I truly believe in a holistic health/wellness approach. Healthy eating habits, healthy sleeping habits, healthy exercise habits, and healthy social life. I think it's all about balance. Too much of good food or too much exercise can be a bad thing, just as too little of those things can be a bad thing as well. You can't be healthy physically if your mind is not healthy.
There is an old Japanese saying: "Yamaiwa kikara," meaning illness starts in the mind. I've started to understand what this means from a physiological standpoint. I often see people who eat just healthy food and avoid eating any "bad" food, and exercise regularly yet they often get sick. One thing I've noticed that those people tend to have a lot of stress in their life (work, family, etc).
There's autonomic nervous system which consists of parasympathetic nervous system and sympathetic nervous system. When we're stressed, parasympathetic nervous system slows down and sympathetic nervous system becomes more active. So they have a yin and yang relationship. When that happens, our digestive system slows down, heart rate and breathing rate go up, immune system slows down. The nervous system is now in "fight/flight" mode. This physiological response prepares us for emergency situations. During those situations, digesting food isn't a priority. Mobilizing muscles so we can fight or flight is a priority. This is a very good thing. However, when someone is chronically stressed, there's a serious problem. Now autonomic nervous system is out of balance. Immune system and digestive system become suppressed. When someone is in this state, no matter how well he/she eats, he/she can't get much nutrition out of food. And, they're much more prone to illnesses due to suppressed immune system. Changes in physical state often reflect changes in psychological state. In Aikido, they say physical body is an extension of mental state.
When our body and mind are out of balance and start to act separately, then we start to develop all sorts of problems. This is why I believe in a holistic health/wellness approach. Mindful movement practice such as Feldenkrais, Tai Chi, Aikido, Yoga, etc is a wonderful way to keep our body and mind in balance.