- We think that we can offset consumption of "unhealthy" foods with consumption of "healthy" foods
- We think that the more "healthier" food we eat, that the more "healthier" we get.
If we define "healthy" food as an food containing essential nutrients (water, proteins, fats, carbohydrates, minerals, vitamins, and so on), and only nutrients (no disease causing agents), then we will see that overeating such "healthy" food will not make us more healthy. In fact, for most foods constituents we have a certain range that our body needs and is able to regulate. The mechanisms with which the body regulates this are hunger and taste.
For the most basic food constituent – water – we will get problems if we consume too little and get problems if consume too much water. And the same goes for everything, from salt to meat, from vegetables to tubers. A healthy individual, with foods from his/her environment of evolution, will choose those foods (and then in the "right" amount) to stay healthy. No animal needs to be fed "healthy" food to stay healthy, no animal needs to be told to not overeat, and no animal needs to be told to exercise (I just need to look at our dog to see how much he likes to run), if it is in its natural environment (or something that is close enough).
Conversly: If an food is not present in the natural environment of an animal, then it has the potential to cause disease. And the same goes for the human animal.