Eating unhealthy food

Hello Rabbi,
I would love to hear your opinion regarding the Halacha regarding eating foods known to be unhealthy for humans.
Does the law permit or prohibit eating them?
If you allow it, why?
And if you forbid it, how come most of us are infected with it?
In general, is it permissible for a person to harm themselves?
What is the difference between eating processed food and smoking cigarettes or even smoking drugs? Is it less harmful, and the destructive effect is not immediate? How much less harmful does it have to be for us to allow it?
It seems to me that this is a complex issue that, as I said, I would be happy to hear the rabbi give his opinion on from a halakhic/moral perspective.
This is worth a column.
Think about it..=)
thanks.