars. It's all about risk management and preventive maintenance.