You Are What You Eat
You really are what you eat. You are also very much affected by the lifestyle you live. Every item that enters your mouth will be digested and reconstructed into your living cells for use. Although it is true that everything is broken down into general building blocks necessary for life (such as sugar, fat, proteins, etc), the quality of what you eat really affects how healthy each of your cells are.
A diet high in processed foods, sugars, fats, chemicals, additives, preservatives, and low quality nutrients is harder for your body to process and leads to the building of lower quality cells. If you rarely exercise, experience high stress, do not sleep well or enough, and do not partake in favorite leisure activities, this also stresses the body and reduces its effectiveness in maintaining balance and health.
It can be difficult in today's world to fit in all that is conducive to an optimal life. Missing one or two steps occasionally will generally not be a problem, as your body is very resilient and made to work under less than optimal conditions for a time. Many people take advantage of this, however, and conclude that since they live the way they do and no signs of illness have appeared thus far, that they are living well enough. It can take days, months, or years for various signs of body 'dis-ease' to begin to appear. By the time these signs appear, it can take just as long to reverse the damage, if reversal is possible.
The key is prevention.