Health is a broad term and difficult to define. This is partly because it includes a wide range of functions and conditions of the human body, and because so many people would choose to assign various aspects of their choosing to that definition.
For one person, health means living long and happily. For another, health may mean being able to run far or fast, while yet another may focus on physical strength or freedom from illness or disease.
For me, health is going to be a condition of physical fitness and lack of disease or illness so that I can live as long and as happily as possible while remaining able to enjoy most of the things which interest or attract me.
To accomplish this, I must be phycically fit, i.e. exercise, eat properly, and get the rest I need. These are the minimums that I can accept in order to achieve my definition of "health". There are many other things which can be included, such as having relationships which can contribute to a healthy condition. It has been shown statistically, for example, that married people, happily married or not, tend to live longer and have fewer health problems than those who are not married.
Even unmarried people with a wide range of good relationships or family ties, and even sometimes people with pets, tend to be "healthier" than those without.
However, these tend to be indirect factors, and the big three are still diet, exercise, and rest.
No single factor is necessarily THE final factor, although doing without food can result in il-health and death sooner than doing without exercise or appropriate rest.
On the other hand, eating the wrong foods, or too much of any food, can easily override the health benefits of the other factors.
For the body to function properly at any time, it requires a wide range of nutrients. The intake of the appropriate levels of vitamins, minerals, proteins, carbohydrates, fats, and other nutritional factors are of utmost importance to naintaining health and fitness and can even cause a regression in the event that the nutritional level is neglected.
The analogy has often been made of the car which is made to run on gas with water in it, or on some other fuel which it was not designed to handle. The day-to-day performance will suffer, and, eventually, the entire mechanism will give out.
If you would be healthy and physically fit, see to the diet you choose to eat and make sure you are getting the appropriate levels of the necessary nutrients.