The bottom line is a primarily acidic diet causes free radicals and other toxin buildup and can eventually cause cancer and other disease in the body. Sugar and most meats (like bacon!) are the primary culprits...
Here's a good list of both alkaline and acidic foods. The key from my point of view is not to go overboard and restrict all foods to alkaline, but to have a balance of foods, leaning mostly toward to alkaline variety. There are some big surprises for the uninformed -- lemons and limes, though "acidic" are alkaline when digested by the body.
