We are what we eat. While trivially true in the sense that the matter which makes up our food will become integrated into our bodies, what we eat has also has a deep relationship with our mental and spiritual well-being.
Our minds, spirits, and bodies are linked together. When we consume things which are harmful to our physical health, it is inevitable that the rest of us will suffer as well. These consequences can be long term, as there are many things which when consumed will act as a slow poison in our bodies.
A proper diet is well balanced. But what does balance mean in this case? Generally speaking, eating has two physiological purposes. The first is to provide energy for our body. Our digestive system breaks down large molecules into smaller ones, and uses the energy released by these chemical reactions to fuel our biological processes. However, the body is poorly equipped to handle excess. While there are three major macronutrients (Fats, Carbohydrates, and Proteins), your body only has one option for long term reserves: fat. Unfortunately, even if you eat only carbs and proteins, you can still see an increase in body weight, as there are mechanisms in your digestive system which will convert these two into fats.
The second purpose of eating food is to provide essential vitamins and other molecules which our body is unable to produce. These are essentially small molecules which are essential for biological function. Even if you have plenty of calories, you will die if you do not have a source of these essential building blocks used by our bodies.
In order to ensure that your diet is healthy, you should seek to moderate your caloric intake while diversifying your diet. This will ensure that you are both well nourished, and that your body has the energy to fuel your life without building up too much extra physical encumberment.
One final thing to remember is that when taking a holistic approach to our health, we can do well to remember that we too are parts of a greater whole. Our interactions with our environment, and the sources from which we gain our sustenance are inextricably linked to the influence different ideologies and commercial enterprises have on our planet. If we feed systems which damage natural ecosystems and break the natural harmony of the world, we are contributing to an overall decrease of health for the entire planet.