In my experience, it really is down to lifestyle choices. I've never been really overweight; the closest I've come is about 200lb (90kg), but I also had a 40in (100cm) waist. What helped is I did diet, then I got a septoplasty for borderline sleep apnea, then I joined a volunteer search and rescue organization in which I hiked every weekend. No special diets (just calorie restriction), no special workouts (just hiking in the mountains regularly).
I inherit high cholesterol (above 220 at one point), but I've managed to lower it through diet alone by drinking a lot of fruit juice (but I also have to watch out for diabetes . . . ), and I've also got it lower through very heavy exercise alone (but I also have to watch out for skeletal/joint injuries . . . ).
As I've gotten older, I've noticed more and more that what I eat has a noticeable effect on my mood, alertness and just physical feeling of well-being. I've also noticed that every time I exercise now, I feel really good afterwords.
I don't think that diet and exercise are a panacea, but they could at least go a very long way to alleviating symptoms and preventing illness. I also think there is way too much pseudoscience and BS going on around exercise and diet that it makes it that much harder, and a lot of programs (high intensity workouts like CrossFit and exclusionary diets like Paleo) are completely irresponsible and very likely worse for people in the long run. It also doesn't help that most of these programs are faddish and very dogmatic, with no tests to back them up.