The impact of lifestyle on physical health is relatively limited; only a few percent of the variation in health outcomes can be attributed to lifestyle factors such as diet and exercise. Moreover, most of the research on the lifestyle and health is correlational, so we should be careful about assuming that the lifestyle factors are the cause of the health outcomes. (Remember how fiber was supposed to reduce the risk of colon cancer? Spurious correlation.)
Sociological aside: given the hyperindividualism of U. S. society,.it's not surprising that people would exaggerate the impact of lifestyle choices on health, We love to pretend that individuals shape their own destinies based on their own choices.
Miles