Foods cause your body to have a certain reaction. So the idea is that foods cause your body to lower its pH, which goes against our slightly alkaline nature.

I think there may be some substance to this, but in general, it's save just to eat a variety of wholesome foods. It's tougher for most Americans to realise this since they're crunched for time, looking for shortcuts, and advertisers are always pushing a new diet or wonderfood every year.