There is no one "American" lifestyle, so it's impossible for it to be dangerous. All lifestyles have dangers, and if someone decides to indulge in more of the dangerous aspects rather than the healthy ones, then it is not the over arching lifestyle that is dangerous--it's the choices the individual makes.
As noted below, there is no singular American lifestyle. Some people eat at McDonalds literally every day, and yes, that's very bad for your health. Most rarely touch the stuff. There are certainly fast, affordable options that are bad for your health offered in this country, but they aren't utilized to excess by a majority of its people.
I don't think you can group everyone in America into one group and call that the American Lifestyle. For example, are you saying that Americans are obese by the foods they eat? Are you saying that they spend too much money? Or are you saying something positive? There needs to be a clear definition of what you are saying is the American lifestyle.
Define the "American lifestyle." The country is far too varied and differing from area to area to make such a simple-minded claim. The general public, and the general perception of the world at large seem to think that every American is fat and lazy. If that was so, then would the US really be at the forefront of the world today?