I've seen this statement, in one form or other, often repeated. But I've never seen any actual studies or data that support the use of such high percentages. Of course it is true that certain behaviors will lead to or worsen some diseases. Most smokers, if they live long enough, will get COPD. Alcohol & drug abusers are also more likely to have certain chronic or deadly health problems. But to say that most chronic disorders are entirely preventable is highly debatable.
In my almost 30 years as an RN, I have seen every disease come in almost every variety of humankind you can imagine. Fat, thin, sedentary, athletic, "health food nut", "junk food junkie", etc. etc. They all get these so-called preventable illnesses. However, there is one thing that does tie the majority of these people together---Most of them are ELDERLY. So I guess we should just not get old. That would be a sure way of preventing chronic illnesses and decreasing health care costs!