Not entirely, by any means.
Living longer is not really the point. The average lifespan was very low because of early deaths, but if you made it to 30 your chances of making it to 80 were not that different from today.
Something has changed because cancer and heart disease were once rather rare in America. Exactly what changes have caused these diseases to increase so is what we don't know. There is a correlation with the rise in consumption of unsaturated vegetable oils and particularly hydrogenated vegetable oils (trans fats, previously used only for soap), but proof of cause / effect is lacking.
Even at the American Heart Association's kick-off event for their "Heart Healthy Diet", one of the older physicians invited said he doubted the research. He said early in his career the American diet was everything the Heart Association said was bad - yet he saw almost no cases of either cancer or congestive heart disease.
