)I would guess the reasons are both diet and lack of physical activity. Ie. Americans (and Australians) eat a lot of "snack food" and a lot of fairly unhealthy food such as fried food or sugary food. Also the main forms of entertainment seem to be watching tv (ie. no physical activity), playing video games (very little physical activity), watching movies (no physical activity), listening to music (little physical activity) and watching sporting games (little physical activity). Things like playing sports, playing music, going to the beach/park etc. seem to be less common. My theory for this is that those activities that are healthy are also basically free - so there is no money to be made from advertising. Getting people to watch tv or buy games and music etc. has the potential to make a great deal of money (and does) so there is much more advertising for those products and so people do them more. Same with food - healthy food is actually fairly cheep - especially if you cook it yourself (ie. fruit and vegetables, rice, pasta etc.) - so there is much less money to be made - hence much fewer advertisements for bananas and lots of ads for McDonalds. There does seem to be a slow move towards promoting foods as healthy in Australia these days but it's unclear if this is having much effect or not. Plenty of items that are not actually very healthy are still advertised as being healthy and good for you too.
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